US3538427A - Alternating current constant rms voltage regulator - Google Patents

Description

Nov. 3,1970 N. OLTE'NDORF .f3;538,427

I ALTERNATING cuannm CONSTANT RMS VOLTAGE REGULATOR Filed may 13, 1968 v I {2 7/ 2/- El ull'iy SUPPLY I I 22 POWER /9 I 4 GATE DIFFERENTIAL CONTROL I mg AMPLIFIER GATE 7 3 I I8 LOAD l5 /5 V R8 Ac. j SUPPLY]! 72 FRO M AMPLIFIER I9 y TO SENSOR R7 //8 RI I OSCILLATOR //VVE"/V7'0/?.

' //7 NORMAN 0L vTo LOADIS TENDORF 3,538,427 ALTERNATING CURRENT CONSTANT RMS VOLTAGE REGULATOR Norman Oltendorf, Algonquin, Ill., assignor to Microdyne, Inc., Rolling Meadows, Ill., a corporation of Illinois Filed May 13, 1968, Ser. No. 728,421 Int. Cl. G05f 1/44 U.S. Cl. 323-24 8 Claims ABSTRACT OF THE DISCLOSURE An A.C. line voltage regulator for supplying a constant [RMS voltage to a load from a varying supply, including a signal controlled rectifier power gate connecting the load to the supply and a control gate for intermittently applying the output of the power gate to an incandescent lamp or other sensing element having a high thermal coeflicient of resistance. The sensing element is connected in a bridge circuit; a differential amplifier senses changes in the resistance of the sensing element during time intervals in which the sensing element is not energized from the power gate. The output signal from the differential amplifier controls the firing angle of the power gate and maintains the RMS Voltage to the load constant.

BACKGROUND OF THE INVENTION There are many industrial, commercial, and laboratory applications requiring a constant-voltage A.C. supply. In some instances, a constant peak-to-peak voltage or a constant average voltage is desirable, but in many applications it is the root-mean-square voltage that must be held at a constant level to assure constant power output from a given load. One example of an industrial or commercial application requiring close regulation of 'RMS voltage is photocopy equipment in which the illumination level afforded by incandescent lamps must be held relatively constant. This is but one example; there are many others.

An effective voltage regulator requires certain specific attributes. High speed of response and consistency of response are of primary importance. For most applications, the voltage regulator must be able to handle quite substantial variations in the supply voltage; for example, a regulator having an output voltage of 120 volts may be required to function for an input range of 105 to 135 volts. The regulator must be effective in performing its function independently of changes in the waveform of the supply voltage and preferably is essentially immune to changes in line frequency. At the same time, and particularly with respect to low-power industrial and commercial apparatus, the cost of the regulator, as an auxiliary component of a larger assembly, must be maintained at a minimum.

Most voltage regulators have been either magnetic or electromechanical devices. Both types have been quite successful but both have been generally best adapted to relatively expensive large scale apparatus. Low cost voltage regulators having high performance levels requisite for many applications have not been geenrally available.

SUMMARY OF THE INVENTION It is an object of the present invention, therefore, to

provide a new and improved high speed, high consistency voltage regulator of relatively small size and moderate cost, suitable for use in relatively small industrial and commercial apparatus, that provides a constant RMS voltage output from a supply that may vary substantially in peak voltage, RMS voltage, and waveform.

A related object of the invenion is to provide a new and improved solid state RMS voltage regulator that is United States Patent 0 ice of inexpensive construction yet affords high performance in connection with loads of small and moderate size.

Accordingly, the invention is directed to an alternating current voltage regulator for supplying a constant RMS voltage to a load having an impedance within a relatively wide range from a supply that may vary substantially in peak voltage, RMS voltage, and waveform. The voltage regulator of the invention comprises power gating means including a signal-controlled bidirectionally conductive semiconductor gate circuit for connecting the A.C. supply to the load and a sensing element, usually an incandescent lamp, having an effective impedance that is a function of the RMS voltage applied thereto, averaged over a short time interval. A control gate, which may comprise a single diode, intermittently applies the output of the power gating means to the sensing element. The regulator further comprises control means, including an amplifier, for detecting the impedance of the sensing element during time intervals when the sensing element is not energized by the output from the power gating means and for developing a control signal representative of changes in the impedance of the sensing element. Coupling means means are included in the regulator for applying the control signal to the power gating means to maintain a constant RMS voltage output therefrom.

