WO1984004018A1

WO1984004018A1 - Device for automatic control of power to an electrical load and circuits therefor

Info

Publication number: WO1984004018A1
Application number: PCT/US1983/000462
Authority: WO
Inventors: Lawson Paul Mosteller Jr
Original assignee: Lawson Paul Mosteller Jr
Priority date: 1983-04-04
Filing date: 1983-04-04
Publication date: 1984-10-11
Also published as: EP0139641A1

Abstract

A device for producing a gradual change in power supplied to a load or plurality of loads which utilizes at least one gated solid state switching device (15). Dimmer switches are well known in the art but these switches are manually operated and the human eye requires approximately 15 minutes to fully adjust from a dark environment to a normal, well-lighted environment, or light to dark. Such problems are solved by the use of a timer select switch (SA) for producing a gradual increase or decrease of the light source (20) such that the human eye has ample time to adjust to the surrounding light conditions.

Description

DEVICE FOR AUTOMATIC CONTROL OF POWER TO AN ELECTRICAL LOAD AND CIRCUITS THEREFOR

This invention relates to an electrical power device and circuits for the device for controlling the level of power and the rate of change in power to an electrical light or load.

BACKGROUND OF THE INVENTION In the past, a number of different types of dimmer switches have been known both for incandescent and fluorescent lighting systems. While the most common control for lights or lighting systems has been the on/off switch, manual dimmer switches have increased in popularity because of the flexibility they offer the user to control the lighting in a room to the desired and most comfortable intensity.

Such manually operated dimmer switches are readily available commercially and are well-known in the art. These manually operated light dimmer switches typically control the brightness of the light by controlling the resistance of a manually operated variable resistor in the control circuit of a silicon controlled rectifier which in turn controls the amount of current to the electric light bulb and thereby controls the brightness or intensity of the light emitted. In addition, such prior art dimmer switches frequently also include an instant on-instant off capability.

The human eye requires approximately 15 minutes to fully adjust from a dark environment to a normal, welllighted environment, or vice versa. Rapid changes from dark to normal lighting conditions causes unpleasant sensations in the human eye and indeed may even cause flash blindness in extreme cases. Upon going from light to near darkness, a person cannot see well until his eyes adjust. This condition can be particularly noticeable by a person developing film in a dark room, for example, where certain operations are performed in total darkness while others are performed at normal light intensity levels, or even by a person trying to locate a door or some other object, such as a bed, in a dark room after the light has been turned off. In addition, there are frequently periods of time after an eye operation when abrupt changes in light intensity from dark to bright could be very painful, if not harmful to the patient.

Further, it is often unpleasant and frequently more difficult for a person to awaken early in the morning before dawn and while it is still dark, than it is to awaken after dawn when the eye is partially adjusted to the light through the eyelid. A more natural method of awakening a person in a pleasurable manner is the gradual lighting of the area around the person as produced by the rising of the sun.

Unfortunately, commercially available dimmer switches, while capable of gradually increasing the light intensity, cannot do so in an automatic manner since an individual must gradually turn the control knob in order to gradually increase the intensity, giving rise to an inconvenience which would ordinarily prevent the dimmer switch from being used in this manner.

The prior art reveals a number of devices for gradually increasing the intensity of a light or gradually decreasing, the intensity. For example, U. S. Patent 3,798,889 discloses an artificial sun rise producing device which utilizes a tapered slit to control the amount of light reaching a light sensitive resistor for controlling the intensity of the lighting system. More recently, a number of solid state devices such as triacs, diacs, programmable unijunction transistors, light emitting diodes, and the like have been used for controlling light dimmers in a progressively variable manner. Some of these devices may also supply power to electrical loads other than lights. Such devices are typified by U. S. Patents 3,898,516, 4,008,416, 4,152,607, 4,152,608, 4,144,478, 4,159,442, 4,082,961, and 3,898,002.

OBJECTS OF THE INVENTION

A primary object of this invention is to provide a control circuit that can maintain a selectable steady state power level to a load or gradually transition the load power between steady state levels and that also can, while the power to the load is disconnected, either maintain a selected control circuit state or transition to another steady state control circuit state.

Another object of the invention is to provide a single control circuit that controls both the positive and the negative half cycle of the supplied power.

Yet another object of the invention is to provide a circuit to control non-rectified current to the load.

Yet a further object of the invention is to provide a circuit that has a low power requirement. Still a further object of the invention is to provide a circuit that does not depend on an electromagnetic relay for timer switch automatic initiation of the gradual-on sequence.

Still another object of the invention is to provide a control system for providing a selectable rate of gradual change between selectable steady state power levels to the load and for providing a means to either maintain a selected control circuit state or transition to another steady state control circuit state while the power to the load is disconnected.

Yet a further object of the invention is to provide a control circuit that does not automatically revert to the full power state when the load is disconnected.

Still another object of the invention is to provide a circuit for providing the gradual-on/gradual-off sequences that can be controlled from two remote locations.

Still a further object of the invention is to provide a circuit that automatically turns on and turns off the power to the load instantly at corresponding specific preset power levels and that automatically varies the rate of change in power and the final steady state power level in accordance with the setting of a selectable switch. Yet another object of the invention is to provide a circuit for controlling the rate of change in power supplied to the load by the positive half-cycle of power while delivering the complete power of the negative half-cycle to the load. Still another object of the invention is to provide a single circuit for providing either a.c. power with the power delivered by each half-cycle controlled by the circuit, or a.c. power with the positive half cycle power controlled by the circuit while the total negative half-cycle is supplied to the load, or half- wave power with the positive half-cycle power controlled by the circuit and the negative half-cycle blocked from the load, while also providing in either case automatic turn-on and turn-off at specific preset power levels, and for providing an option for manual instant disconnect of power to the load or manual instant turn-on to full power with the control circuit by-passed. Still a further object of the invention is to provide a circuit for providing the gradual-on/off sequences in response to the temperature of a temperature sensitive element.

Still another object of the invention is to provide a circuit that provides power that is proportional to the temperature of a temperature sensitive element.

Yet a further object of the invention is to provide a circuit that tends to maintain a constant comfort level in a room by varying the speed of a fan in proportion to the temperature in the room.

Still another object of the invention is to provide a circuit that controls the speed of a fireplace fan so as to regulate the speed according to the temperature of the air blown into the room. Yet a further object of the invention is to provide a circuit that provides power that is inversely proportional to the amount of light received by a light sensitive element.

Still a further object of the invention is to provide a circuit that tends to maintain a constant light level in a room by varying the brightness of a light inversely proportional to light received from other sources such as through a window. Yet another object of the invention is to provide a circuit that provides power that is directly proportional to the amount of light received by a light sensitive element. Yet a further object of the invention is to provide a circuit that tends to maintain a constant visibility of display lights as the brightness of other light sources varies such as a change from indirect sunlight to direct sunlight. A further object of this invention is to provide a control circuit capable of automatically varying over a preselected time interval the rate of change in intensity of a light or lighting system.

Another object of the present invention is to provide a light control circuit capable of gradually increasing the light intensity from a full off position to a preselected level of intensity.

A further object of the invention is to provide a simple electro-optical control circuit capable of automatically varying over a preselected time interval the resistance of an electro-optical device in a continuous and gradual manner from high resistance to low resistance and vice versa upon manual or timer initiation.

Yet another object of this invention is to provide a lighting control circuit wherein the duration of the gradual change may be varied from nearly instantaneous to tens of minutes or longer.

Yet a further object of this invention is to provide a light control circuit for varying the light intensity after manual or timer initiation in a gradual and continuous manner over a preselected time interval. Still a further object of this invention is to provide a lighting control circuit having the capability for manual instant on/off, for manual dimming, and for preselection of the final brightness level of the automatic gradual on/off operation.

Still another object of this invention is to provide an artificial dawn and an artificial dtisk at a preselected time.

Still another object of this invention is to provide an automatic light intensity control switch which may fit into an ordinary electric light switch junction box.

Yet a further object of the present invention is to provide an automatic electric light control for gradual on/off operations which is silent after the sequence is initiated.

Yet a further object of the present invention is to provide an automatic light intensity control switch. These and other objects and advantages of the present invention will become apparent when considered in light of the following description and claims when taken together with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIGURE 1 shows one embodiment of a lighting control circuit according to the present invention using light emitting diodes and photoconductors in the control circuit.

FIGURE 2 shows an alternate embodiment of the circuit according to the present invention using a programmable unijunction transistor with a silicon controlled rectifier.

FIGURE 3 shows another circuit according to the present invention and similar to the circuit of FIGURE 2. FIGURE 4 shows an embodiment of the control circuit according to the present invention that is similar to that of FIGURE 2 in the referenced patent but is different in several important respects. It has a resistor that is placed in parallel with the load when a switch is closed, it has a switchable resistive network instead of a relay subcircuit, it has an ordinary on/off switch and it has a switchable resistive network to provide a choice of charging and discharging time constants for the capacitor in the gate circuit of the programmable unijunction transistor,PUT.

FIGURE 5 shows an alternate embodiment of the circuit according to the present invention using a fullwave rectifier, a half-wave rectifier, and a subcircuit for automatic turn-on and turn-off at preset power levels to the load.

FIGURE 6 shows another embodiment of the circuit according to the present invention using dual remote control locations and an alternate position in the circuit for the electrical load.

FIGURE 7 shows another embodiment of a circuit according to the present invention for controlling fluorescent lights .

