US3002099A

US3002099A - Light intensity controller

Info

Publication number: US3002099A
Application number: US50474A
Authority: US
Inventors: Jr Charles A Gregory; Walter R Bullington
Original assignee: FLIGHT RES Inc
Current assignee: FLIGHT RES Inc
Priority date: 1960-08-18
Filing date: 1960-08-18
Publication date: 1961-09-26
Anticipated expiration: 1978-09-26

Description

Sept. 26, 1961 C. A. GREGORY, JR., ET AL LIGHT INTENSITY CONTROLLER Filed Aug. 18, 1960 2 Sheets-Sheet 1 CONTROLLED RECTIFIER PULSE GENERATOR AMPLIFIER WHEATSTONE BRIDGE SOURCE FIGJ.

INVENTORS Charles A. Gregory,Jr. 8 Walter R. Bulling'ron ATTORNEYS Filed Aug. 18. 1960 c. A. GREGORY, JR., ETAL 3,002,099

LIGHT INTENSITY CONTROLLER 2 Sheets-Sheet 2 NI 8 E m (D a L E N INVENTORS Charles A.Gregory,J|'.8 Walter R. Bullingion 1" ATTORNEYS United St s Pat 9 3,602,099 LIGHT INTENSITY iIONTROLLER Charles A. Gregory, .313, and Walter R. Bullington, Richmend, Va, assignors to Flight Research, Incorporated, Richmond, Va, a corporation of Virginia Filed Aug. 18, 1%0, Ser. No. 50,474 13 (Ziaims. (Q!- 250-205) This invention relates to automatic power regulation, and particularly to the regulation ofv the alternating current power supplied to a source of illumination was to maintain a constant level of illumination. It has particular application as a device for controlling the intensity'of the illumination supplied to material to be photocopied onto microfilm.

In recent years the increasing use of microfilm for. the recording of records, both public and private, has necessitated the development of methods suitable for mass photocopying, preferably using unskilled or semi-skilled operators.

In the procedure currently in use for the photocopying of material on a mass, or commercial basis, the recording camera is mounted above a horizontal table on which is placed the paper to be photographed. The exposure time remains fixed, and variations in the composition. of material, the background color and texture of the papers to be photographed are compensated for by adjustment of the intensity of illumination. This is accomplished by manual manipulation by the operator of a rheostat or variable transformer in the alternating current supply line to the photofiood lamps.

Some photocopying equipment-has been provided with a photocell sensing device mounted on an arm which may be swung out over the table, and an illuminationintensity meter, so that the intensity of illumination reilected from each successive paper placed on the table to be photocopied may be read from the meter by the operator, who may then adjust the r'heostat orvariable trans former controlling the light intensity accordingly. This requires that for each paper to be photographed, the operator must swing the photocell out over a representative portion of the document and adjust the rheostat or variable transformer until a previously determined meter reading is obtained. If the document is not of uniform com-. position, background color, and texture, the operator most exercise considerable care and judgment in arriving at the best compromise rheostat setting.

it has been found that such a slow and exacting proc essis generally not performed well by unskilled or semiskilled operators. Often a considerable portion of the dayfs work must. be re-done because of poor operator judgment, or failure to take sulficient and accurate readings in an attempt to improve theproduction rate.

A technique often used at present represents an attempt to simplify the foregoing procedure. A person skilled and experienced in the art of photocopying examines each batch of work in order to determine the range of variation of: the background colors and textures and of the composition of the papers in that particular batch. 'He then selects a few sample papers representing the range of variation in the batch, determines the setting of the photo. flood lamp voltage required ,for properillumination of each sample, and posts the, samples and their correspond: ing voltage settings for the guidance of the operator, who is usually an unskilled, or at most semi skilled', person: The operator then compares each piece of work with the samples and sets the voltage in accordance. with the sample which appears to be closest. This method relieves the operator of the necessity of making exposure meter readings, but it is still, slow, andinaddition is fairly in-. accurate, resulting in only a rough approximation of the correct exposure for each docmnentto be photographed.

" saunas Patented Sept. 26, 196i.

Another disadvantage of the presently used system is that the rheostat, of the size needed to control the voltage to the photoflood lamps, is a heavy and bulky item, at times difiicult to conveniently locate, and is a large factor in determining the size of the equipment. 1

It is an object of the present invention to provide a power control system, one embodiment of which pro vides an automatic illumination intensity control.

