dc power circuits - nasa · but to the electronics hobbyist as well. additional technical...

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NASA SP-5942(01)

TEelNOLDGY UTILllATlOiN

CASE FILCOPY

DC POWER CIRCUITS

A COMPILATION

AERGNAUBGS AND-SPAGE ADMINISTRATION

https://ntrs.nasa.gov/search.jsp?R=19720011635 2018-07-05T03:47:41+00:00Z

ForewordThe National Aeronautics and Space Administration and the Atomic Energy

Commission have established a Technology Uti l izat ion Program for the dis-semination of information on technological developments which have potentialutility outside the aerospace community. By encouraging multiple applicationof the results of research and development, NASA and AEC earn for the publican increased return on the investment in aerospace research and developmentprograms.

The items in this first of an updated series of compilations dealing withelectronic circuits represents a carefully selected collection of dc power circuits.Many of them are based on well-known solid-state concepts that have beensimplified or refined to meet NASA's demanding requirements for reliability,simplicity, failure resistance, and resistance to environmental extremes. Theitems contained in the sections dealing with power supplies, power conversionand power regulation should be of interest not only to the professional engineerbut to the electronics hobbyist as well.

Additional technical information on individual devices and techniques can berequested by circling the appropriate number on the Reader Service Card in-cluded in this compilation.

Unless otherwise staled, NASA contemplates no patent action on the tech-nology described.

We appreciate comment by readers and welcome hearing about the relevanceand uti l i ty of the in format ion in this compilation.

Technology Utilization OfficeNational Aeronautics and Space Administration

NOTICE • This document was prepared under the sponsorship of the National Aeronautics and SpaceAdministration. Neither the United States Government nor uny person acting on behalf of the UnitedSlates Government assumes any l iabi l i ty result ing from the use of the informat ion contained in th isdocument, or warrants that such use will be free from privately owned rights.

Kor sale by the Nat ional Technical In fo rma t ion Service. Springfield. Vi rg in ia 22151. 51-00

ContentsSECTION!. DC Power Supplies Page

Small, Efficient Power Supply for Xenon Lamps 1Dual-Voltage Power Supply with Increased Efficiency 2Precision Full-Wave Rectifier 2Variable Voltage Supply Uses Zener Diode Reference •. . 3Regulated Power Supply for Varying Loads 4Power Supply with Overload Protection

and Load Isolation 4Stable Power Supply for CC>2 Lasers 5Dual Polarity Power Supply 6Zener Diode Starter for Transistor-Regulated

Power Supply 6

SECTION 2. Power ConvertersHigh Efficiency DC-to-DC Converter 7Efficient, Transformerless, Positive

DC-to-DC Converter 8Transient Suppression in DC Inverters gEfficient DC-to-DC Converter Eliminates Large, Stray

Magnetic Fields 9Eificient, Reliable DC-to-DC Inverter .10Converter Provides Constant Electrical Power

at Various Output Voltages 11Low Input-Voltage DC-to-AC Converter 11

SECTION 3. Current-Voltage Power Supply RegulatorsCircuit Regulates Vidicon Beam Current 12Current-Limited DC Series Regulator .13Regulator Circuit Provides Input-Output Isolation

for High Voltage 14MOSFET Improves Performance of Power

Supply Regulator 15High-Efficiency Step-Up Regulator 15Current-Limiting Voltage Regulator 16

SECTION 4. Overload Protection CircuitsElectronic Warning System for Mechanical StructuresTest Overloads 17

Two-Level Voltage-Limiter Protects Batteriesfrom Overcharging 18

Overvoltage Trip-Out for Variable DC Power Supply 19Under-Voltage Circuit 19Circuit Protects Regulated Power Supply

Against Overload Current 20Improved Limiter for Turn-On Current Transient • . . .20

SECTION 5. DC Constant-Current Power Supplies PageTemperature-Compensated, Constant Current Supply 21Magnetically Coupled Emission Regulator 21Operational Current Source 22Constant-Current Source Converts Temperatureto Analog Voltage 23

in

Section 1. DC Power Supplies

SMALL, EFFICIENT POWER SUPPLY FOR XENON LAMPS

The,unusual power demands of xenon lamps—constant voltage of T5 V when "on," supplyvoltage of 40 V when "off," and ionizing sourceof 25 kV pulse—can be met with a new powersupply, which is smaller and more efficientthan previous models.

Anode

To Lamp

Cathode

High Starter-Inhibiting Voltage

Figure 1. Schematic Diagram of Current Regulator

The supply has four sections: a preregulator,a dc-to-dc converter, a current regulator (see Fig.1), and a high-voltage starter (see Fig. 2). Al-though the individual sections are basically con-ventional, each has specific characteristics that aresomewhat unique.

The preregulator requires fewer .componentsthan other models and can operate at frequenciesbetween 100 and 300 kHz with an efficiency of90%. At high frequencies, smaller transformers andfilter-capacitors can be used.

In the dc-to-dc converter, voltage feedback andcurrent feedback are both used. The currentfeedback is applied from a separate transformerin series with the voltage-feedback transformer.The converter is free running with either feed-back, but voltage feedback predominates at highloads and current feedback predominates at lowloads. The uni t provides 40 V for lamp opera-tion and -5 V for control of the current reg-ulator, which is a ripple-type device.

Another lead from the transformer's —5 Vwinding provides a 10 V square wave to drivethe high voltage starter. Adding secondary wind-ings to the transformer can produce differentvoltages for different purposes.

The high-voltage lamp starter has a balancedvoltage-doubling circuit driven by a 10 V peak-to-peak square wave supplied by the converter.The wave is transformed at 160 V peak-to-peak

From dc-to-dcConverter

High-VoltagePulse Transformer

V

¥rTo StarterElectrode

• . Current LimiterFigure 2. Schematic Djagram of Starter Circuit

before the voltage is doubled. The resulting 160 Vsignal is stored across the voltage-doublingcapacitors until the relaxation oscillator fires asilicon-controlled rectifier. The rectifier dis-charges the capacitor's energy through the primaryof a high-voltage pulse transformer. The oscil-lator allows the voltage-doubling capacitors tocharge and discharge until the xenon lamp lights.The starter-inhibiting circuit then provides a posi-tive voltage to the gate of an N-channel fieldeffect transistor. The transistor turns on andshorts the ' t iming capacitor in the oscillator sothat additional f ir ing of the rectifier is pre-vented.

Source: J. E. Goodwin ofMartin-Marietta Corp.

under contract toManned Spacecraft Center

' (MSC-13637)

Circle I on Reader Service Card.