Other and further objects of the present invention will be apparent from the following description and claims and are illustrated in the accompanying drawings which, by way of illustration, show preferred embodiments of the present invention and the principles thereof and what is now considered to be the best mode contemplated for applying these principles. Other embodiments of the invention embodying the same or equivalent principles may be made as desired by those skilled in the art without departing from the present invention.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram, partially schematic, of a voltage regulator constructed in accordance with one embodiment of the present invention;

:FIG. 2 is a detailed schematic diagram of one specific form of voltage regulator corresponding to the b ock diagram of FIG. 1; and

FIG. 3 illustrates certain modifications of the circuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a voltage regulator 10 constructed in accordance with one embodiment of the present invention and energized from a suitable A.C. supply 11 which may be assumed to 'be a conventional volt 60 Hz. single phase commercial A.C. supply. The two power lines of the A.C. supply 11 are designated by reference numerals 12 and 13 respectively, with lead 13 shown as constituting the system ground.

Voltage regulator 10 comprises a power gating means 14 which constitutes a signal-controlled bidirectionally conductive semiconductor gate circuit. Specifically different forms of appropriate power gate circuits are described and discussed hereinafter in conjunction with FIGS. 2 and 3. Power gate 14 is connected to supply line 12 and to a load 15, the load being returned to the other power line 13. Load 15 may, for example, comprise the lamps of a photocopy machine or some other form of apparatus requiring energization with a constant RMS voltage.

Regulator 10 further comprises a control gate circuit 16 that is connected to the output of power gate 14 and that is used to couple the output of the power gate to a sensing element R7 represented in FIG. 1 as an incandescent lamp. Sensing element R7 may comprise any device having an effective impedance that is a function of the RMS voltage applied thereto, averaged over a short time interval. An incandescent lamp, with its high thermal coefiicient of resistivity, constitutes an inexpensive circuit component that fulfills this requirement. Lamp R7 is returned to the system ground.

Regulator 10 further includes control means for developing a control signal that is representative of changes in the impedance, in this instance the resistance, of the sensing element constituting lamp R7. This control means comprises a bridge circuit 17, a diiferential amplifier 18, and an averaging amplifier 19. The control means is energized from a control supply 21.

Control supply 21 comprises a full wave rectifier or other similar control supply circut that is energized from A.C. supply 11, preferably through an isolating transformer T1. One output terminal of the control supply circuit is connected to an input terminal. 22 of bridge 17; the other output terminal of the control supply is returned to system ground.

Bridge 17 includes a pair of matched resistors R and R6 each connected to the input terminal 22 of the bridge. The other terminal 23 of resistor R5 is connected to one input of differential amplifier 18. Similarly, the remaining terminal 24 of resistor R6 is connected to the other input of the differential amplifier. Bridge terminal 23 is connected through a blocking diode CR2 to the sensing element, lamp R7. In the remaining leg of the bridge, a balancing diode CR3 and a resistor R8 are matched to the combination of the diode CR2 and the lamp R7. Resistor R8, like lamp R7, is connected to the bridge terminal 25 that is returned to ground.

The output of difierential amplifier 17 is connected to the input of the averaging amplifier 19. The control signal output from amplifier 19 is applied to the primary winding of a coupling transformer T2. The secondary winding of transformer T2 is coupled to power gate 14 to control the firing angle of the semiconductor gate circuit incorporated therein.

In considering operation of voltage regulator 10, it may be assumed that load 15 is energized through power gate 14 and that sensing resistance R7 is accurately matched to the corresponding resistor R8 in the other leg of bridge circuit 17. As long as resistances R7 and R8 are balanced, the output of differential amplifier 18 remains constant and the control signal supplied to the power gate 14 through amplifier 19 and transformer T2 holds the firing angle of the power gate constant. This maintains a constant RMS voltage on load 15.

If the input voltage to power gate 14 from supply 11 increases, there is a corresponding increase in the voltage applied to sensing resistance R7 through control gate 16. If this condition is maintained for even a brief interval, the resistance of lamp R7 increases. due to its high thermal coefiicient of resistivity. The resulting unbalanced condition in bridge 17 is detected by differential amplifier 18 and changes the output signal from the differential amplifier. The resulting change in the control signal from amplifier 19 adjusts the firing angle of power gate 14, causing the semiconductor gate to go conductive at a somewhat later point in the signal cycle. This reduces the effective RMS voltage supplied to load 15 and brings the regulator back into balanced condition.

A reduction in the supply voltage, from supply 11, produces an opposite effect. The resulting reduction in the voltage applied to lamp R7 through control gate 16 unbalances bridge 17 and this unbalanced condition is reflected in the output signal from differential amplifier 18.