It is another object of this invention to provide an automatic illumination level control system for photocopying which will not require any participation, at all by the equipment operator.

It is another object of this invention to provide such a control system which will provide accurately the precise level of illumination required for each document.

It is another object of the present invention to provide such an illumination control system which will be light and compact and which will, in particular, eliminate-the necessity'for a bulky rheostat.

It is another object of this invention to provide such an illumination control unit that will efiiciently control the power of the lamps with a'minimum of internal heat dissipation.

Other objects of' this invention will appear from the" following description of the invention and from the draw mgs.

Fundamentally the" invention comprises a sensing elernent, such as a'photocell' or a strain gauge, which senses some characteristic of an electrical load device, such as the light intensity of an electric light, or some characteristicwhich is a function of a load device, such as expan' sion in metal caused by an electric heater, and which sensing-means then produces an electrical parameter, such as'a voltage or current, which parameter is a function, orquantitative measure, of that characteristic sensed. This electrical parameter is then compared with an electrical standard, for instance in a bridge circuit, and another electrical parameter is produced which is a measure of the difference indicated by the comparison. This latter parameter is used-to initiate a series of electrical triggers, which vary in some respect, for instance amplitude of frequency, as a function of the initiating'parameter. This series of triggers controls the conductive cycle of a controlled semiconductor rectifier placed in the line between the alternating current source and the load device, which rectifier in-turn controls the current supplied to the load device. This tends to maintain the sensed parameter; which is-a function out the load device, at a constant-level, determined by the value of the electrical standard.

One embodiment of-t'ne invention, an illumination con troller for photocopying, is shown in the drawings, in which FIGURE 1 is a block diagram; and

FIGURE 2 is a detailed electrical schematic diagram.

The block diagram of FIGURE 1 shows a single lamp L, which represents the one or more. photofiood lamps constituting the load device tobe controlled, with the light from lamp L impinging upon and being reflected from the surface 10, which represents the surface of the document tobe photographed, and with this reflected light being sensed by photocell PC. 'It should be noted that, although none are shown in the diagram, this inention contemplates the placing of whatever lenses and filters in the path of illumination as maybe required for the. desired operation of the system. The Wheatstone bridge comparison circuit is shown as 11, the amplifier as '12, the trigger generator as 13, the controlled semiconductor rectifier as 14, and the alternating current source as 15. v

Following is a description in detail of the circuit schematic shown in FIGURE 2. The photocell PC constitutes one arm or" a Wheatstone mium sulphide photocell manufactured by the Radio Corporation of America, No. 7163, has been found satis factory in the actual operation of this device. This photocell PC, while electrically constituting one arm of Wheatstone bridge 11, is adapted, by virtue of the length of cable18, to be located physically at some distance from the rest of the unit, and connectors 15 and 17 allow the photocell PC and cable 18 to be disconnected as a matter of convenience. Another arm of the Wheatstone bridge consists of the parallel combination of resistor R1 and capacitor C1, one end of which combination is connected to one end of the photocell arm at junction 19. The remaining two arms of bridge 11 consist of the two sections of potentiometer R2: the section from the centertap to the counterclockwise (CCW) terminal of the potentiometer being connected in parallel with capacitor C2 and having its counterclockwise (CCW) terminal connected at junction point 20 to one end of the arm which consists of the parallel combination of resistor R1 and capacitor C1; the section from the centertap to the clockwise (CW) terminal ot the potentiometer being connected at its clockwise (CW) terminal to one end of the arm constituted by the photocell PC at junction point 21. A direct current potential of 150 volts, from a power supply to be described later, is applied across the Wheatstone bridge 11 between

terminals

20 and 21, with terminal Zii being positive with respect to terminal 21.