DC POWER CIRCUITS

DUAL-VOLTAGE POWER SUPPLY WITH INCREASED EFFICIENCY

This circuit can be used in lieu of relativelycomplex and expensive voltage regulators tosupply dual voltages wherever precise voltageregulation is not required.

Power Transformer

\

AC Source

O —

Current Windings

D1

-H

P2

-H-

The primary winding of the power transformer(see fig.) is connected to the ac source, and thesecondary winding is connected to the full-waverectifier consisting of diodes Dl and D2. Theunfiltered output from the full-wave rectifier is fed,in parallel, to a conventional choke-input filterbranch and a diode-capacitor branch. The diode,D3, in this branch conducts on the peaksof the ' ful l -wave rectifier current and charges-

capacitor Cl to the peak voltage across one-halfof the secondary winding of the power trans-former. The voltage at terminal A is approxi-mately 40% greater than at terminal B. The

-O A

C2

HI—In-D3

-M- rHh

-OB

I"required peak inverse-voltage rating of diode D3is only one-half the peak voltage across the fullsecondary winding. For maximum voltage outputat terminal A, a high-conductance semiconductordiode is used in the branch.

Source: Lewis Research Center'(LEW-90107)

No further documentation is available.

PRECISION FULL-WAVE RECTIFIER

Conventional full-wave rectifier circuits whichuse operational amplifiers require at least twooperational amplifiers and six precision resistors.

This circuit uses only one operational amplifier(acting as a linear amplifier and a switch driver)and two precision resistors. The operationalamplifier is operated open loop for switching andclosed loop for linear gain, both simultaneously.

Precision resistors Rl and R2 (see fig.) are ofequal value, and function as the conventionalinput-feedback loop for the operational amplifier;the other resistors can be ±20% carbon types.When the input voltage is positive, the voltageat point B will be equal in magnitude but•f rad (180°) out of phase with the input. Thisrequires the output of the operational amplifierat point A to be more negative by an amountequal to the sum of the forward and reverse

potentials of the zener diodes D3 and D4;this value will be on the order of 6 V for diodeswhich have zener breakdown potentials of about

DC POWER SUPPLIES

5.5 V. Thus, even though the input voltage maybe a very small positive value, the amplifiergenerates at least -5.5 V at point A, and thispotential is more than sufficient to turn offQ2 via the forward biased diode D2. Dl preventstransfer of the voltage at point A to the gateof the FET switch Ql, and since the gate andsource of Ql are at the same potential (thecondition for minimum source-to-drain channelresistance), Ql is on and the input is coupleddirectly to the output.

When the input voltage is negative, Q2 isturned on and Ql is turned off. Hence, full-waverectification (positive output voltage) is achievedby coupling the input to the output terminalin one mode of operation via Ql, and by havingthe amplifier output, which is inverted, appearat the circuit output via Q2 in the other mode.

Since the FET switches in the signal paths havevery low values of resistance and no offsets,the circuit provides linear full-wave rectification.Frequencies up to a few kHz can be handledby the components; higher frequency operation ispossible by replacing the FET switches withMOSFET devices of superior high-speed switchingcharacteristics and using high-bandwidth opera-tion amplifiers.

If Ql and Q2 are interchanged and Dl and D2are each reversed, the polarity of the outputterminal will be negative. The FET switches mini-mize temperature-sensitive offsets.

Source: G. J. Deboo and R. C. HedlundAmes Research Center

(ARC-10101)

Circle 2 on Reader Service Card.

VARIABLE VOLTAGE SUPPLY USES ZENER DIODE REFERENCE

This circuit uses a zener diode as a referenceelement to provide a stable variable referencevoltage. The circuit may have application in lowvoltage power supplies.

Zener diode Dl (see fig.) is used as the ref-erence element. Voltage control is provided by atwo-stage amplifier, consisting of transistors Qland Q2. The output voltage can be varied byselecting a value for R2, such that Voul = Vz

(1 + R1/R2), where Vz is the breakdown volt-age of zener diode D1.

Zener diode D3 is used to start the circuit.The voltage drop across diode D2 must be largerthan the collector-emitter voltage of Q2. A posi-tive feedback loop between the transistors is in-corporated in the circuit; therefore, D3 is cut offwhen the circuit is ON.

Current flow through Ql and Q2 is controlledby the setting of Rl. Increasing the resistanceof Rl increases the current flow through thetransistors, which in turn causes an increasedcurrent flow through Dl and R3. The outputvoltage appears across the series connection ofDl and R3. Since the characteristics of Dl arefixed, an increased output voltage can be ob-

tained by increasing the voltage (1R drop)across R3. This is accomplished by adjusting Rl.The voltage rise at the emitter of Ql limitsthe positive feedback and prevents unrestricted in-crease of the output voltage.

Output

+VdcDiode D2 is included in the circuit to eliminate

voltage dependence upon the emitter-base voltageof Ql. The output voltage may be applied to anemitter-follower circuit to obtain higher operatingcurrents.

Source: R. C. Lavigne and L. L. KleinbergGoddard Space Flight Center

(GSC-262)


DC POWER CIRCUITS

REGULATED POWER SUPPLY FOR VARYING LOADS

While tungsten has many favorable propertieswhen used as a heating element, it has a size-able resistance change in going from low temper-ature to high temperature operation. If a constantvoltage source is used to provide the power,high peak power often requires an additionalpower source, such as a storage battery. A con-stant power regulator (shown in the blockdiagram) delivers constant power to a varyingresistance load such as a tungsten filament, thuseliminating the peak-power requirements and theneed for a storage battery.

A novel pulse-width scheme uses magneticpulse-width control to simplify the control cir-cuitry, and a simple but effective regulator-con-verter combination supplies the amplifier powerand reference voltages. An integrated-circuit,differential amplifier is used to simplify the designand packaging, as well as to increase the re-

Inputo—

+ 28V'in m Switch

1

—^

MagneticPulseWidthControl

RegulatePowerSupply

id

PassFilter

-* Power -Sensor

Differential.Amplifier

liability. The constant power regulated supplycan maintain an output power within ±2% over amoderate temperature range (278 to 313 K.).