Again, the control signal output from the amplifier 19 modifies the firing angle of the power gate, in this instance advancing the firing point of the semiconductor power gate 14.

A better understanding of the operation of voltage regulator can be reached by consideration of a specific circuit for the voltage regulator, such as the circuit shown in detail in FIG- 2. In the circuit of FIG. 2, power gate 14 comprises two signal-controlled semiconductor rectifiers SCRl and SCR2. The two rectifiers are connected in parallel with each other but in opposed polarities, each having one main electrode connected to the A.C. supply line 12 and another main electrode connected to an output terminal 31. It is the output terminal 31 that is connected to load 15 and to control gate 16.

In the embodiment of FIG. 2, control gate 16 is extremely simple in construction. It comprises a series current limiting resistor R1 and a diode CR1. The input of the control gate is connected to the output terminal 31 of power gate 14 and the output is connected to the sensing element constituting lamp R7.

Differential amplifier 18, in the circuit of FIG. 2, comprises two transistors Q1 and Q2. The base electrode of transistor Q1 is connected to terminal 23 of bridge 17. The collector electrode of transistor Q1 is connected through resistor R3 to the input terminal 22 of the bridge. Similarly, transistor Q2 has its base electrode connected to bridge terminal 24 and its collector electrode is connected through a resistor R4 to bridge terminal 22. The two emitter electrodes of transistors Q1 and Q2 are connected through a bias resistor R14 to bridge terminal 25.

In FIG. 2, an additional trimmer resistor R9 is shown in series with the bridge resistor R8, permitting adjustment of this leg of the bridge for an exact match with lamp R7.

The output from differential amplifier 18 to averaging amplifier 19, in the circuit arrangement of FIG. 2, is taken from the collector of transistor Q2. The coupling between the two amplifiers comprises a series resistor R13 which is connected to the base of a transistor Q3 in amplifier 19. The base electrode of transistor Q3 is also connected to a capacitor C1 that is returned to the output terminal 32 of control supply 21. It should be noted that control supply terminal 32 is connected to the input terminal 22 of bridge 17.

The emitter electrode of transistor Q3 is connected to the control supply terminal 32 through a resistor R11. The collector electrode is connected through a series resistor R10 to the gate electrode of a unijunction transistor Q4. The gate electrode of transistor Q4 is also connected to a capacitor C2 that is returned to the AC. supply conductor 13, here represented as system ground.

One of the main electrodes of the unijunction transistor Q4 is connected to the control supply terminal 32 through resistor R12. The other main electrode of the unijunction transistor is connected to the primary winding of the coupling transformer T2, which is returned to system ground. In this circuit arrangement, the coupling transformer is a pulse transformer provided with two secondary windings which are connected, with opposite polarization, to the trigger electrodes of the two signal-controlled rectifiers in power gate 14.

The control supply circuit 21 comprises a full wave diode rectifier bridge including diodes CR4, CR5, CR6 and CR7. The positive terminal 33 of the bridge rectifier is connected through a resistor R2 to the output terminal 32 of the control supply circuit. The negative bridge terminal 34 is returned to the A.C. supply line 13, taken as system ground. A Zener diode VR1 is connected from the output terminal 32 back to system ground.

The two signal-controlled rectifiers SCRl and SCR2 are in series with the controlled load 15 across the A.C. lines 12 and 13. Each of the two SCRs is triggered to conductive condition during alternate half cycles of the input voltage from supply 11 in a manner such that the RMS voltage across load :15 is maintained constant despite variations in the waveform, peak-to-peak voltage, RMS voltage, and average voltage from the A.C. supply.

During those alternate half cycles in which A.C. line 12 goes positive in polarity with respect to line 13, a minute portion of the total current output from power gate 14 is supplied through control gate 16 to the sensing element comprising lamp R7. On alternate half cycles of the opposite polarity, the diode CR1 in control gate 16 blocks current from the power gate to the sensing element. That is, diode CR1 assures that current fiows from the power gate to the sensing element intermittently, the gate being closed on alternate half cycles of the supply voltage. Diode CR2 serves to isolate this portion of the circuit from the differential amplifier 18; the voltage and current to sensing element R7 from control gate 16 are blocked from terminal 23 by diode CR2.

Coupling transformer I11 isolates the control portion of the circuit from the main A.C. line. The bridge rectifier afforded by diodes CR4-CR7 produces a full wave rectified voltage at output terminal 32. Resistor R2 and Zener diode VRl act in conjunction with each other so that the control supply voltage appearing at terminal 32 is a constant positive voltage of the general waveform indicated at reference numeral 35, with negative-going spikes that are in synchronism with the A.C. line input. In a typical circuit, for a 120 volt regulator, signal 35 would have a positive potential of about 20 volts.