Following the Wheatstone bridge 11 is a single stage transistor amplifier 12, whose active element is transistor Q1. The centertap of potentiometer R2 in the Wheatstone bridge circuit is directly connected via wire 24 to the base B of transistor Q1. This transistor may be any general purpose junction P.N.P. audio transistor; a 2 Nl85 has been used successfullyin this circuit. The transistor is operated as a common emitter amplifier, The voltage for the operation of the transistor is obained from an 18 volt power supply 32, whose operation will be describedlater, via

wires

22 and 23, with 22 being I positive with respect to 23. The current path for the transistor circuit is from the positive side of power supply 32 through wire 22, through resistor R3 in the emitter circuit, through the emitter E and collector C of transistor Q1, through resistor R4 in the collector circuit, through the Auto contact and the arm of switch SIA, through the emitter E and base number one B1 of unijunction transistor Q2 in pulse generator circuit 13, whose operation will be described later, thence through resistor R8 in the B1 circuit of unijunction transistor Q2 and through wire 23 back to the negative side of power supply 32. The function of resistor R3, which is con nected at one end to the emitter of transistor Q1 and at the other end through wire 22 to the positive side of the 18 volt power supply 32, is to provide negative feedback and temperature compensation for the transistor amplifier stage in a manner well known in the art. The combination of rheostat R5 and resistor R6, which components are connected in series from the base terminal of transistor Q1 to the positive side of 18 volt power supply 32 through wire 22, permits adjustment of the sensitivity of the transistor amplifier stage by adjustment of the resistance shunting the base and emitter circuits in a well known manner. The function of the diode CR1, whose cathode is connected to the positive side of 18 volt power supply 32 through wire 22, and whose anode is connected to the base of transistor Q1, is to protect the transistor against reverse voltages which might be applied between the base and emitter circuits by running the adjustable center tap of potentiometer R2 to a point where the voltage at the centertap, and thus also at the base of the transistor Q1, is more positive than the voltage at the opposite junction of the Wheatstone bridge 19, which is also the voltage at the positive side of the 18 volt power supply.

'Followingthe stage of transistor amplification 12 is the trigger generator 13, whose active element Q2 is a double base semiconductor diode, or, as it is usually called, a unijunction transistor. A type 2N49l unijunction transistor has been found to work satisfactorily in this circuit. Resistor R7 is connected between base numher two B2 of unijunction transistor Q2 and the positive side of 18 volt power supply 32 through wire 22-. R6.- sistor R8 is connected between base number one 131 of unijunction transistor Q2 and the negative side of 18 volt power supply 32 through wire 23. The emitter circuit charging capacitor C3 is connected between the emitter E of unijunction transistor Q2 and the negative side of 18 volt power supply 32 through wire 23. The junction of the emitter E and capacitor C3 is connected by means of wire 25 with the center arm of switch SIA, and through the Auto contact (when switch SIA is in the Automatic position) with one end of resistor R4 in the collector circuit of transistor Q1. The series combination of potentiometer R9 and resistors R10 and R11 is connected across 18 volt supply 32, with one end of R10 being connected to the positive side of 18 volt supply 32 through wire 22 and one end of R11 being connected to the negative sideof 18 volt supply 32 through wire 23. The variable centertap of potentiometer R9 is connected to the Man terminal of switch SLA. This combination 'acts as a voltage divider placed across 18 volt supply 32, permitting an adjustable voltage, taken from potentiometer R9, to be applied to the junction of capacitor C3 and the emitter E of unijunction transistor Q2 whenever switch SIA is placed in the Man (*Manual") position. Semiconductor rectifiers CR2 and CR3 are placed across capacitor C3, the anode of rectifier CR2 is connec'ted to the junction of capacitor C3 and the emitter E of unijunction transistor Q2, the anode of rectifier CR3 is connected to the end of capacitor C3 which conmeets to the negative terminal of 18 volt power supply 32, and the cathodes of rectifiers CR2 and CR3 are connected together and through wire 26 to one side of resistor R12, the other side of resistor R12 being connected through wire 27 to one side of the alternating current supply. The alternating current supply used is the common volt, 60 cycle, single phase type.

t The junction of resistor R8 with base one B1 of unijunction transistor Q2 is connected through resistor R13 to the control terminal 28 of silicon control rectifier SCRl. Silicon control rectifier SCRl is connected in parallel with rectifier CR4 but in a reverse direction. The junction of the anode of silicon control rectifier SCRl with the cathode of rectifier CR4 is connected through inductance L2 to one side of lamp L which constitutes the load. The junction of the cathode of silicon controlled rectifier with the anode of rectifier CR4 is connected through inductance L4 with one side of the alternating current supply. The path of the current from the alternating current source supplying the load may be traced starting with one contact 29 of a standard utility plug 31, through fuse F, through inductance L3, to the arm of On-Ofi switch S2, from the On contact of switch S2 through inductance L1, through the lamp L, through inductance L2, through the parallel combination of silicon control rectifier SC-Rl and rectifier CR4, through inductance L4 and to contact 30 of plug 31. A voltmeter M is connected in parallel with the series combination of lamp L and inductances L1 and L2, and measures the voltage across the load lamp L in series with inductances L1 and L2. A meter damping capacitor C4 has one end connected to one end of the coil of voltmeter M and the other end connected to the center arm of switch SIB.

