US2155515A - Regulator system - Google Patents

Description

April 1939- D. E. TRUCKSESS 2,155,515

REGULATOR SYSTEM Filed Jan. 31, 1936 INVENTOR B D. E. TRUC/(SESZS A T TORNE Y Patented Apr. 25, 1939 UNITED STATES PATENT OFFICE Bell Telephone Imboratories,

Incorporated,

' New York. N. 1., a corporation of New York Application January 31, 1986, Serial No. 61,681

80lalms.

This invention relates in general to a system of voltage regulation and particularly to a regulated rectifier system of the type wherein the rectifier output is regulated in accordance with the ter- 5 minal voltage of a direct current circuit served by the rectifier. I

In certain types of power plants, such as the small central ofiice of a telephone system, it is sometimes desirable to regulate the voltage of the storage batterieshwhich supply power to the telephone equipment, more closely than is practicable with the well-known rectifier systems ordinarily used for this purpose. A considerable extension in the life of the battery and a reduction in the cost of maintenance may thus be attained.

It is the object of'this invention to simplify and'improve regulated rectifier systems of the type commonly used to fioat storage batteries. maintain them charged and regulate their voltage and to provide for a more precise regulation than has been available heretofore.

This object is attained in accordance with a feature of the invention by the provision of a full wave rectifier of the grid controlled type variations in the terminal voltage of the direct current circuit served by the rectifier, by changing the relative phase relations of the potentials applied to the grid and plate electrodes of the rectifier tubes. e

Another feature of the invention resides in the use of a phase shift bridge circuit whose controlling element comprises the plate-cathode resistance of a three-element vacuum tube and in subjecting the grid thereof to a biasing potential which varies in accordance with variations in the terminal voltage of the direct current circuit served by the rectifier.

A further feature of the invention insures 40 against overloading of the rectifier whichwould otherwise occur upon the resumption of power after a prolonged interruption thereof. This result is attained by automatically transferring the control of the rectifier from voltage regulation to constant current regulation after a prolonged failure of the power supply and the consequent excess drop in voltage of the direct current circuit.

A still further feature of the invention contemplates the use of electromagnetic switching means in the rectifier grid conductors which function in response to power or tube failures to actuate alarm signals.

Another feature of the invention resides in the .1 use of novel means for delaying the fiow of the rectifier anode current until the rectifier filament has been heated to its normal operating temperature.

Still anotherfeature of the invention is emeo bodied in the use of screen grid type tube in the whose output is controlled in accordance with regulator circuit and in subjecting the screen thereof to a potential which varies in accordance with the regulated voltage. The screen grid type of tube has a characteristic such that its plate voltage can be changed over wide limits with 5 little effect on the plate current provided the screen voltage is maintained substantially constant. By supplying the screen with a voltage which varies with the regulated voltage, which is substantially constant, the screen voltage is 10 maintained constant and is therefore unaffected by variations in the alternating current supply voltage. The gain of the tube is also increased by supplying voltage to the screen in this man- 15 ner.

These and other features of the invention not specifically identified, will be readily understood from the following detailed description made with reference to the accompanying drawing which constitutes a complete disclosure of the regulated rectifier system of this invention and its associated charge and discharge circuits.

The rectifier consists essentially of two threeelement mercury vapor tubes which are grid controlled and connected for full-wave rectifica- 25 tion. The output current of these tubes is controlled by changing the relative phase relations of the voltages applied to their grids and plates. When the grid voltage is in phase with the plate voltage, the tubes deliver maximum current and so when 180 degrees out of phase they deliver no current. The phase of the grid voltage is changed by varying the resistance of one arm of a phase shift bridge circuit. For manual control this resistance comprises a variable rheostat 35 whereas for automatic operation the plate-cathode resistance of a three-element vacuum tube. which is controlled by the battery. or direct current circuit, voltage is used. The voltage phase relations and consequently the output current 0 of the rectifier are changed under these conditions by varying the bias on the grid of the tube in the phase shifting circuit. The direct current voltage for biasing the grid of this tube is obtained from a regulating circuit which consists as of a one tube direct current amplifier. This is required to increase the fractional voltage changes of the direct current circuit to several volts before applying it to the grid of the phase shifting tube. As the battery or direct current 5 circuit voltage varies, the regulator changes the bias on the phase shifting tube which in turn varies the output current from the rectifier in a direction to cause the battery voltage to return to its original value.