Source: P. L. WeitzelGoddard Space Flight Center

(XGS-10036)


POWER SUPPLY WITH OVERLOAD PROTECTION AND LOAD ISOLATION

Saturable-Core Transformer

, +28 VInput

(-) Output* + 28 V * InputOutput Ground

DC POWER SUPPLIES

The power supply shown delivers 100 mA at28 Vdc and provides voltage isolation and over-load protection for an extended length of tinie.It has the unique capability of recovering froma short circuit while operating at normal loadconditions. The circuit consists of a bistablemultivibrator which drives a saturable-core out-put transformer that has three secondaries. Thetransformer supports the load voltage and cur-

rent, yet maintains the capability to turn on aftera short-circuit, with a min imum of power and

: limited hysteresis.Source: J. R. Remich of

General Electric Companyunder contract to

NASA Headquarters. . . (HQN-10539)

Circle 5 on Reader Service. Card.

STABLE POWER SUPPLY FOR CO2 LASERS

The design criteria for the laser power supplyis based on the fact that CO2 lasers are in-sensitive to amplitude and frequency variationssuperimposed on a ripple voltage, if the ripplefrequency is relatively high. The high voltage

This power supply has the advantage that noballast resistor : is required to excite a stabledischarge. Therefore, the overall efficiency of thelaser is higher than that of conventional dcpower supplies. Further, only filter capacitors

power supply generates a dc voltage for the ex-citation of the gas discharge in the tubes, whichshow negative resistances. It has high efficiencyand permits operation of the laser with highamplitude and frequency stability.

Experimental measurements have shown that thenegative resistance is present only for frequen-cies below 1 kHz. The fluctuations decrease con-siderably with increasing frequency of the super-imposed voltage, probably due to the long lifetimeof the transition levels and partly due to the highac impedance of the discharge tube at fre-quencies above about 10 kHz.

The conventional design of power supplies forgas discharge tubes takes into account the factthat a stable circuit requires ballast resistors tocompensate for the negative differential resistanceof the discharge tube.

Laser

with small values are necessary, thus reducingthe weight and cost of the power supply. Thesupply circuit shown contains a source, a signalgenerator, and a 50 W amplifier. Since it isdifficult to construct transformers at the operat-ing frequencies between 70 kHz and 100 kHz fort h e - f u l l voltage (about 3 kV), a rectifier is usedto provide'a voltage multiplication factor of four.In place of a conventional ballast resistor, theinductance L is connected in series with the pri-mary' winding. The laser operates with 10 mAat3 .15kV.

Source: G. SchnifferGoddard Space Flight Center

(GSC-11222)


DC POWER CIRCUITS

DUAL POLARITY POWER SUPPLY

A majority of electronic systems use individualoscillator-transformer-rectifier power supplies,operating from 28 Vdc, to supply a positive anda negative voltage for the subassembly units.The dual polarity power supply provides a +14

+4 to+14Vdc

+28 Vdc

28 Vdc Source(Center Tapped)

—4 to—14 vdc

and -14 V to operate the various subassemblyelectronic modules directly, instead of using a 28Vdc supply with the negative terminal grounded.Other 28 V accessories, i.e., motors, relays and so-lenoid valves, can be operated on a 28 V input,with the return to -14 V. Using separate suppliesprovides a measure of redundancy and minimizeselectronic interference from closing relays andswitches.

The circuit performs the function of a powerdistribution network for the other modules, with-out the need of a transformer. Important ad-vantages of the uni t include significant reductionsin weight, size and costs, and internal powerdissipation.

Source: G. O. Bohot, P. E.Fincik, andA. L. Varneau of

North American Rockwell Corp.under contract to

Manned Spacecraft Center(MSC-17072)


ZENER DIODE STARTER FOR TRANSISTOR-REGULATEDPOWER SUPPLY

A starter circuit uses a zener diode in parallelwith a silicon transistor to supply the startingcurrent for a power supply which features highquality silicon transistors as variable impedanceregulators. Zener diodes, Dl and D2 (see fig.),of suitable voltage rating are connected inparallel with the silicon transistors, Ql and Q2,which are used as the variable impedance in eachleg of the regulation portion of the circuit. Thevoltages developed across the diodes provide aninitial current through the transistors which issufficient to turn them on." The silicon transis-tors, Ql and Q2, require an initial starting cur-rent because they have exceedingly small leakagecurrent.

The diodes are selected with zener voltageratings large enough to provide sufficient startingcurrent but small enough to be effectivelyopen-circuited when the transistor is conducting.

Input;RegulatedOutput

D12R2

Starter

Source: G. A. GilmourofWestinghouse Astronuclear Laboratory

under contract toSpace Nuclear Systems Office

(NUC-0015)


Section 2. Power Converters

HIGH EFFICIENCY DC-TO-DC CONVERTER

A new power supply is designed to providehigh efficiency for low output power levels andto provide good regulation with a m i n i m u m ofcircuitry.

Inherently, power supplies with low outputpower levels (less than 1 W) are highly inef-ficient. For a typical conventional design of 0.5 Woutput , the expected efficiency would be about40%. An efficiency of 70% can be obtained withthe new supply, without the use of precision re-sistors or capacitors. The design is applicable toany output power requirement, and at higheroutput power levels, the efficiency is greatlyincreased. As the output power increases, theinternal power loss increases, but at a muchslower rate, making the efficiency increase. Amaximum regulation of 0.2% is possible withoutadditional regulation circuitry.

In the simplified diagram, the input controlcircuitry consists of a voltage-controlled, free-running multivibrator which gates the transistoriswitch in the primary. The multivibrator com-pares a port ion-of the output voltage to a fixedreference voltage and generates pulses whosetime duration is dependent upon the differencein compared voltages. These pulses are appliedto the switching transistor which controls thecurrent through the primary windings of thetransformer. Through control of the pulse dura-tion, regulation of the output is obtained.

The input control circuitry and the transistorswitch also serve as the input dc-to-ac converterby chopping the input voltage. The transistorswitch is either turned completely "off" or "on"in hard saturation. Hence, the switch consumesvery little power.

The transformer uses a core which has alinear hysteresis curve. The primary and secondarywindings are phased so that, when primarycurrent flows, the secondary appears as an opencircuit because the rectifier, diodes are reversebiased. Only a small primary magnetizing currentflows and. power is stored in the core. Whenthe transistor switch is opened, primary current

flow stops and the stored energy is transferredto the secondary windings. Secondary current thenflows unti l all the stored energy is drained fromthe core. The process is repeated when the tran-sistor switch is again turned on. If energy trans-fer takes place during primary current flow, amuch larger primary current is required, and thepower losses in the core are greatly increased.

out

out

The voltage controlled mult ivibrator can havea constant frequency with variable duty cycle,or a constant pulse width with variable fre-quency. For equivalent circuit complexity, the con-stant frequency multivibrator offers better outputregulation. However, the load output variationshould be kept to a factor of about three toone from a nominal load value. This meansthat, for a nominal load resistance of 300 fi,the load variation should be kept to within 100to 900 Q. The constant pulse width multivibratorwith controlled frequency should be used forlarger load variations. Any degree of load varia-tion can be handled by this circuit.