DESCRIPTION OF THE INVENTION Referring firstly to the embodiment of FIGURE 1, a schematic diagram of the circuit of the present invention for use with incandescrnt lights is shown. The circuit utilizes batteries (or power supplies) B1 and B2, typically a 9 volt power supply at B1 and a 9 volt battery at B2, capacitors C1 and C2, resistors R1 through R16, switches SA and SB, timer switch St, relay switch SR, n-channel field effect transistor (JFET) 10, and light emitting diodes 12 and 14, all of which are in a first portion of the circuit.

The second portion of the circuit which is electro-optically coupled to the first portion includes photoconductors 16 and 18, resistors R17, R18, and R19, and switch SA substituted for the manually operated variable resistor of a standard, well-known type of manually operated incandescrnt light dimmer switch which normally also includes choke L1, capacitors C3 and C4, and the disc 13 and triac 15.

With switch SA initially at position 2, switch SB at position 3, and capacitor C1 discharged, the current through capacitor C1 and resistors R1, R2 and R3 (R2 is normally set to full value) makes the gate, G, voltage of the JFET 10 sufficiently negative to prevent the required drain, D, current to light the LED 14. As the capacitor C1 charges through resistors R1, R2 and R3, the gate voltage gradually increases permitting the brightness of the LED 14 to vary gradually from an imperceptible level to the maximum brightness allowed by the current limiting resistor R14 in the drain circuit for a zero gate voltage. The LED 14 reaches full brightness when capacitor C1 becomes fully charged to the voltage of battery B2, i.e. 9 volts. The charging time is determined by the C1 (R1 + R2 + R3) time constant.

Starting with capacitor C1 fully charged to the battery voltage, i.e. 9 volts, LED 14 can be turned off or dimmed gradually by rotating switch SA to position 1 or 3 to discharge the capacitor C1 through resistor R10 and thereby decrease the gate voltage at the JFET 10.

Resistor R10 is selected so that the LED 14 just goes out or reaches the desired minimum brightness upon capacitor C1 discharging to the voltage determined by the resistive network R1, R2, R3 and R10 and the battery B2. When the switch SA is at position 3 and LED 14 is off, the. "gradual on" sequence can be initiated automatically by a timer switch ST which activates relay switch SR to disconnect resistor R10, to connect resistors R7 and R8, and to apply the power supply voltage (9 volts) to the LED 12. Disconnecting the resistor R10 permits capacitor C1 to charge gradually to increase slowly the brightness of LED 14 to the brightness level determined by the setting of variable resistor R7. If variable resistor R7 is set at its maximum value, capacitor C1 charges sufficiently to bring the LED 14 nearly to full brightness, since at that JFET gate voltage, the current through LED 14 is primarily governed by the value of the current limiting resistor R14. If the variable resistor R7 is set to its lowest value (0), LED 14 will not increase in brightness after relay switch SR is activated. However, if the variable resistor R7 is set for some intermediate value LED 14 will increase in brightness to a level governed by the resistance of R7. Applying the 9V power supply voltage to the LED 12 portion of the circuit momentarily lights LED 12. At first, the current in LED 12 is limited by resistor R15, but then decreases quickly as capacitor C2 charges, to the minimum current determined by resistors R15 and R16 for a 9 volt source. Capacitor C2 and resistor R15 are so chosen as to make the LED 12 flash on and off once upon the activation of switch SR by the timer switch ST, and resistor R16 is so chosen that it discharges capacitor C2 quickly without permitting excessive minimum LED 12 current.

The timer is a standard 110V A.C. security light timer that has been modified so that it closes a set of contacts at "on" time and opens these contacts at "off" time without supplying 110V A.C. to the circuit controlled by these contacts. This timer switch ST activates relay switch SR upon closing and deactivates this switch upon opening.

With the timer on and LED 14 at full or intermediate brightness, the "gradual off" sequence can be initiated automatically by the timer opening the circuit to relay switch SR at a preselected time, or manually by rotating switch SA to position 1 so as to connect resistor R10 and to open the circuit to relay switch SR.

The LED 14 will remain on and increase gradually to full brightness, if not already there, when switch SA is rotated to position 2 before the timer open the circuit to the relay switch SR.

The first portion of the circuit thus far described is coupled with the second portion of the circuit by placing the LED's 12 and 14 respectively close to photoconductors 18 and 16. This portion of the circuit is a standard manually controlled incandescent light dimmer switch with the manually controlled variable resistor replaced by photoeonductors 16 and 18, R17, R18, R19, and switch SA. Starting with switch SA at position 1, and LED 14 at minimum brightness, upon moving switch SA to position 2, the light bulb 20 comes on at a dim glow (resistors R17 and R18 are selected such that they, in parallel, provide the maximum resistance that permits the electric light to turn on at minimum brightness). Then, as LED 14 gradually brightens, the resistance of photoconductor 16 gradually decreases which in turn gradually increases the brightness of the electric light 20 to full on as LED 14 reaches maximum brightness. When the switch SA is rotated to position 1 or 3, the electric light 20 gradually dims continuously to completely off as LED 14 dims to minimum brightness. The resistor R17 is selected so that it, in parallel with photoconductor 16 will provide sufficient resistance when photoconductor 16 is about 1M to distinguish the electric light. With switch SA at position 3 and the electric light fully off, upon timer switch ST activation of the relay switch SR, the LED 12 flashes on and dims quickly to lower momentarily the resistance of photoconductor 18 suffuciently for the electric light 20 to come on at minimum "initial on" brightness and then to dim instantaneously to a much lower level before gradually increasing in brightness to full on. The gradual "off" sequence can be initiated automatically at a preselected time by the timer or manually by rotating switch SA to position 1. However by moving switch SA to position 2 before the relay switch SR deactivates, the light remains on at full brightness upon timer switch ST deactivation of relay switch SR.

The variable resistor R2 provides the capability of manually controlling the brightness of the electric light. With switch SA at position 1 or 3 , and the light out, and relay switch SR deactivated, decreasing the value of variable resistor R2 from the normally full values setting, instantaneously turns the light emitting diode 14 on and thus the electric light on, and then continuously increases the brightness of the light emitting diode 14 and the electric light to full brightness as the value of the variable resistor R2 is further decreased to Q. Then, increasing the resistance of the variable resistor R2 decreases the brightness of the LED 14 and the electric light. With the switch SA at position l or 3 and the variable resistor R2 set at an intermediate value, upon rotating switch SA to position 2, the brightness of the light emitting diode 14 and electric light 20, originally at an intermediate brightness, gradually increases in brightness to full on. With the light emitting diode 14 and the electric light 20 fully on, rotation of switch SA to either position 1 or 3 will cause light emitting diode 14 and light 20 to gradually dim to the brightness level determined by the resistive value of variable resistor R2.

Positions 3, 4, 5 and 6 of switch SB provide a choice of different "gradual on" and "gradual off" time durations. The operation of these gradual on/off sequences are identical to that for position 3 of switch SB except for the duration. Table 1 gives suitable values for the various components used in the FIGURE 1 embodiment. With the values as specified in Table 1, position 3 of switch SB provides "gradual on" or "gradual off" duration of about six minutes, position 4 about four and one-half minutes, position 5 about two minutes, and position 6 about three seconds. The time duration for the "gradual on" or "gradual off" sequences could be increased almost without limit by properly increasing the sizes of resistors R1, R2 and R3 and capacitor C1, and the minimum time could be reduced to zero by making R6 and R13 zero, provided that resistors R6 and R13 were decoupled in the "gradual off" mode by using another switching arrangement to avoid shorting of the power supply B2.

Positions 1 or 2 of switch SB and variable resistor R7 provide the capability for the selection of the light emitting diode 14 and thus the electric light 20 terminal brightness at the end of a "gradual on" or "gradual off" sequence and in addition provide the capability for gradual change in the LED 14, and thus the electric light 20, brightness from any brightness level within the full range from off to fully on to any other brightness level. This process is gradual because the state of charge of capacitor C1, which governs the brightness of LED 14 and thus the light 20, changes only by current flowing through the resistive network connected to capacitor C1. With the resistive value of variable resistor R7 set to zero and switch SB at position 1 or 2, LED 14 and thus the electric light 20 gradually dims to completely off with position 1 of switch SB being the slower. Setting the variable resistor R7 to the maximum value causes the light to gradually brighten to fully on when switch SB is at position 1 or 2.

A variable intensity fluorescent light dimmer circuit also has a variable resistor that controls the brightness of the light. By replacing this variable resistor by a set of resistors and photoconductors similar to that of R17, R18, R19, photoconductors 16 and 18, and switch SA, and by controlling the resistive values of these photoconductors with a circuit similar to that of the first portion of FIGURE 1, all of the gradual variations in brightness described above for incandescent light would also be provided for fluorescent light.

Different applications for the present invention utilize different combinations of the switches and networks of FIGURE 1. Other applications could use different combinations or even different sets of networks. In the first application, the complete circuit as shown in FIGURE 1 would most likely be used, and the timer switch plays a key role. A primary function is to provide an articificial dawn in a bedroom before a person is awakened by a preset alarm during non-daylight hours.

When switch SA is set to position 3 to dim the light gradually to completely off with switch SB at position 3, 4, 5, or 6, the switches are properly set for the preset timer switch ST to activate relay switch SR to initiate automatically the "gradual on" sequence to provide the artificial dawn. It is in this application where it is important that the light starts the "gradual on" sequence from a very low level that the flashing of LED 12 plays an important role in initiating "turn on" of the electric light at the minimum "turn on" brightness and then immediately lowers the brightness to a dim glow before starting the gradual increase in brightness to the final level established by the setting of variable resistor R7.