After a prolonged power failure, the grid of the regulating tube is transferred from the volt-" age potentiometer to the terminal of a rheostat which is connected across a resistor in the charge lead to the battery. A constant potential source so is included in the grid lead under this condition. The circuit functions in the same manner as described above except that a constant voltage is maintained across the series resistor which maintains a constant output current from the rectifier. The value of this constant output current may be adjusted by the rheostat at any value but the full load current is usually selected in order to recharge the battery rapidly.

The single figure on the accompanying drawing is a diagrammatic representation of a regulated rectifier system embodying the features of the invention.

Referring to the drawing. a suitable source of alternating current is shown at II from which power is supplied to the load circuit, generally indicated at H by way of the rectified which comprises essentially, the two three-element mercury vapor tubes II and II which are connected for full wave operation. A plate transformer I4 is interposed between the rectifier and the alternating current source III, the plate electrode of the tubes i2 and I8 being connected respectively to opposite terminals of the secondary thereof by way of the choke coils II and I0 respectively. Condenser I1 is bridged across the filament and plate leads of the tube It and a similar condenser It is bridged across the filament and plate leads of tube 3, the condensers I1, is and the choke coils II and it constituting filters which act to suppress radio frequency disturbances caused by the operation of the tubes. The filaments of tubes l2 and II are supplied with heating current from secondary ll of transformer 20. The grids of tubes I! and II are connected to opposite terminals of the secondary of grid transformer 2|, each by way of the winding of the relays 22 and 23 respectively. The purpose of relays 22 and II will be indicated in a later description.

Retard coil 24 is provided in the rectified circuit for filtering the direct current output. A

condenser 25 and an additional retard coil II are used in the connecting charge and discharge circuit for additional filtering.

The phase shift circuit comprises the two halves of secondary 21 of transformer 20, resistances 2s, 29 rheostat III and condenser 3| connected in a Wheatstone bridge arrangement with the primary winding of transformer 2| constituting the galvanometer arm. As shown, one terminal of the transformer secondary is connected to the midpoint of secondary 11 of transformer II and the other terminal to the lead interconnecting condenser 3| and rheostat II.

For manual regulation the switch arm of the rheostat is manipulated to increase or decrease the value of resistance of the bridge arm including resistances II and I! as conditions warrant such adjustments. As the resistance of the rheostat is varied there is a corresponding phase displacement of the potential impressed on the grids of tubes l2 and It by way of transformer 2| with respect to the phase of the potential impressed on the plates of the rectifier tubes by way of transformer I 4 with the result that the rectifier output current is increased or decreased.

For automatic operation, the rheostat switch is actuated to such a position as to cause contact 32 to be closed which results in a substitution of the plate-cathode resistance of tube a for the rheostat resistance 30. The filament heating current for tube 33 is supplied by the secondary 34 of transformer II. The grid of tube 38 is directly connected to the plate of the regulating ,ampiifiertubelwhosegridisconnectedtothe negative pole of battery 80. The tube ll is of the screen grid pentode type.

An input potentiometer, comprising resistors II, 8| and I. is connected across the battery ll, or direct current circuit terminals whose voltage is to be regulated. one side of the potentiometer being connected to the direct current circuit terminal indicated at ii and the other side to ground at the lower armature and back contact ofrelayli,itbeingunderstoodthsttheother direct current circuit or battery terminal is grounded as indicated at N. A rheostat u is connected across one section of the potentiometerandisusedtosdjusttheregulatedvoltage and compensate for a reduction in the voltage of battery It as it wears out. The voltage drop over the combined resistances 81, 38, SI and rheostatflisconnectedinseriesopposingwith the battery It and the voltage difference is appliedtothegridoftheregulstor amplifiertube II. 'Ihis net grid voltagevsries, therefore, in proportiontothevariationsinvoltage acroasthe points ll, ll.

Thisvsryinggridvoltageisamplifiedinthe tube 8| andisimpressedonthegridofthephase shifttubefl, bymeansofrmistsnce Ill tocontrol thebiasthereof. Anychangeinthebias onthegridofthephaseshifttubeliresultsina corresponding change in the plate-cathode resistanceofthetubeandaconsequentshiftin phaseofthepctentialimpressedonthegridsof the rectifier tubes l2 and It. 'Ihe rectifier output current is accordingly varied. It follows. therefore. that any drop in voltage of the direct current circuit or battery ll is accompanied by a change in the phase relation between the potentials impressed onthe rectifier grids and plates respectively, and this change is in such a direction as to increase the rectifier output current which in turn causes the direct current voltage to be raised. Conversely, any rise in the direct current circuit or battery voltage results in a lowering of the rectifier output current and the consequent lowering of the direct current terminal voltage. By selecting suitable values for the resistors 2s and 2! the system may be so adiusted that under normal load conditions the rectifier supplies the entire load and the battery ll merely fioats on the line.