Source: S. Gussow and H. Brey ofSperry Rand Corp.

under contract toMarshall Space Flight Center

(MFS-14392)


DC POWER CIRCUITS

EFFICIENT, TRANSFORMERLESS, POSITIVE DC-TO-DC CONVERTER

The converter provides a source of lower posi-tive voltage derived from a source of higherpositive voltage without the use of transformers.Both voltages are referenced to a common ground.The lower voltage can be 1/2, 1/3, 1/4, etc.,of the higher voltage.

S1

rtad

C11 ( i\\ '

+

1

D1i fcl ,

D3

rD2

+ C2i \(

Switches SI and S2 (transistors represented asswitches) (see fig.) are turned either fu l l on or ful loff at a frequency above 5 kHz. The switches

are synchronized so that SI is open when S2is closed, and vice versa. The complete operationis as follows: With SI closed, Cl and C2 chargeto the positive battery voltage through the pathprovided by SI and Dl. If C2 and Cl haveapproximately the same value, the battery voltagedivides equally across each'capacitor. -One-halfcycle later, SI opens and S2 closes. Both capac-itors then apply their stored voltage acrossthe load. Cl discharges through S2, the load,and D2. C2 discharges through D3, S2, and theload. Assuming ideal diodes and transistors, thevoltage applied to the load is 1/2 of the batteryvoltage. Since the charge time-constant of the filtercapacitor C2 is much less than the dischargetime constant, the load voltage remains duringthe 1/2 cycle when. Cl and C2 are charging.If less than 1/2 of the battery voltage is desired,more stages of capacitors and diodes may beadded.

Source: F. J. Nola and E. H. BerryMarshall Space Flight Center

(MFS-14301)


TRANSIENT SUPPRESSION IN DC INVERTERS

To keep the current transients from occurringin a dc inverter, a saturating magnetic core,LI, is connected to the bases of the powertransistors Ql and Q2 (see fig.). TransformersTl and LI are so designed that LI saturatesbefore Tl, turn ing off the conducting transistorand turning on the nonconducting one. If Tlwere to saturate in the inverter, current transientswould appear in the collector of Ql and Q2and additional circuit losses would occur.

To design Tl for this dc inverter, the follow-ing equation is applied:

v- v if)**N = V|n x 1U— n\4 F B m a x A

where N is the number of turns, Vjn is thevoltage applied across the turns, F is thedesired frequency of operation, Bma.\ is the

T1

NfSJ

| A A A.vw

L1 I

| VVV :

' •(/

n

if(7. vk

> k0o0o

-H+ I

I i1. O O o

Vjn E 2o

> 7•

Nf ^

Output

POWER CONVERTERS

saturated f lux density in gauss, and A is theeffective cross sectional area in square centime-ters. To keep Tl from saturating, 'equation (1)is modified as follows:

N = x 10'

2/3 (4 F B max A)(2)

Once the turns N are determined, the remain-ing windings of Tl can be calculated from thevoltage ratio requirements. A desirable voltagerange for the feedback windings is 2.5 to 3.5 V,allowing sufficient biasing of Ql and Q2 with-out excess power losses.

The advantages of this design are simplicityand increased reliability. The connection of LIto the bases of Ql and Q2 provides the neces-sary control for eliminating the undesirable currenttransients. If a precise inverter frequency is de-sired, the windings of LI can be modified tocompensate for inaccuracies of core material anddesign approximations.

Source: W. D. Steele ofSperry Rand Corp.


(MFS-21194)


EFFICIENT DC-TO-DC CONVERTER ELIMINATESLARGE, STRAY MAGNETIC FIELDS

A converter avoids the excessive waste of powerand the generation of stray magnetic fields byusing two concentric cores with the same number

SecondaryVoltage

O

Outer Core

Outer Core

Inner Core

Primary andSecondary Windings

Inner CoreV SecondaryWinding

of magnetizing turns (see fig.). The inner coresaturates before the outer core and provides thepositive feedback for switching. To assure that

the magnetizing current is the same for bothcores, the current in the feedback winding isminimized. A high input impedance amplifieris used to amplify the feedback signal. If Qlis "on," a positive voltage is applied from Bto A and a voltage is induced at C, keepingQl on. Since the inner core saturates before theouter core, the voltage at C collapses and Qlturns "off". The f lux in the inner core beginsto change in the other direction, turning Q2"on." Q2 remains "on" until the inner coresaturates in the opposite direction.

After the inner core saturates, the rate ofchange of magnetizing current does not increasegreatly because the f lux .in the outer unsaturatedcore can still increase substantially before the lat-ter core saturates. Since the rate of change ofmagnetizing current is not increased greatly, thetransistor turns off immediately after saturationof the inner core, and switching occurs.

Source: E. O. Turns ofEnrico Fermi Institute for Nuclear Studies,

The University of Chicagounder contract to

Goddard Space Flight Center(GSC-463)


10 DC POWER CIRCUITS

EFFICIENT, RELIABLE DC-TO-DC INVERTER

Certain types of primary electrical powersources have output voltages as low as one ortwo volts. A conventional practice is to step upthe output voltage with a dc-to-dc inverter thatemploys large-area, high-current transistors.

T2

mining loop-function interval. Other advantagesinclude: (1) Total circuit switching time ap-proaches the intrinsic transistor component riseand fall time; (2) overlap and switching invertercrossover ripple is minimized for a wide range of

r,

| uy.,

vfc

Frequency--, Determining3 Feedback

Loop

1•J

SR L

y"X"X"^U 9

r1— i

?ii

The long rise, decay, and storage times inherentin this type of transistor produce switching andcircuit overlap losses which degrade inverter ef-ficiency and reduce its operational lifetime. Thisproblem has been eliminated by a novel fre-quency-determining feedback loop added to astandard inverter circuit, as shown in the schematic.The feedback loop contains an inductor, L, inseries with a saturable reactor, SR; the propervalue of inductance in this loop permits theinverter power transistors Ql and Q2 to beswitched in a controlled and efficient manner.It is important to note that this inverter designis protected against Tl core saturation effectsas well as storage time overlap.

The dc-to-dc inverter can be used with atransformer current feedback inverter or with avoltage feedback-driven inverter, since the switch-over is controlled only by the frequency-deter-

Filter

LoadVoltageRegulator(OverloadProtected)

load demands and input voltages; and (3) properbase-drive current-shaping for optimum inverterperformance is provided during the entire cycle.