It should be noted that neither resistors R4, R5 nor R6 is in parallel with the R1, R2, and R3 resistive network when the relay switch SR is activated to provide a "gradual on" duration of six minutes or greater which is independent of the "gradual off" selection.

In addition to providing an artificial dawn light, this invention also provides a manually initiated "gradual on"/"gradual off" control circuit with preselected time durations, a control circuit to gradually vary the brightness of a light from one level to another, an ordinary on/off switch by using the fast "gradual on/gradual off" selection and a manually operated dimmer switch that provides instant variation in brightness over the complete range from off to full on by rotating a knob.

In another application of the invention thus far described, the artificial dawn option need not be provided and variable resistor R2 could be replaced by a fixed resistor and variable resistor R7 could be omitted. This version of the invention provides a two switch electric light control circuit to provide up to six "gradual on" and six "gradual off" time durations ranging from about three seconds to six minutes or longer if desired. The control system could be housed in a container which could be easily inserted into an ordinary wall switch box. This embodiment of the invention could be used in a bathroom or any other place where the light normally would be turned on after a person's eyes have adjusted to the darkness or where it would be convenient for a person to see for a short time after the light switch has been turned off. For example, the device could be used in an infant's room to avoid the instant turning on of a light to full brightness while providing instant light at a low level sufficient for seeing, and then gradually increasing to a brighter level. The "gradual off" mode would also avoid awakening an infant as often occurs when a room light is turned off instantaneously shortly after the infant is put to bed for the night. Of course it would also lessen the likelihood of awakening a child when the light is turned on instantaneously. Alternatively, a single two-pole/six-throw switch could be used in place of the two switches mentioned above to provide three different "gradual on/gradual off" times. Another embodiment of the present Invention is shown in FIGURE 2, and has the major advantage of operating directly on ordinary household line voltage, e.g. 110V A.C. without the need for batteries, transformers, power supplies or the like.

The circuit of FIGURE 2 includes a silicon controlled rectifier (SCR) 22, a programmable unijunction transistor (PUT) 24, an incandescrnt light bulb or lighting system 26, diodes D21-D24, capacitors C21-C24, resistors R21-R33, switches S20-S21, relay switch SR' , timer switch ST' and coil L20.

With the circuit in steady state condition with relay switch SR' open and switch S20 at position 1, 2 or 3, the voltages at the anode A and the gate G of the PUT 24 are nearly equal. In this state, PUT 24 does not conduct, as it only conducts when the voltage on the anode A is greater than the voltage on the gate G by the threshold value. If switch S20 is now moved to position 5, the light 26 comes on at a dim glow and gradually increases in brightness to full or nearly full brightness over a time interval equal to about three minutes. During this time interval, the conduction initiation point of the PUT 24 is initially at about the 180° point of the positive half cycle and then gradually decreases to about zero degrees. During this same time interval, the SCR 22 starts conducting initially at about the 180 ° degree point of each positive half cycle and then gradually changes the conduction initiation point to about the zero degree point of each positive half cycle. Since the SCR 22 conducts only for the remainder of each positive half cycle after the SCR gate G has been triggered by voltage which is positive with respect to the cathode, the fraction of the positive half cycle during which the SCR 22 conducts, varies gradually from zero when the PUT 24 is in the off state for whole cycle to 100% when the PUT 24 conducts at the beginning (zero degree point) of the positive half cycle.

If switch S20 had been moved to position 6 instead of position 5, the light would have immediately come on at a low intensity, brighter than the initial low intensity of position 5 and gradually increased to full brightness in about 30 seconds. Likewise if switch S20 had been moved to position 4 instead of position 5, light 26 would have come on at a faint glow and then gradually increased in intensity to full brightness in about 15 minutes provided that the variable resistor R30 is rotated to a full resistance position in the circuit. The purpose of the variable resistor R30 will be described below. With the light at maximum brightness for switch S20 at positions 4, 5 or 6, upon rotating switch S20 to positions 1, 2 or 3, the light gradually dims to completely off in about 30 seconds, three minutes or 15 minutes respectively.

The gradual on and gradual off times given above are for the values of the components as listed in Table 2 however other times may be provided by using intermediate values for resistors R26-R32 provided that the sum of R26 R27 and R28 remains constant. Sufficiently large relative increases in resistance in parallel with capacitor C21 when switch S20 starts at position 1, 2 or 3 causes the light when off, to turn on immediately at full brightness. Likewise, a sufficiently large relative decrease in the resistance in parallel with capacitor C21 causes a fully on light to go out immediately.

The intensity of the light 26 at initial on, when switch 20 is turned to position 4, 5 or 6 from position 1, 2 or 3 varies in direct proportion to the resistance in parallel with capacitor C21. At position 4, the light comes on so faintly that the glowing filament in the electric light bulb is just visible in a dark room. At position 5, the initial brightness of the light is sufficient for a person whose eyes are sufficiently adjusted to the dark to see most objects in a previously dark room without unpleasant sensations to the eye. At position 6, the light intensity starts at about half of full brightness.

By adjusting the variable resistor R30, the final brightness of the light may be preselected for position 4 of switch S20 without changing the gradual on/off timing sequence for any other setting of switch S20. The variable resistor R30 provides the full range control of the final light brightness from fully on to completely off. Further, the light gradually changes from one steady state partial brightness level to another steady state partial brightness level when R30 is adjusted.

Capacitor C22 partially charges during each positive half cycle when the SCR 22 is non-conducting, and partially discharges during the time that SCR 22 is conducting, and during each negative half cycle. When the circuit is in the "gradual on" sequence, capacitor C22 discharges more than it charges, thereby producing a net effect of a slow discharge. On the other hand, during the gradual off sequence, capacitor C22 charges more than it discharges thereby producing a net effect of a slow charge. When the control circuit is set to produce a steady state "partial on" condition, and the circuit has reached this state, capacitor C22 charges and discharges an equal amount during each cycle, thereby maintaining a constant bias on the gate of the PUT 24 within a range such that the PUT 24 anode voltage control circuit causes the PUT 24 to fire at the same phase point in each cycle of the applied 110V A. C. power.

If the values of resistors R22 and R24 or of capacitor C22 are doubled (or halved), the time duration for gradual-on and gradual-off sequence will increase (or decrease) by a factor of about 2. Therefore, it is possible by using the proper choice of resistors R21 through R32 and capacitors C21 and C22, to obtain essentially any desired duration for the gradual-on or gradual-off modes. Also by using different suitable values for the resistors R21 through R32 and capacitors C21 and C22, for each case any gradual-on or gradual-off duration from seconds to hours may be achieved with the brightness of the light 26 traversing the full range of intended change in illumination from fully off to completely on and vice versa, or between any two dimmed or partially on steady state levels.

Since the circuit of FIGURE 2 controls only the positive half-cycle of the applied voltage while blocking the negative half-cycle, the electric light bulb when fully on, as described above, emits light at less than half the rated intensity. By using a sufficiently high wattage light bulb, adequate intensity can be produced with increased bulb life, and of course if necessary, additional light bulbs may be placed in parallel. The circuit of FIGURE 2 may be used as a dawn light control system when switch S21 is closed. Under normal operating conditions, the timer switch ST' would energize the relay switch SR' only when switch 20 is in positions 1, 2 or 3 so that the light bulb would be off. With the light off (switch S20 at positions 1, 2 or 3) , and switch S21 closed, upon automatic closure of the timer switch ST' at the preset time, the light comes on at a very faint glow and gradually increases in brightness to full intensity in about 15 minutes to simulate the natural dawn.

If timer switch ST' or switch S21 is opened after the light is completely on, the light will gradually dim to completely off in about 15 minutes, three minutes, or 30 seconds when switch S20 is respectively in positions 3, 2 or 1. If switch S20 is in position 4, 5 or 6 and the light is fully on when the timer switch ST' or switch S21 is opened, the light remains fully on. If the light 26 is partially on when relay switch SR' is activated, the light 26 gradually increases in brightness to full intensity in time, less than 15 minutes, that is inversely proportional to the brightness of the light upon relay switch SR' activation. If relay switch SR' is deactivated when the light is partially on, the light gradually increases from that brightness to full intensity in less than 15 minutes, three minutes, or 15 seconds if switch S20 is respectively in position 4, 5 or 6, where alternatively it decreases from that brightness to completely off in less than 30 seconds, three minutes or 15 minutes if switch S20 is respectively in positions 1, 2 or 3.

Capacitor C24 and coil L20 provide a filter which greatly decreases or eliminated any interference with radios, and these components prevent sharp spikes of current through the light 26. They are similar to and provide the same function as the coil and capacitor filter on standard, manually operated variable intensity switches. Thus the circuit of FIGURE 2 provides an automatic artificial dawn producing device when the timer switch ST' energizes the relay switch SR' with the light initially completely off and switch S20 in position 1, 2 or 3. Further, the device produces an automatic artificial dusk, when the timer switch ST' de-energizes relay switch SR' while the light is on, and switch S20 is in positions 2 or 3.

In addition, the light control circuit can keep the light intensity at any desired level between completely off and fully on, and can gradually vary the light intensity from any intermediate level to any other immediate level including fully on and completely off, thereby performing functions similar to those of the circuit of FIGURE 1.

Since the circuit of FIGURE 2 only illuminates light during one half-cycle of the current, at a very low intensity a light bulb will produce a slight flicker giving the appearance of candlelight. Of course this may be avoided, if not desired, by adjusting the PUT anode voltage control resistors so that the initial-on brightness is beyond the slight flicker range while still at a very low brightness.