The two halves of bridge circuit 17, and the related halves of differential amplifier 18, are fully balanced in their component values. Potentiometer R9 is employed to adjust that leg of the bridge between terminals 24 and 25 to a total resistance that is approximately equal to the normal operating resistance of lamp R7 when the regulator is functioning at the desired constant RMS voltage. As the voltage regulator warms up, which takes only a brief time interval, the bridge approaches balance whenever the A.C. line voltage is matched to the desired RMS voltage required for load 15. Under these circumstances, the collector currents of transistors Q1 and Q2 approach equality.

Any change in the resistance of the sensing element R7 changes the balance of the bridge and causes a corresponding change in the signals applied to the base electrodes of the transistors Q1 and Q2. The changes in bridge conditions cause corresponding changes in the control signal supplied to power gate 14. I The collector current from transistor Q2 in the differential amplifier flows through resistor R4 and determines a voltage drop that is averaged and stored by capacitor C1 in the averaging amplifier 119. This establishes a bias voltage for transistor Q3 which serves as a constant current source for charging capacitor C2 through resistor R10. The unijunction transistor Q4 is triggered to conduction whenever the charge on capacitor C2 reaches the firing point for the unijunction transistor. The discharge of capacitor C2 through transistor Q4 and the primary winding of the coupling pulse transformer T2 provides a pulse output signal, in the secondary windings of transformer T2, that triggers either SCR1 or SCR2 into conduction, depending upon the momentary line polarity. In this circuit, resistor R12 is chosen so that transistor Q4 will function as a trigger device; moreover, resistor R12 determines the firing voltage level of transistor Q4 and provides some temperature compensation for its operation.

If it is assumed that a line voltage drop occurs, across the supply lines 12 and 13, it is seen that the current to sensing element R7 is reduced. In a very short time interval this produces a lower filament temperature in the sensing lamp. Because of the high thermal coefficient of resistivity of lamp R7, a lower resistance is produced across the lamp. This lower resistance produces a lower bias voltage for transistor Q1, causing the collector current of that transistor to be reduced with a corresponding increase in the collector current of the other transistor Q2 in the differential amplifier. This results in an increase in the voltage drop across the resistor R4 in the collector circuit of transistor Q2, increasing the bias voltage supplied to the averaging amplifier transistor Q3.

The increased bias on the base electrode of transistor Q3 increases the operating current for capacitor C2, triggering transistor Q4 to conduction earlier in each cycle of operation. As a consequence, each of the two signal controlled rectifiers SCR1 and SCR2 is triggered to conduction at an earlier point in the operating cycle, increasing the RMS voltage supplied to load 15. This also increases the current supplied to the sensing element, lamp R7, through gate 16, and the voltage regulator returns to balanced operation when the RMS voltage is restored to its initial value. It will be seen that the essentially reverse operation occurs in the circuit for an increase in line voltage.

The regulated voltage supplied to load 15 is always somewhat lower than the line voltage but can approach of the input voltage on a practical basis. Regulation of better than one percent is readily obtained. Of course, if a relatively high voltage for load 15 is necessary, a step up transformer can be incorporated in the A.C. supply ahead of lines 12 and 13.

From the foregoing description, it will be apparent that the resistance of the sensing element R7 is effective ly measured only during those alternate half cycles in which current does not flow through the sensing lamp from the power gate 14. It is important that there be definite time intervals available in which there is no current flow through lamp R7 from the power gate in order to permit the control means comprising bridge 17 and amplifier 18 to sense instantaneous changes in resistance of the sensing lamp.

FIG. 3 illustrates certain modifications that can be made in the circuits of FIGS. 1 and 2 without departing from the present invention. In FIG. 3, there is shown a modified power gate 114 that can be substituted for the power gate circuit 14 specifically illustrated in FIG. 2. Power gate 114 comprises a single triac, a signalcontrolled bidirectionally conductive semiconductive rectifier, having its input and output electrodes connected between supply line 12 and load 15. In the circuit arrangement of FIG. 3, the pulse transformer T2 has only a single secondary winding which is connected to the trigger electrode of the triac gate.

The modified circuit of FIG. 3 also includes a different circuit for the control gate of the regulator. The control gate 116 shown therein comprises a transistor 117 having its collector electrode connected to resistor R1 and its emitter electrode connected to sensing element R7. The base electrode of transistor 117 is connected to a control oscillator 118 operating at a substantially higher frequency than the A.C. line frequency. For example, the frequency of oscillator 118 may be of the order of 600 hz.