The new crossover technique can be used withboth high and low impedance sources. The basecurrent control and shaping, used with push-pulloscillator inverters, improve total inverter cross-over performance and eliminate most of thetransient voltage and current spiking conditionsnormally present at the source output. Outputripple conditions at the load are also reducedwithout the need for additional filters. Thus,minimal input/output ripple can be obtainedwithout incurring a size and weight penalty.

Source: E. R. PasciuttiGoddard Space Flight Center

(XGS-06226)


POWER CONVERTERS 11

CONVERTER PROVIDES CONSTANT ELECTRICAL POWERAT VARIOUS OUTPUT VOLTAGES

The power converter transfers electrical energyat a constant rate from a dc source to a numberof individual batteries which are to be chargedone at a time. For some applications, the powerconverter can invert the polarity of the source

the initial turn-on pulse to Q3. At the end ofthe time required to energize the inductor (L),the transformer saturates. Under this condition,Q3 is turned off and the stored energy in Lis passed to the output circuit which has pre-

and provide the correct charging voltage, whichmay range from 3 to 25 V. The circuit usesan inverted flyback switching circuit as a meansof transferring energy to the battery to becharged. Energy is stored in an inductor duringthe first part of the cycle and is released to thebattery during the remainder of the cycle. Thepower (product of the input voltage and cur-rent) corresponding to the peak power point ofthe voltage vs. current characteristic of the dcsource is transferred to the battery, and does notchange as a function of battery voltage.

The saturable core transformer (consisting ofthree windings on one core) and transistorQ3 comprise the flyback switching circuit (seefig.). The transformer determines the energy-storage period of the inductor L. Q2 supplies

viously been switched to the desired battery.During this charging period, the transformer coreis reset through the transformer winding as-sociated with resistor R2.

The operating level of the input voltage isdetermined by the input sensing circuit com-prised of Dl, Rl, Ql, and D2. These componentsare selected for operation at the peak-power pointon the voltage vs. current curve of the dcsource. Resistor Rl limits the maximum currentof Ql. A potentiometer may be substituted forRl for setting the circuit to compensate for anyvariation in the peak power point of the source.

Source: J. PaulkovichGoddard Space Flight Center

(GSC-519)


LOWER INPUT VOLTAGE DC-TO-AC CONVERTER

The dc-to-ac converter circuit shown is par-ticularly suited for operation with a very lowinput voltage. Reliability is improved by e l imina t -

ing current surges through the transistors. Effi-ciency is maintained with variations in inputvoltage and load current.


The operation of this circuit as a self-os-cillating current-feedback converter depends on thecombined effects of three nonlinear characteris-tics: (1) the current-voltage characteristic of therectifying emitter-base junction of the transistors;(2) the current-voltage characteristics betweenthe collector and emitter terminals of the transis-tors; and (3) the hysteresis loop of transformerT2. Transformer Tl serves merely as the output

+ Vdc Input

transformer for the inverter. The two transistorsin this circuit, Ql and Q2, are alternately turnedon and off to apply the input voltage acrossturns Nl and then across turns N2 of the outputtransformer Tl, inducing a square wave of alternat-ing voltage in the output turns of N3. Switchingof the transistors is caused by the cyclic satura-tion of the square-loop core of transformerT2.

Transformer T2, with its windings, is a satu-rable current transformer. With one ,'transistorconducting and T2 unsaturated, the load im-pedance limits the transistor collector current,and the base current is dictated by the turnsratio, N4/N5 = N7/N6, of the current trans-

former. This ratio is chosen in accordancewith the base-drive requirements of the transis-tors.

When transistor Ql is conducting with givenvalues of collector and base currents, its emitter-to-base voltage drop appears across N5 and es-tablishes the rate-of-change flux in the core oftransformer T2. Transformer T2 becomes sat-urated and N4 and N5 are decoupled. The basecurrent to transistor Ql then decreases and Qlturns off. Some energy is stored in the core oftransformer T2 after saturation. This energy is re-turned to the circuit and transistor Q2 is turnedon. The circuit is self-oscillating and is symmetrical,with events in alternate half cycles being com-plementary.

It is important to use closely matched transis-tors to avoid a loss in efficiency. TransformersTl and T2 are hand wound on saturable toroidcores. The oscillating frequency is given bythe relation:

f =4NAB

x 108

where Vbe is the base-to-emitter voltage, whichis a function of the load current, and whereN = N5 = N6, A = the cross sectional areaof core T2 in square centimeters, and Bm = sat-uration flux density of core T2 in gauss.

Source: E. T. Moore and T. G. WilsonGoddard Space Flight Center

(GSC-130)


Section 3. Current-Voltage Power Supply Regulators

CIRCUIT REGULATES VIDICON BEAM CURRENT

The circuit shown regulates the beam currentof a vidicon camera in a television system andmaintains the vidicon cathode at a constant po-tential.

The vidicon cathode potential is held constantby the reference diode D.I, in the base circuit

of Ql. D2 in series with Dl compensates forthe base-to-emitter voltage variations of Ql.

The voltage drop across the emitter resistors,R2 and R3, is proportional to the sum of thevidicon cathode current and the emitter currentof Ql. Since the beta of this transistor is

CURRENT-VOLTAGE POWER SUPPLY REGULATORS 13

greater then 100, the base current may be ne-glected in computing this voltage drop. In opera-tion, the circuit maintains Vbe of Ql constant.Any increase in the vidicon cathode current resultsn a decrease in The voltage drop across R2

and R3, tending to turn Ql "on." This lowersthe collector voltage and provides less base driveto Q2. When this occurs, the Q2 collector volt-age increases to the value of the supply. Thenegative output is applied to the grid Gl, andtends to cut off the vidicon to compensate forthe increased cathode current. D3 protects thebase-to-emitter junction of Q2 from reverse volt-ages, while D4 and D5 limit the maximum andminimum excursions of Gl. Cl reduces rippleand noise on the sensitive G1 line.

The technique described may be used whereverthe cathode current of a thermionic emissive de-

vice must be regulated and maintained at a con-stant voltage.

-o-HV

-r4.5 V

Source: Radio Corp. of Americaunder contract to

Goddard Space Flight Center(XGS-10023)


CURRENT-LIMITED DC SERIES REGULATOR

The current-limited dc series regulator shutsoff a series regulated power supply when a steadystate overload is present. The circuit (see fig.)also senses the removal of the overload condition,

back on automatically when the current hasbeen restored to a safe value.