It will be understood that the regulator delivers no current, only a voltage to the control circuit of the grid controlled rectified.

Transformer ll supplies filament voltage to the screen gridtypeamplifiertubetlbywayofthe secondary winding II. The plate of tube II is connected to the positive terminal of battery I by way of resistance Ill and conductors Ill and Ill and since the cathode of tube ii is grounded. as is the negative terminal of the regulated battery, the amplifier tube plate voltage is furnished by the battery ll whose voltage is being r sulated. The screen voltage for tube 88 is supplied by the voltage drop across resistance which is supplied by battery II and since the voltage of the battery is regulated and therefore substantially constant the screen voltage of tube OI will be maintained substantially constant.

A description of the operation of the complete system, including the charge and discharge circuits will now be made.

Itwillbenotedthatassoonasthesource I. has been connected to the equipment herein disclosed by the' operation of any suitable switching means, the primary windings of all the transformers utilised become energised. The plate potentials for the tubes l2 and I! are therefore applied at the same time that the heating current is applied to the filaments. These plate and grid potentials however, are 180 degrees out of phase so that the rectifier anode current is blocked and thetubes if and I! do not fire. The phase displacement of the plate and grid potentials is obtained by maintaining the resistance arm of the phase shift circuit open. This open is effected at the outermost armature and front contact of relay ll.

When the source It is effectively associated with the equipment an obvious path for the 1s heater winding II of the thermostatic device is completed. The passage of current through the heater winding 52 for a predetermined period of time causes the flexing of element ll and the consequent closure of contact ll whereupon relay so u is connected across the terminals of the supply source and operates. Relay it loclrs up directly to the source ll independent of the thermostatic element l5 and opens the heater circuit allowing the heater element to restore to its normal condition. In actuating its outermost armature, relay it closes the resistance arm of the phase shift circuit to cause the phase of the grid potential to shift in such a direction as to permit the rectifier tubes tofire. The advantage so of this arrangement over well-known time delay expedients heretofore employed is that the relay contacts need handle only a small current, in the order of milliamperes, instead of the main rectifier current of many amperes. The cost of as the relay is -materially reduced and the same relay may be used for all sizes of rectifiers.

The rectifier tubes i2 and I3 function as a full wave "rectifier delivering rectified current to the direct current load circuit indicated at II.

o It will be assumed, at this stage of the description that the voltage of battery and the load conditions are such that the battery merely fioats on the line and the load is handled entirely by the rectifier.

grid leads accordingly operate causing relay 5.

to operate and maintain the alarm circuit indicated at 51 open. The operating circuit for relay 5' extends from ground battery, winding of relay front contacts and armatures of relays so 22 and 28 to ground at the point 0. The relays 22 and 23 remain operated as long as the tubes l2 and I! are functioning.

The path for the rectified current may be traced from the plate electrode of tube i! to the filament thereof, conductor 60, lower half of the transformer secondary l9, conductor 0|, meter '3, charge fuse 64 to the battery terminal ll, through the counter cells 65, 66 and 81, through the load ii to ground at It, resistance OI, coils w 26 and 24, upper half of the secondary of transformer I4, choke coil it to the plate electrode of tube 12. the rectified current from tube It. The load II is accordingly supplied by the rectifier and 65 the battery Ill floats on the line.

The counter cells 65, 66 and 81 are inserted in the discharge lead to maintain the load voltage within limits of the voltage responsive relay 10.

70 As hereinbefore described, any variation in the voltage of the direct current circuit results in a corresponding variation in the net voltage impressed on the grid of regulator tube II and a corresponding change in the bias on the grid 15 of phase shift tube 33. The overall effect is to Relays 22 and 23 in the rectifier A similar path may be traced for vary the rectifier current output whereby the voltage of the direct current circuit is brought back to normal.