However, by using one of these control circuits to supply current to the light bulb during the positive half-cycle of the applied A.C. power and another identical control circuit to supply current to the light bulb during the negative half-cycle of the supplied power, the light bulb when fully on will emit light at its rated capacity. Such an arrangement is shown in FIGURE 3.

Referring to FIGURE 3, the light bulb 46, capacitor C44, and coil L40 are common to each control circuit. Capacitor C43, diode D44, resistor R53, switch S41, timer switch ST' ' and relay switch. SR' ' are shown in the top half of the circuit only. Both circuits can be controlled by using a double-pole double-throw relay SR''. With the switch S40 of one circuit ganged with switch S40' of the second circuit, the gradual on and gradual off sequences as described above for one circuit produce the full swing in light intensity from off to full rated illumination and vice versa. By using resistive values for R41, R42, R44 and R46-R52 closely matching the values for R41', R42', R44' and R46'-R52' respectively in the circuit, the light intensity can be made to stop at or between partial values as described above for a single control circuit. The automatic dawn producing circuit still consists of the top half of the circuit only, but could involve both halves as indicated.

When the two control circuits are thusly combined, the values of resistors R42, R44, R42' and R44' must be reduced to prevent one circuit from interfering with the trigger circuit for the PUT of the other circuit. To compensate for the reduction of the values of these resistors, the value of capacitors C42 and C42' must be increased proportionally. Suitable values for these components are: R42, R42' = 235K , R44, R44' = 90K and C42, C42' = 200 MF. The values of the other components may be the same as given in Table 2.

The connections to coil L40 and capacitor C44 are slightly different from FIGURE 2. Here, the cathode of the SCR 42' is connected to resistors R44' , R45', and R46' , and capacitor C42' and also connected to the anode of SCR 42 and to the anode of diode D41. Likewise, the anode of SCR 42' is connected to the anode of diode D41' , capacitor C42, and resistors R44, R45 and R46, as shown. Referring to the embodiment of FIGURE 4, a schematic diagram is shown of a circuit based upon the circuit in FIGURE 2 for use with half-wave rectified current to the load, and for timer activation of the gradual-on process. This circuit has the capability to provide multiple selectable rate of change in load power and final steady state power level. It also permits the load to be disconnected without changing the steady state condition of the control circuit. Furthermore it permits the full a.c. power to be supplied to the load by setting a switch while removing all power to the control circuit.

To simplify operation of the circuit all manually operated switches have been ganged so that they can be controlled by two knobs. Switch S40 represents a segmented variable resistor with detents at the numbered positions. The resistance does not change over the small region represented by the straight lines near the detents to facilitate alignment of the detents with the resistive values. The arc between positions 3 and 4 has been enlarged to facilitate selection of steady state intermediate levels. Intermediate resistive values between other detented positions are also selectable. Switch S40' is ganged with switch S40 in the standard manner. Switches S40'' and S40''' are ganged with switch S40 by utilizing cams on the shaft of switch S40. The normal positions for these switches are shown in FIGURE 4. At detent 0 of switch S40, switch S40" opens thereby disconnecting power to the load. It is closed at all other positions. Switch S40' ' ' is at the upper contact with switch S40 below the two-thirds point between positions 3 and 4. It is open above this position and below detent 7. It moves to the lower contact at detent 7, where it supplies full a.c. power to the load by-passing the control circuit. Switches S42 , S42' , and S42'' represent a standard three- pole four-throw switch. Switch ST' ' ' is an ordinary security light timer. With switch S40' ' closed, switch S40''' at the upper contact and switch S42 at position A, the gradual- on, gradual-off sequences and final steady state load power levels provided by selectable switch S40 at position 1 through 6 are essentially identical to those provided by the circuit shown in FIGURE 2. With the above switch settings and with no power to the load and switch S40 at position 1, 2, or 3, the circuit provides about a 15 minute, 5 minute, or 30 second gradual-on sequence upon rotating switch S40 respectively to position 4, 5, or 6. The rms voltage obtained at these positions are respectively about 80V, 83V and 84V. Witih full steady state power to the load at position 6 or 7 of switch S40 the present circuit provides about a 30 second, 5 minute, or 15 minute gradual-off sequence upon the rotation of switch S40 from position 6 or 7 respectively to position 1, 2, or 3. With full steady state power to the load at position 4 or 5 of switch S40 , the present circuit provides a slightly more rapid gradual-off transition than cited above upon the rotation of switch S40 to position 1, 2, or 3 due to the slightly lower initial power. Transition between any two steady state intermediate power levels between a rms voltage of about 20V and about 80V is provided by varying the position of switch S40 between position 3 and 4. The duration of the transition between some of these intermediate power levels is as great as thirty minutes. Upon rotating switch S42 to position B, all of the timing sequences are decreased by a factor of about 10. In this case the load power can be reduced from full power to half power in about 3 minutes by placing switch S40 near the midpoint between positions 3 and 4. With switch S42 at position C, upon positioning switch S40 to any position the power to the load obtains the steady state level so rapidly it appears to be instantly.

With switch S42 at position A' the timing sequence and final steady state load power levels are identical to that with switch S42 at position A for all positions of switch S40 with timer switch ST' ' ' open. The same is true for position 4, 5, or 6 of switch S40 when timer switch ST' ' ' is closed. If the load power is zero and switch S40 at position 1, 2, or 3 , upon the timer closure of timer switch ST''' the power is gradually increased to full power in about 15 minutes. Three different timer switch initiated gradual-on sequences could be provided if desired by adjusting the values of the resistors R52, R53 and R54. Resistor R55 provides a path for current flow from the power source to the control circuit when switch S40'' is open or when the load is physically removed with switch S40' ' ' mt the upper contact. Without resistor R55, upon opening switch S40'' or removing the load, the control circuit quickly reverts to the full load-power state if not already there with capacitor C42 completely discharged. Then upon closing switch S40'' the full rectified current is supplied to the load or load terminals. This not only greatly reduces flexibility in the control of the load power, but also creates a dangerous situation. For example, if the load is an electric light as shown in FIGURE 4, and if this bulb is removed when the light is off with switch S40 at position 1, 2, 3, or the lower region of the resistor between 3 and 4, without resistor R55, a person would be shocked upon intentionally or unintentionally inserting a finger into the empty light bulb receptacle when the power appeared to be disconnected from this receptacle. With switch S40''' at the upper contact, upon opening switch S40'' or removing the load, the state of charge on capacitor C42 remains essentially the same as if the load had not be disconnected. Consequently switch S40'' can be opened to disrupt power to the load and later be closed to supply instantly essentially the same power to the load as was being supplied before the switch S40'' was opened. Also the control circuit can be transitioned from one steady state condition to another with switch S40'' open. Furthermore, a person would not be shocked upon inserting a finger into an empty light receptacle if the bulb is removed after the control circuit is in the steady state off condition.

The value of resistor R55 should be small relative to the charging resistor for capacitor C42 when the current through the load is disrupted. On the other hand, it should be as large as possible to reduce power dissipation when the SCR42 conducts. For the circuit shown in FIGURE 4, a good compromise is 25K. With switch S42 at position C and a 25K resistor for R55, the change in the charge on capacitor C42 would not be insignificant when switch S40'' is opened or the load removed, but in this case it does not matter. The control circuit will remain in the off state if a finger, which has a large resistance, is inserted into the empty light bulb receptacle. Furthermore, since the circuit responds essentially instantly, the load power upon closing switch S40'' returns immediately to the steady state level corresponding to the setting of switch S40. Other gradual on/off durations can be obtained by appropriate choice of circuit components as descibed above.

The trim resistors RT44A, RT44B and RT44C are provided so that the gate resistive network of the programmable unijunction transistor, PUT 44 can be balanced with the PUT 44 anode resistors for each of the settings of switch S42. These resistors are balanced when the gradual-on duration with switch S40 at position 4 is nearly equal to the gradual-off duration when switch S40 is at position 3. When the circuit is balanced the load power is about half full power at steady state conditions with switch S40 near the midpoint between positions 3 and 4.

The component values for the circuit illustrated in FIGURE 4 are given in TABLE III.

The embodiment shown in FIGURE 5 is similar with three major differences to that of FIGURE 4 with switch S42 in position A. This circuit can control half-wave rectified power to the load with or without the other half-wave blocked and can control full-wave (a.c.) power to the load. In addition it can instantly turn on or off the power to the load, automatically, at preset power levels. By means of selection switches either half-wave power alone, with the other half-wave blocked, half-wave power with the other half-wave supplied directly to the load, full-wave power alone, half-wave power with or without the other half-wave supplied to the load with automatic preset power level turn-on and turn-off, or full-wave power with automatic preset power level turn-on and turn-off can be controlled.

Switches S50, S50', and S50'' are ganged and operate in the same manner as switches S40, S40'', and S40''' in FIGURE 4. Switches S51 and S51' are ganged so that one is closed while the other is open. Switches S52 and S52' are also ganged so that one is closed while the other is open. Relay switch SR50 is normally at the lower contact position. It moves to the upper position when the relay is energized.