If an overload condition is present for thelength of time required to charge C (through R2

ConventionalSeriesRegulator

_L R5 R60-15 Vdc

and automatically resumes normal operation. Theunique feature of this circuit is that it shuts offthe regulator circuit to avoid excessive powerdissipation in the regulator, and senses the loadon the output to allow the regulator to turn

and Ql) to a voltage level higher than El,the output of ampiifier Al is driven negative.Dl becomes reverse biased and the loss of thefeedback signal causes El to decrease, dependingon the nature of the overload. The negative output


of Al is coupled into the series regulator toreverse bias the transistors and cut off the reg-ulator. Since no output current flows with theregulator cut off, Ql also turns off. Al is heldin the .negative output state long enough forthe output voltage of the regulator to drop tozero after it cuts off. When the output goesto zero, El is reduced to a very low value,the biasing amplifier output goes negative and theregulator turns off.

Once the regulator is turned off, due to over-load, a small amount of bleeder current is pro-vided to the load through RIO and D2. D3 isused to keep the output voltage from feeding

back into the +15 V supply during normaloperation of the regulator. When the overloadcondition is removed, the current through RIOand D3 develops a small voltage cross the load.If the load is restored to a safe value, thevoltage developed across the load makes Elmore positive than E2, providing positive feed-back which turns on the regulator.

Source: D. J. Dixon and R. T. Mattle ofGeneral Electric Company

under contract toNASA Headquarters

(HQN-10342)


REGULATOR CIRCUIT PROVIDES INPUT-OUTPUT ISOLATIONFOR HIGH VOLTAGE

Complete isolation is provided between the in-put voltage and the control circuits of the regula-tor shown. This is particularly important in ap-plications where a common connection between

VdcoInput

tion between this voltage and the control systemvoltage was necessary.

The operation of the circuit is as follows: Theoutput voltage V between points A and B, or

«V0ut

I Voltage Regulator

the output and input of a system is not per- the current through R between points B and C,missable. The circuit has proven useful in a develops a voltage across Cl. This voltage isregulated high voltage power supply where the impressed across Rl and C2, causing C2 tovoltage to be measured was 2500 Vdc and isola- charge to the value of the voltage on Cl. When


the breakdown voltage of the four-layer deviceDl is reached, Dl conducts and allows C2 todischarge into the primary of the pulse trans-former Tl. The pulse appearing on the secondaryof Tl is rectified by D3 and filtered by C3and R2 to an average value. When the chargeon C3 exceeds the voltage of the referencediode D4 a current flows through R3 into the baseof Ql, causing Ql to conduct. The state ofconduction of Ql is used as the error signalbetween -points D and E that is fed back to

other circuits to provide accurate control of thevoltage from A to B (or the current from B toC). A change in the error signal changes thepulse frequency of Dl, which in turn increasesthe conduction of Ql.

Source: R. P..Putkovitch and L. E. Staley ofWestinghouse Electric Corp.

under contract toLewis Research Center

(LEW-90318)

No further documentation is a variable.

MOSFET IMPROVES PERFORMANCE OF POWER SUPPLY REGULATOR

The circuit shown provides a higher degree ofpower supply voltage regulation and temperaturecompensation than a conventional circuit using

^

D

VRR5

-VW

0.3

R3

a zener diode as a voltage reference. The im-provement is made possible by using a MOSFET,Q4, as the voltage reference in place of the zener

diode. As in the case of the conventional regula-tor, the improved regulator utilizes a bridge cir-cuit Rl, R2, Q4 (in place of a zener diode)and R3, and a difference amplifier consisting ofQl, Q2, and R4, with R5 allowing init ialoperation at power turn-on. The regulator per-formance is determined by the voltage differencebetween VI and V2 produced by a change inregulator supply VR. The difference amplifiergain and the current gain of transistor Q3 amplifythis voltage difference to determine the closedloop performance. Cross coupling of the gateof Q4 to the base of Ql allows Q4 to servealso as an additional amplifier.

Source: D. C. LokersonGoddard Space Flight Center

(GSC-10022)


HIGH-EFFICIENCY STEP-UP REGULATOR

An improved step-up regulator, developed for apower supply which operates from an inputof 28 t 4.2 V, provides a constant output of35 V from the variable input. The single-ended,step-up regulator-chopper power supply combinesthe advantages of chopper and switching regulatorcircuits to achieve a conversion efficiency of97%.

The step-up regulator, which converts the inputto a 35 V output, consists of Ql, Q2, R4 andC2. The 35 V can be held within ±0.25 Vwithout the use of filtering. The output fromthe step-up regulator is applied to the choppersection^ (Q3, Q4. R4 and R5) where the in-ductance of the transformer in the chopper tendsto eliminate the spikes from the regulator. Since


Input 0 rfflflfv.

(28 ± 4.2 V) L1

L2Input A /v.Return °— Ul&L/—^

L3

Q1

c

-po

~

D1

M

*TIr

:-

C2

HI-

>R2;

; y

•

'

4

•R1

, Do

I<<<

I>R3

R4

-"*€D4

R5 sy

U^

03

Q4

Winding#1

ooo

o•oooooo

oo

oo

o

Oop

Ij

T 1

; o Output 1

S Output 2o

JS Output 3

o Output 4

S Output 5

§ Output 6

-2.3Vthe chopper operates at 35 V, instead of thecustomary 18 V, the current in the primary ofTl is reduced to one-half, resulting in less heatdissipation and greater efficiency.

The voltage drop across the series regulatorsin the output circuits can be maintained at1.0 ± 0.003 V. Si\ isolated outputs are availableat the secondary of Tl.

Source: L. R. Lister ofSperry Rand Corp.


(MFS-20049)


CURRENT-LIMITING VOLTAGE REGULATOR

A voltage regulator operates within preset cur-rent limits and acts as a circuit breaker to pre-vent overload failure. Automatic reset occurswhen the overload is removed.

Normal voltage control is accomplished by thevoltage sampling circuit consisting of Q4, R4, R6,C2, R7, R5, and zener diode Dl. An increasein the output voltage V0 causes an increase inthe current through Q2, resulting in an outputvoltage drop to the level set by R4.

Transistor Q3 is the current-limiting controldevice. Current sampling resistor R12 controls thecurrent available through D4 to the base of Q3.

Zener diode D4 is forward biased as a conven-tional diode except under very light load condi-tions. Under normal operating conditions, Q3performs as an output-voltage regulating elementto supplement the action of Q4. The currentlimiting level is controlled by the choice of R9,and the combination of R9 and R3 sets theoperating level at the emitter of Q3 below current-limitirtg levels. Above the current-limiting level,the combination of R9, RIO, and R3 sets thevoltage at the emitter of Q3 such that, whenQl is shut off, excess power is dissipated in theparallel circuits of Rl, R3 and Q2, and RIO,


D6 and R3. The combination of R8 and D3 andthe elements D7, R l l , and D4 serve to providetemperature compensation.