In the event of a power failure or rectifier tube failure, relays 22 and restore their ar- I matures. opening the circuit to relay II which relay thereupon releases establishing an obvious circuit to the alarm ll. Relay I. may also cause the functioning of an auxiliary charging circuit if so desired. As this feature does not con- 10 stitute a part-of the present invention no further reference thereto will be made.

At its lower armature and back contact relay N completes a circuit for the energization of relay 4!. this circuit extending through the upper 15 armature-and back contact of relay I2. Relay 42 operatesand locks up through resistance II to ground. At its lower armature relay 42 connects the cathode of tube II to oneterminal of the rheostat ll andatfits upper outer armature 9o transfers the grid lead of tube II from the nesative terminal of battery II to the negative ter minal of battery ll whose positive plate is connected to the other terminal of rheostat II. The

control of the regulated rectifier has accordingly as been transferred from voltage regulation to constant current regulation. It will be noted that the cathode grid circuit of tube 3' is now connected across the constant potential II and a portion of rheostat 14 so that the bias on the so grid of tube 88 is maintained constant with the result that the rectifier current output is held at a constant value and the battery ll charged accordingly, assuming that the power or tube failure has been corrected and the rectifier again '5 in operating condition.

If the failure is of such a duration as to permit the voltage of battery 40 to fall to such an extent as to operate relay II, the low voltage contact I. thereof is closed to battery. Relay II o thereupon operates over an obvious circuit and causes the operation of relay ll. Relay 1', at its lower armature and front contact establishes an obvious circuit for the alarm II which functions to give a low voltage alarm. At its upper 5 armature, relay 1! places battery on the lead by way of the lower armature and front contact of relay Ii thereby causing the release of the normally operated relay I2. At its upper armature and back contact relay .2 established an obvious energizing circuit for relay II whichrelay operates and short-circuits the counter cells 8' thereby raising the output voltage.

Relay 83, at its lower armature, removes battery from the winding. of relay 8i causing this a relay to release.

If the power failure continues until the low voltage contact I. of relay II is made a second time, relays I1 and I. again operate, the latter, at its lower armature and front contact establishing a circuit for the alarm It. At its upper armature and front contact relay ll places battery on the conductor I by way of the back contact and lower armature of relay Ii and the lower armature and front contact of relay 8'. Relay ll accordingly restores its armature establishing an obvious circuit for relay It. Relay ll at its lower armature removes battery from the winding of relay II causing this relay to release and at its upper armature and front contact short- 7 circuits the counter cells I causing a further increase in the output voltage.

Upon resumption of power or replacement of the defective tube. the rectifier operates under constant current output under control of relay 4! 1s as hereinbefore described. As the rectifier functions, relays 22 and 28 operate in the tube grid circuits, as previously pointed out, to cause the operation of relay II which, in turn, opens the circuit to the rectifier alarm II. The silencing of the alarm 51 indicates the correction of the condition which manifested itself in the failure of the rectifier.

The battery 40 continues to be charged at a constant current from the rectifier until its voltage and that of the direct current circuit reaches a value sufiiciently high to cause relay 10 to function and close its high voltage contact 90. Relay 9i accordingly operates in an obvious circuit and establishes an operating circuit for relay 92. At its lower armature and front contact relay 92 causes the operation of alarm 19 and at its upper armature and front contact establishes a circuit extending from grounded battery, winding of relay 9l, back contact and upper armature of relay 89 (released), upper armature and front contact of relay 92, front contact and armature of relay 9i to ground. Relay 81 operates and locks up through resistance 99 to ground and opens the energizing circuit to relay 88. Relay 89 upon release of its upper armature removes the short circuit from the counter cells 95 causing them to be effectively inserted in the discharge circuit and to reduce the output voltage. At its lower armature and back contact relay 88 connects battery to the winding of relay 89 causing this relay to operate.

The voltage of battery 40 continues to rise under constant charging current from the rectifier and the output voltage increases until the high voltage contact 90 of relay 10 is closed a second time. Relays 9| and 92 again operate, causing the actuation of the alarm 19 and establishing a circuit for relay 82 extending from grounded battery, winding of relay 82, back contact and upper armature of relay 8| (released), front contact and upper armature of relay 86, upper armature and front contact of relay 92 to ground on the front contact and armature of relay 9i. Relay 82 operates in this circuit and locks up through resistance 95 to ground. At its upper armature relay 82 opens the energizing circuit for relay 88 which relay restores its armatures thereby effectively inserting the counter cells 66 in the discharge circuit to lower the output voltage and connecting battery to the winding of relay 8| to cause this relay to operate.