With switches S52 closed and S52' open, the circult operates without the turn-on and turn-off action provided by the relay switch SR50, With switch S53 open, it operates in the half-wave mode with switch S51 open and switch S51' closed and in the full-wave mode when these latter two switches are reversed. In the full-wave mode, a.c. power is delivered to the load while full-wave rectified power is supplied to the control circuit. The zener diode Z50 clamps the voltage at the zero point for sufficient time for PUT54 to convert to the non-conduction state between half-cycles of power. Resistor R54 prevents the gate of PUT 54 from becoming completely depleted of charge at power levels below about 38V rms. Excessive flickering of the light could occur without this resistor. The full range of gradual-on and gradual-off sequences as described above for the circuit in FIGURE 4 is provided. For the circuit components listed in TABLE IV, the power levels provided at position 4, 5, 6 or 7 of switch S50 correspond respectively to a rms voltage of about 110, 117, 119, or 120 volts. Steady state intermediate power levels can be obtained as low as that corresponding to a rms voltage of about 84V. The action of the particular value of resistor R54 prevents lower steady state power levels in the full-wave mode. A larger value of resistor R54 would permit a lower steady state power; however, if resistor R54 is too large, the lower power instability would be excessive. With the control circuit in the fully off state, upon rotating switch S50 from any of the off positions 0, 1, 2, or 3 , to position 4, 5, 6 or 7, the power to the load reaches the maximum value for the corresponding position in about 17, 8, 1, and 0 minutes. The initial voltages supplied to the load upon rotating switch S50 to position 4, 5, and 6 are respectively about 0, 48, and 106V rms. With the circuit in the fully-on state with switch S50 at position 6 or 7, upon rotating switch S50 to position 3, 2, 1, or 0, the load power reaches the off state respectively in about 20, 6, 1, or 0 minutes. It reaches the off state in slightly reduced times starting from positions 4 and 5 due to the lower initial power level. In the half-wave mode with the control circuit in the fully off state, upon rotating switch S50 from position 0, 1, 2, or 3 to position 4, 5, 6, or 7, the power to the load reaches the maximum value for the corresponding position in about 17, 8, 1, or 0 minutes. The power level obtained corresponds respectively to about 78, 83, 84, or 120 volts rms. The initial voltage to the load upon rotating switch S50 to these positions is about 0, 34, 78, or 120V rms. With the circuit in the fully on state with switch S50 at position 6 or 7, upon rotating switch S50 to position 3, 2, 1, or 0, the load power reaches the off state respectively in about 17, 5, 1, or 0 minutes. It reaches the off state in slightly reduced times starting from position 4 and 5 due to the lower initial power levels. Intermediate steady state levels as low as that corresponding to about 60V rms can be obtained. Again, the action of the particular value of resistor R54 presents lower steady state levels. Even though this resistor is not required for the half-wave mode, it is advantageous to leave it in the multipurpose circuit represented by FIGURE 5. It permits the duration of gradual-on and gradual-off sequences for the half-wave mode to be reasonably close to that for the full-wave mode. Appropriate choices of circuit components other than those listed in TABLE IV can provide a greater deviation in corresponding durations.

In the operation of the control circuit the swing in voltage levels for capacitor C51 is much greater each half-cycle (each cycle for the half-wave mode) than the swing in voltage on capacitor C52. The average voltage in capacitor C52 gradually increases as the load power is decreasing, gradually decreases as the load power is increasing and remains constant when the load power is in steady state. In the half-wave mode the swing in voltage on capacitor C52 from the fully-on state to the off state is about half the corresponding voltage swing for the full-wave mode. For the components listed in TABLE IV the maximum voltage on capacitor C52 is about 7.5V for the half-wave case and about 15V for the full-wave case. The voltage on capacitor C52 is about zero at the fully-on state for both the half-wave and the full-wave modes. For example, if the circuit is operating in the half-wave mode with the power off and with switch S50 at position 3, upon switching the circuit to the full-wave mode, the load power instantly becomes about the half-power level for the full-wave mode. This is a consequence of capacitor C51 also receiving charge each half-cycle when the circuit is in the full wave mode and the power delivered to the load being dependent on the relative voltage on capacitor C52 with respect to the turn-off voltage. With switches S52 open and S52' closed, and power being supplied to the load, relay switch SR50 deactivates and thereby effectively turns off the power to the load when the power decreased to about 45V rms for the full-wave mode or to about 32V rms for the half-wave mode. With this relay deactivated, power is still supplied to the control circuit through resistor R66 and the relay coil subcircuit. Under this condition the resistance of resistor R66 is so much greater than that for the load that the voltage to the load is essentially zero. While the relay is deactivated the electrical state of the control circuit continues to change in accordance with the setting of switch S50 with the timing sequence nearly the same as it would have been if the relay had not deenergized. Upon adjusting switch S50 to a gradual-on position, the electrical state of the control circuit gradually increases the conduction of SCR52 each subsequent cycle thereby increasing the charge on capacitor C54. When the electrical state for the control circuit increases to the point where about 50V rms for the full-wave mode or about 36V rms for the half-wave mode can be delivered to the load, capacitor C54 charges to a voltage sufficient to energize relay SR50, thereby instantly supplying this voltage level to the load.

When the relay SR50 is de-energized, more voltage is required on capacitor C54 to energize it than is required to maintain it in the energized state. Resistor R65 provides additional charge to capacitor C54 when the relay is de-energized to reduce the difference in the voltage supplied to the load at the energizing and de-energizing points for relay SR50. Resistor R66 provides circuit continuity in the full-wave mode when relay SR50 is de-energized so that capacitor C52 can charge some each half-cycle of supplied power. This permits relay SR50 to energize near the load voltage at which it de-energizes in both the full-wave and half-wave cases. Without resistor R66, relay SR50 would energize when capacitor C52 is charged to a level at which about 84V rms would be supplied to theload in the full-wave case. The energizing and de-energizing voltages for relay SR50 can be varied by proper selection of resistors R64, R65, and R66, capacitor C54, relay coil resistance and maximum relay wattage rating. When switch S53 is closed with the other switches as shown in FIGURE 5, only the positive half-cycle of power is applied to the control circuit. The negative half-cycle of power is delivered directly to the load through diode D53. This circuit then provides power to the load that gradually varies between half power and full power and can maintain steady state power levels at half power, and at any other power level greater than about three quarters power where full power is 120V rms to the load. If switches S51 and S51' are reversed from that shown in FIGURE 5, the power range would still be between half power and full power. The durations of power change, however, would be modified for the various settings of switch S50. The durations would shorten when the power is increasing while they would lengthen when the-power is decreasing. The relay subcircuit performs the same function in the range between half power and full power as it does for the range between no power and half power.

The particular zener diode required in the circuit depends to a great extent on the value of resistor R52 used. If the zener breakdown voltage is too small for a particular value for resistor R52, the positive and negative half cycles of the power to the load may be slightly mismatched. On the other hand the zener breakdown voltage should be as small as possible to increase the maximum power delivered to the load.

The gradual on/off durations can be changed by varying the values of the circuit components. It can vary from essentially instantly to extended durations. When the full-wave mode is used to provide power to a light bulb, the extention of the durations has to be achieved primarily by means other than increasing the value of resistor R52. An increase in the value of R52 increases the low power instability which eventually would become excessive. The sequence durations for the half-wave mode with or without the other half-wave blocked can be extended to several hours if desired. The same is possible for the full wave mode with the relay turn-on/turn-off circuit included. In this case power to the load would be turned off before the low power instability region is reached, thereby permitting resistor R54 to be omitted. Without resistor R54 this a.c. control circuit could maintain a steady state partial power level as low as that corresponding to about 50V rms provided the relay switch SR50 is set to de-energize below this value. Furthermore, if there are any uses for the a.c. mode in which the low power (below about 30V rms) instability is unimportant, both resistor R54 and the relay subcircuit could be omitted to provide extended sequence dμrations and a significantly lower steady state partial power level.