Switch SIB is the second section of the switch of which 81A comprises the first section. The two sections are mechanically ganged together, but electrically insulated, and taken together constitute a double pole, double throw switch S1. Each section has a center arm and two contacts, one Man (Manual) and one Auto (Automatic).

Mea an The; Man contact of switch SlB- is not connected; the Auto contact is connected to the other side of the coil of voltmeter M from that to which one side of capacitor C4.is connected, so that when switch S1 is put in the Auto position, capacitor C1 will be connected in parallel with voltmeter M. Switch S2 is a single pole switch whose purpose is to turn the equipment on or oil. Inductances L1, L2, L3 and L4 are radio frequency chokes, one of which is located in each of the two wires from the power plug 31 and in each of the two wires to the load lamp L, and whose purpose is to prevent any high frequency electrical signals generated in the control circuitry from being :fed into the alternating, current supply or into. the load lamp.

Power for the transistors is provided by 18 volt power supply 32. In this supply half wave rectification is provided by rectifier CR6, the anode of which is connected to: the side of the alternating current supplycorresponding tocontact 29 of the power plug 31. Thecathode of rectifier CR6 is connected to filter capacitor C and one end of a dropping resistor R14. The other end of resistor R14 is connected to the cathode of an 18 volt Zener diode ZCRl. The anode of Zener diode ZCRl is connected to the other end of filter capacitor C5, so that the series combination of resistor R14 and Zener diode. ZCR]. is

in parallel with filter capacitor C5. The positive voltage is taken from the cathode of Zener diode ZCRI and is connected to the transistor circuitry via wire 22. The negative voltage is taken from the 18. volt supply at the anode of Zener diode ZCRI via wire 23, and is connected to the side of the alternating current supply corresponding to contact 39 of the power plug 31. This is a standard power supply configuration. The alternating current, which receives half wave rectification from rectifier CR6, charges filter capacitor C5, and the filtered 'voltage is impressed across a voltage divider composed of dropping resistor R14 in series with Zener diode ZCRI, across which latter component the required 18 volts is developed.

The direct current voltage applied across

junction points

20 and 21 of the'Wheatstone bridge is supplied by 150 volt direct current power supply 33. The primary winding. 34 of transformer T1 is connected across the alternating current supply. In the circuit of the secondary wind ing of transformer T1, rectifier CR5, with its anode connected to one end of the secondary winding 35, provides halfwave rectification, and supplies the rectified current to filter capacitor C6, one end of which is connected tothe cathode of rectifier CR5. The required ISO volts direct current is taken from across filter capacitor C6,

with the side of capacitor C6 which is connected to the Theory of operation As an aid in understanding the operation of the circuit, it will be assumed that a piece of paper having a relatively light background has just been photographed by photocopying equipment incorporating this invention, and that the next piece of paper to be positioned -on the horizontal stage in readiness for photographing hasa much darker background than the prior "piece. While the intensity of illumination emanating" from the photo flood lights remains the same, the darker piece of work requires more illumination to obtain the required correct exposure. This is indicated to the control circuit by the fact that there is less light reflected" from the surface of the recently positioned piece of paper, and since photocell PC is positioned in the path of the refiected light, the light impinging upon photocell PC will he. of a. lower intensity than. that. previously impinging upon it;

6 The electrical resistance of a photocell increases the intensity of the incident light decreases; Here photocell PC is in series with resistor R1 and thetwo act as a voltage divider across the volts direct cur rent applied between