The power gate modification illustrated in FIG. 3 is generally self-explanatory. Because triac 114 conducts bidirectionally, only a single connection to the pulse transformer T2 is required. operationally, the circuit is fully equivalent to the two signal-controlled rectifiers used in the circuit of FIG. 2.

The modification of the control gate comprising transistor 117 and oscillator 118 simply turns the control gate on and off at a higher frequency than is effected by the diode CR1 in the circuit of FIG. 2. This high speed switching of the input to sensing element R7 provides some improvement in sensitivity of the circuit and also makes it possible to reduce the size of the averaging capacitor C1 (see FIG. 2) because the capacitor is not required to average the output from differential amplifier 18 over as long a period.

In a practical embodiment of the circuit of FIG. 2, employed to maintain a constant RMS voltage on load 15 of 120 volts within limitations of 0.5 volt over a range of input voltages from 105 to volts, and with variations of load current in a ratio of 4:1, the components set forth in tabular form hereinafter have been employed. It should be understood that these data are furnished solely by way of illustration and in no sense as a limitation on the invention.

Table of Components Resistor R11500 ohms Resistor R25 kilohms Resistors R3, R4, R5, R6, R4.7 kilohms Resistor 8-180 ohms Potentiometer R9200 ohms Resistor R11--220 ohms Resistors R12, R14-47-O ohms Resistor R'13330 ohms Capacitor C1300 microfarads Capacitor C20.1 microfarad Diodes CR1, CR2, CR3 -1N4003 Diodes CR4, CR5, CR6, CR7-1N3493 Transistors Q1, Q2-2N4123 Transistor Q3--2N4125 Transistor Q42N2646 Rectifiers SCRl, SCR2C20B Lamp RSType 1839 Hence, while prefrerred embodiments of the invention have been described and illustrated, it is to be understood that they are capable of variation and modification.

I claim:

1. An alternating-current voltage regulator for supplying a constant RMS voltage to a load having an impedance within a relatively wide range from a supply that may vary substantially in peak voltage, RMS voltage, and waveform, comprising:

power gating means, comprising a signal-controlled bidirectionally conductive semiconductor gate circuit, for connecting an alternating-current supply to a load;

a sensing element having an eifective impedance that is a function of the RMS voltage applied thereto, averaged over a short time interval;

a control gate for intermittently applying the output of said power gating means to said sensing element;

control means, including an amplifier, for detecting the impedance of said sensing element, and developing a control signal representative of changes in the impedance of said sensing element;

and coupling means for applying said control signal to said power gating means to maintain a constant RMS voltage output therefrom.

2. An alternating-current constant RMS voltage controller according to claim 1, in which said sensing element is incorporated in one leg of a bridge circuit comprising a part of said control means, and in which said control means amplifier is a difierential amplifier con- 8 nected across said bridge circuit, said bridge circuit including blocking means for isolating said difierential amplifier from said power gating means during time intervals in which said sensing element is energized by the output from said power gating means.

3. An alternating-current constant RMS voltage regulator according to claim 2 in which said control means further comprises an averaging amplifier for developing a substantially continuous A.C. control signal from the output of said diflerential amplifier.

4. An alternating-current constant RMS voltage regulator according to claim 2 in which said sensing element constitutes an incandescent filament lamp.

5. An alternating-current constant RMS voltage regulator according to claim 2 in which said control gate comprises a diode in series between the output of said power gating means and said sensing element and said blocking means comprises a diode in series with said sensing elementin said one leg of said bridge circuit.

6. An alternating-current constant RMS voltage regulator according to claim 2 in which said control gate is actuated by a high frequency source operating at a substantially higher frequency than the AC. supply frequency.

7. An alternating-current constant RMS voltage regulator according to claim 2 in which said power gating means comprises two signal-controlled semiconductor rectifiers connected in parallel with each other in opposed polarities and in which said coupling means comprises a coupling transformer having two secondary windings each coupled to the trigger electrode of a respective one of said rectifiers.

8. An alternating-current constant RMS voltage regulator according to claim 2 in which said power gating means comprises a triac.

References Cited UNITED STATES PATENTS 3,295,053 12/1966 Perrins. 3,372,328 3/ 1968 Pinckaers.

3,389,328 6/1968 Janson 323-24 X 3,408,558 10/1968 Peterson et al. 3,444,456 5/ 1969 Codichini. 3,461,376 12/1969 Wanlass 323-22 J D MILLER, Primary Examiner G. GOLDBERG, Assistant Examiner U.S. Cl. X.R. 323-37, 40

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