As the value of resistors R52, R55, and RT55 are reduced to decrease the durations of the gradual-on/off sequences, the low voltage instabilities decrease, thereby permitting larger values for resistor R54. An increase in the value of resistor R54 would lower the minimum steady state level with switch S50 between positions 3 and 4. Even without increasing the value of resistor R54, upon decreasing resistors R52, R55 and RT55, the minimum steady state voltage is decreased. When these resistors are respectively reduced to 100K, 20K, and 6K, with all other components of the circuit the same as listed in TABLE IV, the minimum steady state voltage reduces to about 48V rms for the full-wave mode and to about 20V rms for the half-wave mode. For this particular set of com ponents the circuit provides gradual-on/off responses that are so rapid that they appear to be instantaneous. The timer switch initiation of the gradual-on sequence to produce an artificial dawn can be provided by the addition of several different subcircuits to the full-wave circuit. The capability can be provided by a relay subcircuit similar to that shown in FIGURE 2 with the gradual-on duration and maximum load voltage the same as that for the full-wave mode with switch S50 at position 4. A second method would be to include a subcircuit similar to that associated with switch S40' of FIGURE 4. In this case no resistor would be between positions 2 and 3 of the switch corresponding S40' , and the right end of the resistor corresponding to R52 would be connected to the cathode instead of the anode of diode D55 in FIGURE 5. The resistors corresponding to R52 and R53 would be respectively 47K and 180K. At positions 1 or 2 of the control switch the load would be provided respectively a maximum voltage of 60V rms in 4 minutes or 71V rms in 7 minutes. In this case half-wave power only would be supplied to the load even though the non-timer initiated operation of this circuit is the full-wave mode. A third subcircuit similar to the second would supply both half-cycles of the power through diodes to the center terminal of a switch similar to switch S40'. In this case a double-pole single-throw timer switch would be required to control both half-cycles of current to the switch corresponding to S40' . The resistor corresponding to R52, R53, and R54 of FIGURE 4 would be respectively 78K, 470K, and 940K. The right end of resistor corresponding to R52 would be connected through a zener diode, oriented to operate in the breakdown mode, to the cathode of diode D55. At posi tions 1, 2, or 3 of the switch/ the load would be supplied a maximum of 120V rms after a gradual-on duration of 4, 10, or 14 minutes respectively if a 20 volt zener diode is used. The higher maximum voltage and the difference in the gradual-on durations for the three positions of the selection switch is a consequence of this subcircuit charging capacitor C51 for the total portion of each half- cycle during which the input voltage is greater than 20V. With a large resistor of about 10M connected between positions 3 and 4 of this selection switch that is similar to S40' of FIGURE 4 and another switch by by-pass the timer switch when the selection switch is at position 4, the maximum voltage supplied to the load will be increased when switch S50 is at position 4. The embodiment shown in FIGURE 6 provides a control that can be operated from two remote locations. It provides all of the functions provided by the circuit in FIGUTE 4 with switch S42 in position A. The control circuit is similar to that of FIGURE 4 with one position for switch S42, with two important differences. First, there are two portions of the control circuit, with the second one located at a remote location with respect to the first. The main portion of the control circuit, housed at the first location, contains a 24 VDC, 350 Ohm, dual coil latching relay, LR60. A single coil latching relay could be used with a slightly more elaborate switching arrangement. The dual coil relay is latched to the left contact when switch S61 is momentarily depressed, thereby returning control of the complete circuit to switch S60. When switch S61' at the remote location is depressed momentarily, the latching relay, LR60 is latched to the right contact thereby returning the control of the complete circuit to switch S60' at the remote location. The relay switching and latching action is so rapid that only half-wave pulsed current is required. The power to the load varies with the setting of switches S60 or S60', whichever is in control, in a manner similar to the variation in power to the load for the circuit illustrated by FIGURE 4 with corresponding setting of switch S40 with switch S42 at position A. For the remainder of this discussion it will be assumed that switch S60 is in control. The second difference is that the current that charges capacitors C61 and C62 does not flow through the load as it does for the circuit in FIGURE 4. Therefore, opening switch S60'' in FIGURE 6 or removing the load, a light bulb for example, does not change the state of charge on capacitor C62 which primarily determines the state of the control circuit. Consequently, there is no need to place a resistor in parallel with the load as was required for the circuit in FIGURE 4 to permit the load to be disconnected without changing the state of the control circuit. Also, this resistor is not needed for the control circuit to transition from one state to another when the load is disconnected. With the load placed in the circuit as shown in FIGURE 6, circuit components diode D64, resistor R66 and SCR68 are required. The functions of these components are discussed below.

In the operation of the circuit illustrated in FIGURE 4, the voltage between the right end of coil L40 and the bottom side of the 120 VAC power source remains essentially at the voltage of the power source whenever SCR42 is not conducting. For the remainder of each half cycle after SCR42 is triggered by the cathode voltage of PUT 44, the voltage between the right end of coil L40 and. the bottom side of the 120 VAC source reduces to about 1 volt depending on the amount of current flowing through the SCR42. Consequently, whenever SCR42 conducts, capacitor C42 will not charge unless its voltage is less than the voltage between the anode and the cathode of the SCR42. This latter feature is important to the operation of this control circuit. The circuit depicted by FIGURE 6 also has this latter feature as will be explained next. In FIGURE 6 the voltage between the right end of coil L60 and the lower side of the 120 VAC power source is always essentially equal to the voltage of the power source independent of whether switch S60'' is open or closed or whether the bulb is in or out of light 66. Consequently, without the addition of resistor R66 and SCR68 the charging or discharging rate of capacitor C62 depends only on the setting of switch S60, the phase of the applied power, and the state of charge of capacitor C62. While a control circuit of this design can be made to function, its response is not as appealing as the response of the control circuit illustrated in FIGURE 4. With the addition of resistor R66 and SCR68, the charging rate of capacitor C62 once again also depends indirectly upon the state of conduction of SCR62. When SCR62 is triggered by the cathode voltage of PUT64, SCR68 is also triggered and then both of these SCR's conduct for the remainder of the half cycle of applied power. If the sum of the voltage drops across resistor R66 and SCR68 is equal to the voltage drop across SCR62, the state of charge of capacitor C62 in the circuit illustrated by FIGURE 6 would vary in a manner similar to that of capacitor C42 in the circuit illustrated by FIGURE 4. In reality, it is only important to have the sum of the voltage drops across resistor R66 and SCR68 equal to the voltage drop across SCR42 when the light 66 is at full brightness. At slightly less than full brightness of light 66, capacitor C62 will not charge while SCR62 conducts because both the voltage across SCR62 and the sum of the voltage drops across resistor R66 and SCR68 are less than the voltage on capacitor C62.

With a zero value for resistor R66, when light 66 is at full brightness, SCR68 effectively prevents capacitor C62 from charging when switch S60 is rotated to positions 1, 2, or 3 (the three gradual-off positions). This is accomplished as follows. Capacitor C61, which has a shorter charging time constant than capacitor C62, quickly charges to the point where the anode 'A' to grid 'G' voltage for PUT64 exceeds the threshold for conduction and thus causes PUT64 and in turn SCR62 and SCR68 to conduct . The small amount of charge that accumulates on each of the capacitors, C61 and C62, during the first instant of each cycle effectively discharges during the remainder of each power cycle after PUT64 and SCR68 start conducting. With the proper values for resistor R66, when switch S60 is rotated to position 1, 2, or 3 with light 66 at full brightness, capacitor C62 charges very slowly during the complete first half cycle of power. Resistors R64 and RT64 are selected so that capacitor C62 does not completely discharge during the second half-cycle of the power. During the second power cycle, capacitor C61 must charge to a higher value to trigger PUT64 than required during the first power cycle. The voltage must be the sum of the charge on capacitor C62 at the beginning of the second power cycle, the charge that will be accumulated on capacitor C62 during the second power cycle before PUT64 conduction, and the anode 'A' to grid 'G' threshold voltage for PUT 64 conduction. During each additional power cycle until the light goes out, a longer time duration is required for capacitor C61 to charge to the point where PUT64 will fire. This ever increasing charging time for capacitor C62 before SCR68 fires enables it to charge more each power cycle than it discharges, resulting in a slow charge of this capacitor. When switch S60 is at positions 4, 5, or 6, capacitor C62 discharges more each power cycle than it charges until the light obtains full brightness. These two operations, of course, are a consequence of the values of resistors R62, R64, RT64, R67-R72 and capacitors C61 and C62. When switch S60 is at certain intermediate positions between positions 3 and 4, capacitor C62 reaches a steady state condition in which it charges and discharges like amounts each cycle, while light 66 maintains a steady state intermediate brightness level.

The diode D64 prevents current flow from the PUT64 to the SCR68. Without it, light 66 flashes while it is supposed to be increasing in brightness. This flashing would continue until capacitor C62 is completely discharged. With diode D64 in the circuit as shown, there is no detectable flashing of light 66. Component values for FIGURE 6 are shown in TABLE V.

This embodiment, with the control circuit current not passing through the load, could be used at a single location by removing the latching relay subcircuit and the remote circuitry. As a single station control, it could be combined with a timer activated gradual- on circuit to provide an automatic gradual-on sequence. The three-way control circuit can also be made with a control circuit as illustrated in either FIGURE 4 or FIGURE 5 by adding a latching relay subcircuit and the remote station. If the timer activation capability and the multiple time sequence selector, switch S42 in FIGURE 4, are included they would be housed in and controlled at the first (main) location only.

When a positive temperature coefficient thermistor with a 25°C resistance of 15K is substituted for the 15K resistor R57, in FIGURE 5, the operation of the control circuit becomes sensitive to the ambient temperature of this thermistor. When the temperature of this thermistor is greater than 25°C each gradual-on sequence becomes faster, each gradual-off sequence becomes slower and each intermediate steady state power level is obtained at a smaller value of variable resistor R60 than for the thermistor at 25°C. At sufficiently elevated temperatures for the thermistor, some gradual-off settings will provide a gradual-on sequence instead. Conversely, when the temperature of the thermistor decreases below 25°C each gradual-on sequence becomes slower, each gradual-off sequence becomes faster and each partial power steady state condition is obtained with a larger value of variable resistor R60 than for the thermistor at 25°C. Furthermore, at sufficiently reduced temperatures some gradual-on settings will provide a gradual-off sequence. This same effect could be accomplished by replacing resistor R55 with a negative temperature coefficient thermistor having the appropriate temperature gradient while using a standard 15K resistor at R57. Also one or more of resistors R58 through R62 could be replaced by positive temperature coefficient thermistors to provide similar effects. The reverse effect could be obtained by using negative coefficient thermistors for one or more of resistors R57 through R62 and a positive coefficient thermistor in place of resistor R55. The temperature sensitive control circuit has utility when controlling the speed of an electric fan, for example, with a control circuit utilizing the a.c. mode with the relay turn on/off capability and a large value for resistor R54. With a positive coefficient thermistor in place of resistor R57 and with switch S50 set between positions 3 and 4 so as to run the fan at about half speed, the speed of the fan could increase as the temperature increases and would decrease as the temperature decreases. If the temperature decreased sufficiently the fan would turn off automatically and then subsequently turn on automatically when the temperature increased sufficiently. For example, with the appropriate temperature gradient for the thermistor, the speed of a ceiling fan would automatically vary so as to maintain a constant comfort level over a range of temperatures in the room. The response of the fan to the change in temperature will be instantaneous with appropriate values for resistors R52, R55, and RT55 and capacitor C52.