An important feature of this circuit is theapproximately constant power dissipation in theseries transistor Ql, from normal load to short-circuit condition. In the conventional current-limiting approach, the power dissipation of Qlincreases from normal load to short-circuit condi-tion.

Source: E. F. Cleveland ofLockheed Electronics Co.


(MSC-11824)


Section 4. Overload Protection Circuits

ELECTRONIC WARNING SYSTEM FOR MECHANICAL STRUCTURESTEST OVERLOADS

Strain GageInputs

Visual WarningSystem

-12V

Mechanical components undergoing structuralintegrity tests can be damaged if overloaded bya driving force (hydraulic power supply). Withthe use of strain gages applied to critical load-carrying links and electrically connected to the

Relay

visual warning system shown in the schematic,the forcing operation can be removed beforedamage occurs.

The mechanical members are selected for criticalload-path transmission and are monitored with


strain gages connected to a recorder. The gageoutputs are individually buffered, amplified, andthen applied to Schmitt triggers for transmissioninto a lamp-driver circuit. When an overloadcondition occurs in one of the members, theappropriate lamp lights up indicating a pre-selected danger level. The lamp-lighting circuitsare controlled by relays which also apply

control signals to shut off the hydraulic powersource of the forcing function.

Source: P. G. Smith ofNorth American Rockwell Corp.


(MSC-11441)


TWO-LEVEL VOLTAGE-LIMITER PROTECTS BATTERIESFROM OVERCHARGING

The two-level voltage-limiter operates in con-junction with a constant-current charge sourceand a battery. Initially, all of the current from

level, additional circuitry within the limiter isactivated to produce a new (and lower) voltagelimit. This new voltage limit is chosen so that

J2

Lower VoltageStep Adj. 19v

the source flows into the battery. As the batterycharges and the voltage rises to a predeter-mined level of 19.6 V, the voltage limiter startsto divert a portion of the charging currentfrom the battery. At this point, a fur ther risein battery voltage results in the diversion of morecurrent, which results in less current to the bat-tery. When the battery current is reduced to low

all of the current from the source now flowsthrough the voltage-limiter and the battery cur-rent is reduced to zero.

Source: N. H. Potter and R. L. MorrisonGoddard Space Flight Center

(GSC-10696)


OVERLOAD PROTECTION CIRCUITS 19

OVERVOLTAGE TRIP-OUT FOR VARIABLE DC POWER SUPPLY

The overvoltage, trip-out delay for a variabledc power supply prevents the removal of inputpower during sudden decreases of the dc outputvoltage.

. . - I . i a m m m m m O i /'out

The voltage divider, Rl, R2, and Dl (see fig.),provides the proper biasing to transistor Ql.Should the output voltage suddenly be loweredby an external operation, the capacitor Cl willprevent instant turn on of Ql. Approximately10 msec elapse before Cl is ful ly charged,allowing Ql to turn on and actuate the over-voltage device. By this time, the output voltagehas decreased sufficiently to prevent the over-voltage sensor from being actuated.

Source: North American Rockwell Corp.under contract to

Marshall Space Flight Center(MFS-16948)


UNDER-VOLTAGE CIRCUIT

An under-voltage, cut-off circuit acts quicklyto shutdown a power supply when a suddenvoltage drop occurs. The power supply is so de-signed that removal of a frequency reference re-duces the output to zero. The under-voltagecut-off is essentially a complementary flip-flopin which both Q2 and Q3 (see fig.) are in theconducting state. Q2 is normally saturated and,in this state, provides the ground return forbuffer amplifier Ql. The circuit is supply-voltagesensitive so that, when the minus 12 Vdc out-put is reduced to a preset level, Q2 becomesunsaturated. The flip-flop then drives itself intoa non-conducting condition whereby both Q2and Q3 are turned off . The action occurs in afew microseconds once the cut-off level is reached.The ground return for Ql is thus opened, andthe frequency reference input to the power supplyis removed, thereby shutting off all outputs andremoving the dc supply from the under-voltagecut-off circuit.

To restore operation, the return path for Ql isreestablished through a control relay. CapacitorCl makes the circuit immune to bus voltagetransients or momentary losses of the frequencyreference by providing a sufficient time constant

to sustain Q2 in the saturated state for approx-imately 0.1 sec.

dc Input01:

FrequencyReferencelnput-rC

LJK1\

AAA— \\ \ f — «•, l̂lr*: 3IE_' T1

)Q1

PowerSupply

<<

D3

<<

1

:D2 Q2(1 0

TelemetryCircuit

'( Exc'J Ou

0 +1°

' S

HHi

)D4

•>>

Q3 «

Cl:»

:itatitputVdcVdc

>

Source: T. Wood ofSperry Rand Corp.

under contract toGoddard Space Flight Center

(GSC-10139)



CIRCUIT PROTECTS REGULATED POWER SUPPLYAGAINST OVERLOAD CURRENT

A tunnel diode controls a series-regulator toprotect a low-voltage transistorized dc supplyfrom damage by excessive load currents. R4,Q3 and D3 (see fig.) form the series-regulator

OUnregulatedDC Voltage

Load

R1 LoadCurrent

driver stage, and Q2 is the regulator serieselement. In case of overload, the overload cir-cuit, composed of Rl, R2, R3, Dl, D2, and Ql,shunts the base current of Q2 through Ql,

thereby shutting off Q2 and limiting the faul tcurrent. The volt-ampere characteristics of Dl areused to provide the voltage threshold detectionand the voltage across Rl is used to detect themagnitude of the load current.

Typical changes of the threshold-detection cur-rent are ±10% over a range from 273 to 343 K.(0° to +70° C). Any change with temperaturein the base-emitter voltage threshold of Ql iscompensated for by a similar change in thethreshold voltage of D2. This circuit providessharp detection of overload currents at verylow voltage levels and limits the short circuitcurrent to less than 10% above the detectorthreshold current.

Source: H. B. Airth ofWestinghouse Electric Corp.

under contract toGoddard Space Flight Center

(GSC-453)


IMPROVED LIMITER FOR TURN-ON CURRENT TRANSIENT

This circuit limits the turn-on current tran-sient to a specified amplitude and provides a low-impedance path between supply voltage and load

The new circuit (see fig.) automatically con-trols the initial peak current that can flow intoa high-capacity load when voltage is applied.

after a prescribed time interval. The circuit offersa wide range of flexibility in adjusting the peakcurrent and the time to complete the connec-tion to the load. It is more compact andlighter than a comparable limiter circuit whichuses a large choke.