junctions

20 and 21, with junction; 20 being positive with respect to junction 21. With an increase in the photocell PC resistance, more voltage; will be dropped across the photocell, and less will be dropped across resistor R1, giving. the junction point 19 of photocell PC and resistor R1 a more positive potential than it had previously. This causes the Wheatstone bridge to become unbalanced; the voltage at junction 19 becomes greater than the standard voltage atthe centertap of potentiometer R2. Since vertex 19 of the bridge is connected to the emitter E, of transistor Q1 through resistor R3, and the vertex of the bridge; formed by the centertap of potentiometer R2v is, connected to the base B of transistor Q1, this change in: the voltage between these vertices tends to makev the emitter voltage more positive with respect to the base. voltage. With a P.N.P. transistor utilized in a common emitter configuration, this voltage change causes the transistor to conduct an increased amount of current. This increased current will charge capacitor C3 in the emitter circuit of unijunction transistor Q21 at a faster rate. Capacitor C3 may only charge during the half cycle of applied alternating current during which the. anode of control semiconductor rectifier SCRl is positive and the cathode negative. During the other halfi cycle, the voltage at the junction of the cathodes, of rectifiers CR2 and CR3 will be negative with respect. to the voltage at their anodes, causing both. rectifiers. to conduct, and effectively short circuiting capacitor C3.

It is characteristic of a unijunction transistor that when voltages below a critical value are applied to the; emitter terminal, there will exist a very high. resistance, essentially on open circuit, between the emitter and either of the bases. The value of the critical voltage is afunction of the type of transistor and. of the voltages at the two bases; when the voltage at the emitter equals.- or exceeds this critical voltage, the resistance from the. emitter to base number one B1 decreases suddenly'to a, very low value. In this circuit then the capacitor C3 is charging toward the critical value. When thevoltag on capacitor C3 reaches the critical value, the electrical. energy stored in the capacitor will be discharged from, the emitter througlrbase number one B1 and through resistor R8 in the base one B1 circuit, impressing .a sharp voltage spike or trigger across resistor R8. This. voltage spike will be positive as. viewed from the end. of resistor R8 next to base one with respect to the other end of resistor R8, and its duration will be roughly on the order of live microseconds. Since, as described previously, the capacitor is completely discharged by rectifiers CR2 and CR3 during alternate half cycles, the charge cycle just described begins anew every other half cycle. The speed with which the capacitor is able to; charge from no voltage to the critical voltage. is dependent primarily upon the size of the capacitor and the amount of charging current which is supplied bythe circuit of transistor amplifier 12. lt-is desired that. the capacitor C3 charge to the critical voltage at least; once during the time interval of a half cycle of sixty cycle alternating current, or during 1/120 of a second, and the size of capacitor C3 and the parameters at transistor amplifier 12 are chosen accordingly. There. is thus a sharp trigger pulse generated across resistor, R8 during every half cycle in whichthe voltage across the silicon control rectifier SCRl is such that the rectifier may conduct.

In the example being followed by way of explanation, a decrease in intensity of reflected light has resulted in an increase in photocell resistance and an unbalance. in. the bridge circuit, causing transistor Qlto conduct more,

heavily and charge capacitor C3 more. quickly.

causes the trigger pulse to occur closer to the beginning of the half cycle.

' The trigger pulses generated across resistor R8 are coupled via resistor R13 to the control terminal 28 of silicon control rectifier SCRl. The purpose of resistor R13, which has a relatively small resistance value, is to protect the silicon control rectifier against damage from current overload which might result from a failure in trigger generator circuit 13.

A silicon control rectifier has incorporated in it an extra terminal, a control terminal, in addition to the normal anode and cathode of a rectifier, and it requires as a condition for conduction, in addition to the voltage polarity required by standard rectifiers, a positive pulse applied to the control terminal (that is, a pulse during which the control terminal is made positive with respect to the cathode). Once this pulse is impressed upon the control terminal, the rectifier will conduct as long as the voltage on'the anode remains positive with respect to that on-the cathode. Thus in the circuit of this device; the series of triggers coupled from resistor R8 to the control terminal of silicon control rectifier SCRl occur only during those half cycles in which the polarity impressed across the rectifier allows conduction, and willresult in the silicon control rectifiers being triggered into conduction at some time during each such half cycle, and remaining in conduction during the remainder of each such half cycle. If each trigger pulse is caused to'occur nearer the beginning of its half cycle, it will result in the silicon control rectifiers conducting during a larger portion of each such half cycle.

Since the parallel combination of silicon control rectifierS CRl and conventional type semiconductor rectifier CR4 is located in the current path between the alternating current source and lamp L, all of the power delivered to the lamp L must pass through one of the two rectifiers. During the half cycles when the voltage polarity permits conduction of conventional rectifier CR4, full power will be delivered to the load. During the alternate half cycles, when the polarity across the silicon control rectifier SCR l is such as to permit conduction, the amount offipower delivered to the load depends upon the portion of each such half cycle for which the control rectifier conducts, and this portion is in turn controlled by the position of the triggering pulse relative to the start of e achjhali cycle.