This control of the fan speed in response to the ambient temperature can be combined with a standard gradual-off sequence of about one hour to control the comfort of aperson going to sleep at night. The one hour gradual-off sequence would tend to compensate for both the change in room temperature and the reduction in metabolic rate when a person goes to sleep. The thermistor adds an additional control. If the temperature decreases more rapidly than anticipated, the fan speed would decrease faster. Conversely if the temperature increases, the speed of the fan would decrease more slowly. In the extreme case where the room temperature increases significantly, the speed of the fan would increase up to the full power speed to provide greater comfort. The control circuits as illustrated by FIGURES

4 and 6 also would respond as described above if corresponding resistors are replaced by thermistors of the appropriate temperature coefficient.

The circuit illustrated by FIGURE 5 with components selected to provide instant response becomes a light sensitive control circuit that tends to maintain a constant light level in a room when one or more of the resistors R57 through R62 are combined with photo-resistors. In particular if a photo-resistor, which commonly has a dark resistance greater than 1M ohm and a resistance as low as 4K in normal room light, is placed in parallel with resistor R57, the combined resistance of the two would range between about 15K and 3K depending on the amount of light detected by the photo-resistor. This change in resistance is equivalent to rotating switch S50 from position 5, where the steady state condition is fully on, to position 3 where the steady state condition is off. With an appropriate light filter and with the components of the control circuit selected to provide instant response, the circuit could be adjusted to vary the power to the light bulb so as to range from full brightness when no light is provided by other sources to completely off when other sources, such as other lamps or light through a window, pro vide ample light. When other sources provide partial light, this control circuit would provide partial power to the light bulb to provide partial light inversely proportional to the level of light provided by the other sources. Such a light control would tend to maintain a constant level of light in the vicinity of the photo-resistor, thereby producing a savings of energy when light is provided by other sources.

With resistor R55 combined with a photo-resistor, the brightness of the light bulb controlled by the circuit varies directly as the light intensity received by the photo-resistor. This control circuit could be utilized to control display lights. The display lights would vary in brightness in direct proportion with the ambient light level thereby tending to maintain a constant visibility of the display.

The circuit illustrated by FIGURE 1 could also be made temperature sensitive by appropriate insertions of one or more thermistors. The value of the thermistors to be used would depend on the intended functions of the control circuit. There are a number of locations in this circuit where a thermistor could be utilized. It could be substituted for or combined with the resistors associated with switch SB but this requires many thermistors to make each circuit function temperature sensitive. On the other hand only one or two thermistors would be needed when combined with resistor R17 and/or capacitor C4 to make each function of the circuit temperature sensitive. In this latter case the response to temperature would be instantaneous. Resistor R17 would be replaced by a negative coefficient thermistor alone or by the thermistor in series with a resistor. A positive coefficient thermistor would be placed in parallel with capacitor C4. This circuit would be utilized in the same manner as described above for temperature sensitive versions of the circuit illustrated by FIGURES 4, 5 or 6.

With the circuit illustrated by FIGURE 1 made temperature sensitive by combining a thermistor with resistor R17 and/or capacitor C4, the circuit output could be made to respond to temperature changes even when the light emitting diode is dark. Consequently, the outpower could be made to be temperature sensitive even when the circuitry associated with the light emitting diodes and the photoconductors are removed, leaving only resistor R17, capacitors C3 and C4, diac 13, triac 15, coil L1 and load 20.

When this simplified circuit is made temperature sensitive by replacing resistor R17 with a negative coefficient thermistor that is 100K at 25 °C and about 30K at 50°C, and when it is also modified by placing a variable resistor, with a range from 15K to 30K set to 20K, in parallel with capacitor C4, it provides power to the load that changes progressively from no power to full power as the temperature changes from 25°C to 50°C. The power initially comes on at a low level when the resistance of the thermistor becomes less than 80K and obtains full power at 30K for the thermistor. This circuit also provides a progressive reduction in power to the load as the thermistor cools from 50°C to 25°C. No power to the load occurs when the resistance of the thermistor becomes greater than 85K near 25 °C. The variable resistor provides manual adjustment of the turn-on, turn-off and full power temperature points.

One application for this circuit is to control automatically a fan used to blow air through heat conduction pipes in a fireplace. After the fire is started the fan would come on at a low speed as the fire becomes hot enough to reduce the resistance of the thermistor below 80K, and progressively increase to maximum speed at 30K for the thermistor near 50°C. When the fire dies down, the speed of the fan progressively decreases. If the temperature approaches 25°C where the resistance of the thermistor becomes greater than 85K, the fan will stop. With such a fan speed control, the fan would automatically start at a low temperature and progressively change in speed for temperatures up to 50°C where it will reach maximum speed. Also, the fan would turn off when the temperature decreases to a but 25°. With the fan speed being proportional to the temperature of the air being blown by the fan, there would be no noticeable cool drafts in the room when the fire is low.

Another application for this reduced circuit would be to provide a comfort control for a ceiling fan. The fan would be set at a speed that is comfortable at 82°F. for example. Then the speed of the fan would vary with the temperature to maintain a constant comfort level. In this case resistor R17 would be replaced by a variable resistor with values ranging from 10K to 250K in series with a 30K at 25°C negative coefficient thermistor. In addition, a 15K at 25°C positive coefficient thermistor would be placed in parallel with capacitor C4. At any normal temperature in the house the full range of fan speeds could be obtained by adjusting the variable resistor. After a comfortable speed has been set for the fan, it will tend to maintain a constant comfort level as long as the required fan speed remains in the range of speeds of the fan.

This reduced circuit of FIGURE 1 can also be converted into a light sensitive control circuit that tends to maintain a constant light level in a room when a second light source contributes a varying amount of light to the area. This is accomplished by connecting the series unit of a variable resistor of about 50K and a photo-resistor in parallel with capacitor C4. This variable resistor is adjusted so that the light goes out near the midrange of the manually controlled variable resistor R17 when the photo-resistor detects strong light from another source. Alternatively, when a photo-resistor with a proper filter is combined with resistor R17 the circuit tends to maintain a constant visibility of a light bulb in varying illumination from other sources.

This concept of tending to maintain a constant visibility of a display light in the presence of varying illumination from other sources can also be extended to fluorescent lights. FIGURE 7 without resistor R77 and photo-resistor P.R. is a standard manually operated fluorescent light dimmer. The addition of resistor R77 and photo-resistor P.R. converts this circuit into one which tends to maintain a constant light level in a room when other sources contribute to the illumination of the area of the controlled light. By placing the series combination of the variable resistor and photo-resistor in parallel with resistor R75 instead of capacitor C71, the circuit, when properly adjusted, tends to maintain a constant visibility of a fluorescent light in varying external illumination. Furthermore, by replacing these photo-resistors in the fluorescent light control circuit with appropriate negative coefficient thermistors, the response of the circuit would become temperature sensitive. It should be understood that although the foregoing description illustrates supplying power to but a single electrical load, the circuits of FIGURES 1, 2, 3, 4, 5, 6 and 7 may also be used to supply power to a plurality of electrical loads provided that the rated current of the silicon controlled rectified is not exceeded. Suitable SCR's exist with rated capacity ranging from a fraction of an ampere to several thousands ampere. Thus, with a properly selected SCR, the control device may also be used to gradually vary the power supplied to an electric stove and thus the temperature of a heating element, or to gradually change the temperature of a heating lamp.

While this invention has been described as having a preferred design, it will be understood that it is capable of further modification. This application, is therefore, intended to cover any variations, uses, or adaptations of the invention following the general principles thereof and including such departures from the present disclosure as come within know or customary practice in the art to which this invention pertains, and as may be applied to the essential features hereinbefore set forth and fall within the scope of this invention or the limits of the claims.

Claims

WHAT lS CLAIMED IS:

1. A device for producing a gradual change in power supplied to a load comprising: a first solid state switching device having first and second power electrodes and a gate electrode, a variable timer network connected to said gate electrode and comprising a chargeable capacitor and a switchable resistive network, a first light emitting diode connected to receive current passed by said switching device and emit light proportional to the received current, a first photoresistor positioned to receive light emitted by said first light emitting, a second solid state switching device having first and second power electrodes and a gate electrode and controlling the supply of current to said load, said first photoresistor being connected to the gate of said second switching device whereby power is supplied to said load as a function of the light received by said first photoresistor from said light emitting diode, said resistive network being adjustable so that with said capacitor, said timer network controls the rate of change of voltage to the gate electrode of said first switching device and thereby the rate of change of power supplied to said load.

2. A device as in Claim 1 and wherein: said switchable resistive network includes a first switch (SA) having a first position (SA2) for enabling charging of said capacitor and a second position (SAl or 3) for enabling discharge of said capacitor whereby in said first position said light emitting diode increases in light intensity and in said second position said light emitting diode decreases in intensity.

3. A device as in Claim 2 and wherein: said switchable resistive network includes timer switch (ST) means, a second light emitting diode (12) connected to receive electric current upon actuation of said timer switch, a second light emitting diode (12) connected to receive electric current upon actuation of said timer switch, a second photoresistor (18) positioned to receive light emitted by said second light emitting diode, means for limiting the current to said second light emitting diode so that said second light emitting diode will flash on and off once upon actuation of said timer switch and momentarily lower the resistance of said second photoresistor and enable the power supplied to said load to turn on at a minimum initial level.

4. A device as in Claim 3 and wherein: said switchable resistive network includes a plurality of selectable resistors for selectively enabling different rates of increase and different rates of decrease in supplied power, and a second switch (SB) for selecting the desired of said resistors.