The voltage of battery 48 continues to rise from the constant current output of the rectifier increasing the output voltage until the high voltage contact 90 of relay I0 is closed a third time giving an alarm at 19 and causing relay I2 to operate in a circuit extending from grounded battery, winding of relay I2, front contacts and upper armatures of relays II and 96, upper armature and front contact of relay 92 to ground on the front contact and armature of relay 9|.

Relay I2 at its lower armature and front contact places battery on the locked up contact of relay 42 causing relay 42 to restore. Relay 42 at its upper outer armature disconnects the grid of tube 35 from the negative terminal of battery 15 and connects it to the negative terminal of bat- 'tery 39 thereby returning the control of the regulated rectifier from constant current to voltage regulation at normal fioating limits of the battery 49. A manually operated switch 91 is provided for the purpose of short-circuiting the counter cells 61 to permit operating the battery to a higher overcharge voltage after a power failure, if des Y It is apparent from the foregoing description that applicant has devised a regulated :ectifier system which provides for precise voltage regulation of the rectifier under normal operating conditions and which guards against overloading of the rectifier in the event of a prolonged power failure and a subsequent resumption of power. In the latter instance it will be noted that during a power failure, the battery 48 furnishes power to the load and the longer the failure persists the lower the terminal voltage attained by the battery. Unless provision is made for the transfer from voltage regulation to constant current regulation under this condition the phase of the potentials impressed on the grids of tubes l2 and I8 would be displaced with respect to the phase of the potentials impressed on the rectifier plates upon resumption of power, to such an extent that the rectifier current output would experience an abnormal increase, with the result that the rectifier is overloaded and the fuse N in the charge circuit becomes overtaxed and blows.

vA further guard against overloading the rectifier is efiected by selecting resistors 28 and 28 to ive a high overload current when the rectifier starts to recharge after a power failure, which will continue until the transformer heats to the allowable limit. A thermostat, indicated at Ill, attached to the core of transformer M will be operated by the heat of the tramformer and its contacts lli operated to remove the short circuit from resistor 28, reducing the current to a safe limit. The addition of this thermostat permits the rectifier to be adjusted for a high charging rate to bring the battery 49 back quickly after a power failure by using the rectifier at its highest capacity during the heating period of the tramformer i4, and then reducing the charging current to a value which the transformer will carry continuously.

The condensers 99 and ill are interposed between ground and the charging conductors for the purpose of further insuring against radio interference.

What is claimed is:

1. In a. regulator system, a direct current circuit, an alternating current supply circuit, means comprising athree-element space discharge rectifier tube connected to supply rectified current to said direct current circuit, means including a potentiometer connected across said direct current circuit for regulating the output current of said rectifier in accordance with the voltage of said direct current circuit, means for maintain ing the output current of said rectifier at a constant value irrespective of the voltage of said direct current circuit, a relay for rendering said last-mentioned means effective and means for indicating a failure of said supply circuit controlling said relay.

2. In a regulator system, a direct current circuit, an alternating current supply circuit, means comprising a thermionic rectifier connected to supply rectified current to said direct current circuit, means including a phase shifting bridge circuit for varying the output current of said rectifier, a vacuum tube included in said phase shifting bridge circuit and having a grid whose bias determines the extent to which the phase shifting circuit functions to vary the rectifier output current, means for biasing the grid of said vacuum tube in accordance with output voltage of said rectifier, and means responsive to an interruption of the alternating current supply circuit for rendering said biasing means effective upon a subsequent completion of said alternating current supply circuit to maintain the output current of said rectifier constant after a power failure.

3. In a regulator system, a direct current circuit, an alternating current supply circuit, means comprising a thermionic rectifier connected to supply rectified current to said direct current circuit, means including a phase shifting bridge circuit for varying the output current of said rectifier, a vacuum tube included in said phase shifting bridge circuit and having a grid whose bias determines the extent to which the phase shifting circuit functions to vary the rectifier output current, means normally biasing the grid of said vacuum tube in accordance with the voltage of said direct current source, means for biasing the grid of said vacuum tube in accordance with the output current of said rectifier, switching means for interchanging said grid biasing means and means for indicating an interruption of the alternating current supply circuit and controlling said switching means whereby the normal voltage regulation of said rectifier is replaced by a constant current regulation after a failure of the supply circuit.