Following the example through, it was found that a decrease in reflected illumination resulted in the displacement of each trigger pulse toward the beginning of the corresponding half cycle. This causes the silicon control rectifier to conduct for a large portion of the alternate half cycles, and results in more power being delivered to the lamp L. This increase in the brightness of the lamp increases the intensity of the reflected illumination. The intensity of illumination of the lamp L will continue to increase until the intensity of the reflected illumination causes the resistance of photocell PC to decrease to the point where the Wheatstone bridge is again in balance, that is, where there is no voltage differential between vertex 19 and the centertap of potentiometer R2, except fora small fraction of a volt bias needed for operation of the transistor amplifier 12.

The setting of potentiometer R2 in Wheatstone bridge circuit ll determines the left of intensity of reflected illumination which the device will maintain. The more the centertap is rotated toward the clockwise (CW) terminal, the higher the intensity of illumination.

' By adjusting the sensitivity control, potentiometer R in the transistor amplifier circuit, the system may be made more or less sensitive to fluctuations in the level of reflected illumination. As the centertap of potentiometer R5 is moved toward the clockwise (CW) terminal, the resistance shunting the base and emitter circuits of transister Q1 is increased, the sensitivity of the transistor 'am plifier isincreased, and the controller circuit will respond y r r to smaller light intensity fluctuations and will respond more quickly.

All of the above description has been of normal circuit operation in which the controller automatically regulates the level of reflected intensity, and it has been assumed accordingly. that switch S1 has been in the Auto (Automatic) position. which the operator may, if he wishes, disable the automatic operation and adjust the intensity of illumination manually, without having the device correct for suraces of varying reflectivity. By putting switch S1 in the Man (Manual) position, the Wheatstone bridge and transistor amplifier circuits. are disconnected, and the centertap of potentiometer R3 is connected to the junction of the capacitor C3 with the emitter E of the uni junction transistor Q2 inthe trigger generator circuit. By varying the position of the centertap of potentiometer R9, which ispartof a voltage divider placed across 18 volt transistor-supply 32, the resistance in the charging pathof capacitor C3 maybe varied, thus varying the current charging-capacitor C3. This, as described previously, will control the intensity of the load lamp L. In manual operation also, the damping capacitor C4 isremoved by switch SIB from across the voltmeter M-since it is not needed. The rapid fluctuations in the voltageacross the load which occur in automatic operation are not present when the light intensity is controlled manu ally, and so meter damping is not needed. 1

The following components have been found to perform satisfactorily in actual operation-of the circuit described:

61 10 mfd. C2 0.2 mfd. C3 0.1 mfd. C4 2000 mfd. C5 8 mfd. C6 50 mfd. C7 0.05 mfd. R1 33K ohms. J R2 50K ohm potentiometer. R3; Selected empirically. R4 470 ohms. R5 1K ohm potentiometer. R6 470 ohms. R7 470 ohms.

R8 47 ohms. R9 50K ohm potentiometer. R10 4.7K ohms. R11 K ohms. R124 10K ohms. Rl 3.;..' 22 ohms. Rl4 L ....Q 5K ohms. CR1 1N1695.

CR2 1Nl695.

CR3 1N1695. CR4 1N2156.

CR5 1N1695.

ZCR1 1M18Z. Q1 2N185. Q2 2N491.

General Electric SCRI type 036B. L1 3.35 #11. L2 3.35 .11. L3 3.35 1.11. L4 3.35 uh.

Radio Corporation of RC America No. 7163.

. Simpson Model 47 M voltmeter.

The embodiment of the power controller described above provided for the use of one silicon control rectifier, connected in parallel with a conventional rectifier, to provide control overthe power .supplied to the load during alternate half cycles. This provides a rangeof There is provided a means by.

power control of one half of the total power capability of the system; i.e., the power level supplied to the load may be varied from one half of full power to full power. The invention also contemplates increasing the range of power control over that of the described embodiment by using two silicon control rectifiers in parallel and providing control over the delivery of power to the lamp load during each half cycle.