5. A device as in Claim 4 and wherein: said timer switch means includes a timer switch and a secondary switching device whereby actuation of said timer switch activates said secondary switching device and connects said first and second switches and thereby increases power supplied to said load at a rate determined by a variable resistor (R7).

6. A device as in Claim 2 and wherein: said switchable resistive network includes a variable resistor (R2) for adjusting the minimum voltage supplied to said gate electrode and thereby the minimum power supplied to said load.

7. A device as in Claim 5 or 6 and wherein said first switching device comprising a junction field effect transistor.

8. A variable power intensity control circuit, comprising: a power input line, a power output line adapted to be connected to an electrical load, electrical conducting means connecting between the power input and power output lines for passing a variable amount of electrical energy from the input to the output line to thereby control the load intensity supplied to the electrical load, a control element connected to the electrical conducting means for regulating the amount of electrical energy passed therethrough dependent upon the electrical state of the control element, and control circuit means connected to the control element and having a plurality of manually selectable settings for supplying a selected variably increasing or decreasing power output dependent upon such settings and present power output so as to vary the electrical state on the control element to provide an instantaneous change ranging from negligible to significant in the power supplied followed by a gradually increased or decreased power supply through the electrical conducting means at a pre selected instantaneous change in power and rate of change in power dependent upon such settings to a given preselected power level such that the power supplied to the load varies in accordance therewith.

9. The variable power intensity control circuit as set forth in Claim 8, wherein: the electrical conducting means being a first gated solid state switching device having an anode and a cathode, the control element including a second solid state switching device having a gate connected to the output of said first switching diode, variable timing network connected to the anode of said first switching device and comprising a chargeable capacitor (C21) and a switchable resistive network for controlling the rate of change of voltage to the anode of said first switching device, diode means for blocking one half-cycle of current to said variable timing network, means for maintaining a voltage on the gate of said first switching device within a range such that the anode voltage control circuit causes said first switching device to fire as a function of the variable timing network, whereby said second switching device conducts current to said load when said first switching device fires and blocks current flow to said load when said first switching device does not fire.

10. The variable power intensity control circuit as set forth in Claim 9, and wherein said switchable resistive network comprises a plurality of serially connected resistors (R26-32) and a switch member (S20) for selective tivaly connecting at least one of said resistors in parallel with said chargeable capacitor and with timer switch means.

11. The variable power intensity control circuit as set forth in Claim 10, and wherein said plurality of resistors includes a first set of said resistors for controlling the rate of increase of power supplied to said load and for obtaining and maintaining the full power steady state condition, and a second set of resistors for controlling the rate of decrease of power supplied to said load and for obtaining and maintaining the no power steady state condition.

12. The variable power intensity control circuit as set forth in Claim 11, and wherein the variable timing network includes a timer associated with the chargeable capacitor and the switchable resistive network for thereby increasing power supplied to the load at a controlled gradually increasing selective rate.

13. The variable power intensity control circuit as set forth in Claim 12, and wherein said serially connected resistors include a variable resistor (R30) for controlling the final steady state condition of partial power supplied to said load and for controlling the rate of change in power supplied to the said load during the transition from one final steady state condition of partial power to the said load to another such steady state condition.

14. The variable power intensity control circuit as set forth in Claim 13 , and wherein said means for maintaining a voltage bias on the gate of said first switching device comprises a parallel connected resistor and a second capacitor circuit connected through a blocking diode to the gate of said first switching device, whereby when said first set of resistors of said switchable resistive network is selected, said second capacitor of said circuit discharges more than it charges while the supplied power is increasing and when said second set of resistors of said switchable resistive network is selected, said second capacitor charges more than it discharges while the supplied power is decreased and said second capacitor dis- charges and charges at substantially equal rates when power is upplied to said load at a constant rate.

15. The variable power intensity control circuit as set forth in Claims 8, 9, 10, 11, 12, 13, or 14 for supplying alternating current to said load and including: a third gated solid state switching device,

(44') having an anode and a cathode, a fourth solid state switching device (42') having a gate connected to the output of said third switching device, a second variable timing network connected through a blocking diode to the anode of said third switching device and comprising a third chargeable capacitor (C41') and a second switchable resistive network for determining the discharging time of the said third capacitor. a resistive network (R41) connected between the power source and the anode of the said third switching device, for determining the charging time of the said third capacitor and in conjunction with said second switch- able resistive network determines the maximum fraction of supplied voltage to be across the said third capacitor and on the anode of said third switching device, second diode means for passing said one half- cycle of current to anode and grid circuits of said third switching device and blocking the other half-cycle of current, and means for maintaining a voltage on the gate of said third switching device within a range such that the anode voltage control circuit causes said third switching device to fire as a function of the ratio of the resistance in said charging network of said third capacitor to the resistance of the said discharging circuit for the said third capacitor, whereby said fourth switching device conducts current to said load for the remainder of the half-cycle of supplied power when the said third switching device fires and blocks current to the said load during the portion of each half-cycle of supplied power prior to the firing of the said third switching device.

16. A device as in Claim 15 and wherein said first and third switching devices are programmable unijunction transistors and said second and fourth switching devices are silicon controlled rectifiers.

17. A device as in Claim 8, 9, 10, 11 or 12 and wherein said first switching device is a programmable unijunction transistor and said second switching device is a silicon controlled rectifier.

18. A variable intensity control unit comprising: a power input line, a power output line adapted to be connected to an electrical load, electrical conducting means connecting between the power input and power output lines for passing a variable amount of electrical energy from the input to the output line to thereby control the load intensity supplied to the electrical load, a control circuit connected to the electrical conducting means for regulating the amount of electrical energy passed therethrough dependent upon the electrical state of the control element, and control circuit means connected to the control element and having a plurality of manually selectable settings for selecting electrical elememts for supplying a selected variably increasing or decreasing power output dependent upon such settings, state of such electrical elements, and present power output so as to vary the electrical state on the control element to provide an instantaneous change ranging from negligible to significant in the power supplied followed by a gradually increased or decreased power supply through the electrical conducting means at a preselected instantaneous change in power and rate of change in power to a given preselected power level, dependent upon such settings and of such electrical elements, such that the power supplied to the load varies in accordance therewith.

19. The variable power intensity control unit as set forth in claim 18, wherein: switching means is disposed within the control circuit for selecting the type of electrical power to be supplied to the load for instant returning power on or off at preset power levels independently of the control circuit means, and the electrical state of the control circuit means Is not affected by the load being disconnected by removal of the load or by operation of an ordinary on/off switch.

20. The variable power intensity control unit as set forth in claim 19, wherein: means connected to the control circuit means for permitting adjustment to produce the same change and the same manner of change in the electrical state of the control circuit with or without the load connected in the circuit.

21. The variable power intensity control unit as set forth in claim 19, wherein: the electrical conducting means being a first gated solid state switching device having an anode and a cathode, the control circuit including a second gated solid state switching device having an output connected to the gate of the first switching device, variable timing network connected to the anode of said second switching device and comprising a chargeable capacitor and a switchable resistive network connected in parallel for controlling the rate of change of voltage to the anode of said second switching device for maintaining a voltage on the gate of said second switching device within a range such that the anode voltage control circuit causes said second switching device to fire as a function of the variable timing network, whereby said first switching device conducts current to said load for the remainder of each half-cycle after said second switching device fires and blocks current flow to said load in each half-cycle until said second switching device fires.

22. The variable power intensity control unit as set forth in claim 4, and wherein: said switchable resistive network comprises a plurality of connected resistive sections and a switch member for selectively connecting at least one of said resistive sections in parallel with said chargeable capacitor and with timer switch means.

23. The variable power intensity control unit as set forth in claim 5, wherein: said plurality of connected resistive sections includes a first section for controlling the rate of increase of power supplied to said load and for obtaining and maintaining the full power steady state conditions, a second resistive section for controlling the rate of decrease of power supplied to said load and for obtaining and maintaining the no-power steady state condition, and a third resistive section connected serially between the said first resistive section and said second resistive section for controlling the final steady state condition of partial power supplied to said load and for controlling the rate of change in power supplied to the said load during the transition from one final steady state condition of partial power to the said load to another such steady state condition.

24. The variable power intensity control unit as set forth in claim 23, wherein: said resistive sections contain at least one resistive element which is fixed or is variable with respect to ambient conditions.

25. The variable power intensity control unit as set forth in claim 24,wherein: the division between the particular elements contained in the said first resistive section, the said second resistive section and the said third resistive section depends upon the sensitivity of these resistive sec tions to temperature and to the intensity of light impinging upon them, as well as upon the resistive values of a charging resistive element connected in series with and a discharging resistive element connected in parallel with a second chargeable capacitor in a gate bias circuit for the first said switching device.

26. The variable power intensity control unit as set forth in claim 25, wherein: said gate bias network of the said first switching device as a second switchable resistive network for selecting a charging and discharging pair of said resistive elements for said second chargeable capacitor.

27. The variable power intensity control unit as set forth in claim 26, wherein: said means for maintaining a voltage bias on the gate of said second switching device comprises a circuit, consisting of a said resistive element in parallel with the said second capacitor connected through a block- ing diode to both the gate of said second switching device and a charging resistive element whereby when said first resistive sections of said first switchable resistive network is selected, said second capacitor of said bias network discharges more than it charges while the power sup- plied to the load is increasing and when said second section of said first switchable resistive network is selected, said second capacitor charges more than it discharges while the power supplied to the load is decreasing, and said second capacitor discharges and charges at substantially equal rates when the said third section of said first switchable resistive network is selected.