..'RfLoad)

Peak-current limiting is controlled by the valuesof resistors R4, R5, and R6, and the time forcompletion of the cycle is determined by the timeconstants R1C1, R2C2, and R3C3. The smallchoke, L, offers a high impedance to smallstray-capacity charging currents and short-

OVERLOAD PROTECTION CIRCUITS 21

duration voltage changes. When power is removed,the sequence of steps is automatically readyfor the next power turn-on.

In one modification of the basic circuit, a relay(switched on by SCR3) is used to eliminate theunnecessary gate current.

Source: F. C. HallbergGoddard Space Flight Center

(GSC-10413)


.Section 5. DC Constant-Current Power Supplies

TEMPERATURE-COMPENSATED, CONSTANT CURRENT SUPPLY

The current supply is designed to maintaina constant output current for temperature changesas great as 413 K (140° C). Unlike conven-tional power sources, which have a nonlinearchange in current as a function of temperature,this circuit has a current which increases (ordecreases) linearly as the temperature changes.

The temperature coefficient of the currentsource can be determined by analyzing the equiva-lent circuit schematic shown in the figure.A constant current is maintained through re-sistor Rl if the voltage across it is constant. Thisvoltage is determined by amplifier A and theresistor ratio R2/R3. The relative gain-error ofamplifiers A and B is fed back to the inputof amplifier A. To ensure a constant voltageacross Rl, it is only necessary to set the ratioR2/R3 to provide the correct gain error as-sociated with amplifiers A and B. Amplifier Bhas a high input impedance and maintains asmall but constant current drain through R.

The supply output current increases linearlywith increases in temperature if the ratio ofresistor R2/R3 is made less than one; con-

oVccEquivalent Circuit of Current Source

versely, the supply output current decreaseslinearly with increases in temperature if the re-sistor ratio is greater than one. The amount ofcurrent increase or decrease for changes in tem-perature is dependent on the relative ratio.

Source: Electronic Components Div.United Aircraft Corp.


(MFS-20653)


MAGNETICALLY COUPLED EMISSION REGULATOR

A new type of high performance emission reg-ulator provides a constant current for ion sources.A magnetic amplifier provides dc isolation betweeninput and output circuitry, with a significantreduction in circuit complexity. This innovationhas particular application in the design of elec-tronic power supplies for use in mass spectrom-

eters, vacuum ionization gages and similar in-strumentation.

High voltage isolation is achieved through theuse of magnetic coupling between the input andthe power handling circuits (see fig.). A feed-back regulator, consisting of an error sensingamplifier and zener reference, samples the ion


source bias current and provides deviation signalsto a magnetic amplifier pulse modulator. Thepulse modulator controls the dc to ac powerinverter, which in turn controls the emissioncurrent.

Drive voltages (20 V peak-to-peak at 5 kHz)are applied to the primary of the magneticamplifier T2, through blocking diodes Dl andD2. The secondaries bias the transistor powerswitches Ql and Q2 "on" for equal periods,determined by the pulse source. The length ofthe periods is determined by the amount ofcurrent flowing from the error amplifier, Q3and Q4, through the magnetic amplifier controlwinding. The input to the isolated error amplifieris derived from the voltage difference betweenthe zener diode D3 and the voltage across R3caused by the electron collector current. As afunction of this control current, the magneticamplifier goes into saturation at some point duringeither half-cycle period. The inrush of currentthrough the primary increases the voltage dropacross R2, reducing the primary voltage. Thesecondary voltage is thus reduced unt i l the ap-propriate power switch is turned off.

The phasing of the error correction loop issuch that an increase in current (appearing asa negative going voltage at the junction ofQ3-R3) tends to cause a decrease o f ' t he "on"periods of the power switches.

Power transfer efficiency has been measured at93% with a 2.5 A fi lament load. The regulator,as designed, can handle f i lament currents up to

3 A. The emission current can be regulated from10 MA to 100 MA with a total regulation errorof 0.1%.

MassSpectrometerIon Source

(15-25Vdc)

Vin/o

Vdc

Square Pulse SourceInputs -5 KHz20 V P-P

D2 \

H2<

Source: Consultants and Designersunder contract to

Goddard Space Flight Center(GSC-10056)


OPERATIONAL CURRENT SOURCE

CurrentInput R2

CurrentOutput

Through the use of a matched, dual bipolartransistor, the output of an operational amplif ieris converted to a current source suitable for

driving a low impedance such as a voltage-to-frequency converter.

The circuit (see fig.) provides an output currentproportional to the input current. RL and thefeedback resistor are selected for a given inpu tcurrent range, and Rl and R2 are selected fora given output-current range. The current flowingthrough Rl is equal to the current through R2since Ql and Q2 and Rl and R2 are matchedcomponents. Therefore, the collector current of Qlis equal to the collector current of Q2. Adjust-ing RL provides control over the current sen-

DC CONSTANT-CURRENT POWER SUPPLIES 23

sitivity of the amplifier. The amplifier could bean electrometer or a similar amplifying device.The active input is not at ground potentialin applications using an input current source.Typical output currents range from 1 to 1000>A,although lower input currents may be used ifthe amplifier is stable.

Source: R. W. O'Neill ofLockheed Electronics Co.


(MSC-13490)

No further documentation is available.

CONSTANT-CURRENT SOURCE CONVERTS TEMPERATURETO ANALOG VOLTAGE

Temperature sensor devices such as thermistors,diodes, and carbon resistors require an extremelyaccurate current supply in order to generate ananalog output voltage. Normally, a large powersupply is required as the energy source. A novel

QIAi

Constant Current Source

current source has been designed which iseconomical, stable and accurate. The circuit (seefig.) contains a matched differential amplifier,Ql, a temperature-compensated zener diode, Dl,and a field-effect transistor, Q2.

Excellent voltage regulation is maintainedacross Dl by the use of Q2 as a constant cur-rent source. Resistor Rl is selected so that Ic

compensates for the desired temperature range.For a resistance variation between 50 and

1500 ft, the accuracy obtained is 0.43%. Tem-perature changes over a range of 248 to 363 K(-25° to +90° C) create an error of less than0.08%.

Source: J. R. PadillaofCaltech/JPL

under contract toNASA Pasadena Office

(NPO-10733)


NASA-Langley, 1972

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