4. In a regulator system, a direct current load circuit, an alternating current supply circuit, means comprising a grid controlled rectifier connected to supply rectified current to said load circuit, means comprising a phase shifting bridge circuit for varying the potential impressed on the grid of said rectifier, a vacuum tube included in said phase shifting circuit and having a grid whose bias determines the extent to which said phase shifting circuit functions to vary the potential impressed on the grid of said rectifier, a potentiometer connected across said load circuit which is subject to voltage variations experienced by the load circuit, a resistance-rheostat combination included in the output circuit of said rectifier and subject to the current output of said rectifier, an amplifier having its plate circuit connected to the grid of said vacuum tube, switching means connecting the grid of said amplifier with said potentiometer whereby the grid of said vacuum tube is biased in accordance with the voltage of said load circuit, and means responsive to an interruption of said supply circuit for actuating said switching means to connect the grid of said amplifier to said resistance-rheostat combination whereby upon a subsequent completion of said supply circuit, the grid of said vacuum tube is biased in accordance with the output current of said rectifier.

5. In a regulator system, a direct current load circuit, an alternating current supply circuit, means comprising a grid controlled rectifier connected to supply rectified current to said load circuit, a battery connected across the output terminals of said rectifier and adapted to be charged by current from said rectifier, means comprising a phase shifting bridge circuit for varying the potential impressed on the grid of said rectifier, a vacuum tube included in said phase shifting circuit and having a grid whose bias determines the extent to which said phase shifting circuit functions to vary the potential impressed on the mid of said rectifier, a potentiometer connected across said battery and subject to variations in the voltage thereof, a resistance-rheostat combination included in the output circuit of said rectifier and subject to the output current of said rectifier, an amplifier having its plate circuit connected to the grid of said vacuum tube, switching means connecting the grid of said amplifier with said potentiometer whereby the grid of said vacuum tube is biased in accordance with the voltage of said battery and the charging current to said battery varied accordingly, means responsive to an interruption of said supply circuit for actuating said switching means to connect the rid of said amplifier to said resistance-rheostat combination whereby, upon a subsequent completion of said supply circuit, the grid of said vacuum tube is biased in accordance with the output current of said rectifier and said battery charged at a constant rate, and means effective upon a predetermined increase in the battery voltage for releasing said switching means and reconnecting the grid of said amplifier with said potentiometer whereupon the charging current to said battery is again regulated in accordance with the voltage of said battery.

6. In a regulator system, a direct current circuit, an alternating current supply circuit, means comprising a three-element space discharge rectifier tube connected to supply rectified current to said direct current circuit, means for regulating the output current of said rectifier in accordance with the voltage of said direct current circuit, and means responsive to the failure of said altematlng current source for causing the rectifier output to be regulated independent of the voltage of said direct current circuit upon reestablishment oi the alternating current supply.

7. In a regulator system, a direct current load circuit, an alternating current supply circuit, means comprising a grid controlled rectifier connected to supply rectified current to said load circuit, means comprising a phase shifting bridge circuit for varying the potential impressed on the grid of said rectifier, a resistance arm in said phase shifting circuit including a vacuum tube having a grid whose bias determines the extent to which the phase shifting circuit functions to vary the potential impressed on the grid of said rectifier, a potentiometer connected across said load circuit which is subject to voltage variations experienced by the load circuit and means including an amplifier for amplifying the voltage variations of said potentiometer and impressing corresponding biasing potentials on the grid of said vacuum tube whereby the potentials impressed upon the grid of said rectifier and therefore the output 01' said rectifier are varied in accordance with the voltage of said load circuit.

' 8. In a regulator system, a direct current load circuit, an alternating current supply circuit, means comprising a grid controlled rectifier connected to supply rectified current to said load circuit, means for regulating the output of said rectifier in accordance with voltage variations of said load circuit, said means including a phase shifting bridge circuit having the plate-cathode resistance of a vacuum tube included in one arm thereof, a potentiometer connected across said load circuit which is subject to voltage variations experienced by the load circuit, a screen grid amplifier tube for amplifying the voltage variations of said potentiometer and impressing corresponding potentials on the grid of said vacuum tube and a connection from the screen of said amplifier tube to a point on said potentiometer whereby said screen is supplied with a substantially constant grid voltage.

DAVID E. TRUCKSESS.

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