We claim as our invention:

1. A power control system, for use in controlling the power supplied from an alternating current supply to a load device, comprising: a means for sensing a physical characteristic which is a function of a load device and transforming into an electrical parameter a quantitative measure of said physical characteristic, means for comparing said electrical parameter with a standard and producing an output electrical signal which is a function of the diiference indicated by said comparison, means for converting said output electrical signal into a series of functionally dependent electrical triggers, at least one semiconductor device located in the current path between said alternating current supply and said load device and adapted to be controlled by said series of electrical triggers so as to vary in accordance therewith the amount of power supplied from said alternating current supply to said load device, whereby said quantitative measure of said physical characteristic which is a function of said load device tends to be maintained at a constant level.

2. The invention of claim 1 wherein the comparison means is a Wheatstone bridge.

3. The invention of claim 1 wherein the sensing means is a photocell.

4. The invention of claim 1 wherein the means for converting the output electrical signal of the comparison means into a series of functionally dependent electrical triggers comprises an amplifier followed by a trigger generator.

5. The invention of claim 4 wherein the amplifier uses one or more semiconductor devices as its active elements.

6. The invention of claim 4 wherein the trigger generator uses one or more semiconductor devices as its ac tive elements.

7. The invention of claim 6 wherein the semiconductor device usd as an active element in the trigger generator is a unijunction transistor.

8. The invention of claim 4 wherein the amplifier and the trigger generator each contain only one active element.

9. The invention of claim 1 wherein the means for converting the output electrical signal of the comparison means into a series of functionally dependent electrical triggers includes a trigger generator adapted to generate a series of electrical trigger pulses having the characteristic that the time of occurrence of each trigger pulse with respect to the beginning of the corresponding half cycle of alternating current during which it occurs varies as a function of the electrical output signal of the comparison means.

10. The invention of claim 1 wherein the semiconductor device located in the current path between the alternating current supply and the load device comprises a controlled semiconductor rectifier adapted to control the power delivered through it by having the duration of its periods of conduction varied in accordance with the series of electrical triggers applied to it.

11. The invention of claim 1 wherein the semiconductor device located in the current path between the alternating current supply and the load device is a controlled semiconductor rectifier.

12. An illumination regulating device, for use in regulating the current supplied from an alternating current supply to a source of illumination, comprising: a sensor producing an electrical parameter which is a function of the intensity of the illumination impinging upon said sensor, means for comparing said electrical parameter with a standard and producing an output electrical signal which is a function of said comparison, means for converting said output electrical signal into a series of functionally dependent electrical triggers, at least one semiconductor device located in the current path between said alternating current supply and said source of illumination and adapted to being controlled by the said series of electrical triggers in such a manner as to vary the amount of alternating current power which is delivered to said source of illumination, whereby the intensity of the illumination impinging upon said sensor tends to remain constant.

13. An illumination regulating device, for use in regulating the current supplied from an alternating cur rent supply to a source of illumination, comprising: a photocell sensor adapted to provide an electrical measure of the intensity of the illumination impinging upon it, said photocell comprising a part of a bridge comparison circuit wherein the electrical measure of the intensity of said impinging light provided by said photocell is compared with a standard, said standard being easily adjustable, and wherein is generated a voltage which is a function of said comparison; an amplifier adapted to receive said comparison voltage and to supply an output signal functionally dependent upon the magnitude of said comparison voltage; a trigger generator adapted to receive said amplifier output signal and to generate a series of electrical trigger pulses having the characteristic that the time of occurrence of each trigger pulse with respect to the beginning of the corresponding half cycle of alternating current during which it occurs varies at a function of said amplifier output signal; at least one controlled semiconductor rectifier located in the path of current between the alternating current supply and the source of illumination and adapted to having the duration of its periods of conduction controlled by the said series of elec- References Cited in the file of this patent UNITED STATES PATENTS Machlet June 16, 1953 Beck Aug. 2, 1960 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,002,099 September 26, 1961 Charles A. Gregory, Jr. 9 et al.

hat error appears in fiche above numbered pat- It is hereby certified 1; r

that the said Letters Patent should read as ent requiring correction and corrected below.

Column 7, line 51, for "large" read larger line 64 for "left, read level column 9, line 23 for "be" read being line 44, for "usd" read used column 10, line 46, for "at" read as Signed and sealed this 13th day of February 1962.

(SEAL) Attest:

ERNEST w. sWIDE Attesting Officer DAVID L. LADD Commissioner of Patents