8/3/2019 7 Nov 1983 Programmable Power Supply

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programmablepower-supply

E. Stohr

digital/analoguepower supplyinterface

technical characteristics input voltage: 5 V DC (TTU;

12 V DC (CMOS) resolution: 8 bits output voltage: 0 ... 30 V pro-grammable in three ranges programmable ranges:o . 5 V (1 bit = 19.5 V)

0 13V(lbit= 50mV)O 30 V (1 bit = 117 mV)

switchable output current: 2 A;0.5 A; 50 mA

drive: via signals SELECT andENABLE (these can be obtainedby address decoding)

Many integrated circuits are now so familiar to most of us that we tend tooverlook some of their remarkable characteristics. Here we take a well-knownvoltage regulator, the 723, and hook up a digital/analogue converter to its inputand the result is that we can program the output voltage very precisely and canalso select the maximum output current (with a digital command, no less!).This circuit should interest anybody who wants to use the 'digital' accuracyof a microprocessor system to meet stringent analogue requirements.programmablepower supp lyWeare not dealing here with an ordinarydigital/analogue converter: its conversiontime of 5 JJ.ss, of course, pretty good butits high output current of 2 A is far fromordinary (see technical characteristics). Theprogrammable output voltage is dividedinto three digitally switched ranges as is thecase with the output current.

Circuit descriptionThe heart of the circuit is the 8-bit digital/-analogue converter, IC!. The output of thisIC, Eo, supplies a high impedance analoguesignal which is proportional to the valueof the binary word applied to its inputsBl ... B8. This binary word, supplied viathe data bus of the programming system,travels to latches IC5 and IC6 which arecontrolled by signals which we will talkabout later.The 'power part' of the circuit consists ofintegrated circuit voltage regulator IC3which compares (and corrects) the outputvoltage with the reference voltage suppliedby IC2. Darlington T9 ensures that theoutput current is usefully large: about2A. Resistors R18 R20 and presetpotentiometers P3 P5 adjust the maxi-mum output current and maximum outputvoltage.As we are dealing with a power supply, itwill not come as a surprise that the circuitcontains a bridge rectifier and a smoothingcapacitor for the supply of IC3 and T9 aswell as a second regulator which providesa stable reference voltage (Uref = 10.0 V)for IC!.

SwitchingBesides the two latches already mentioned(IC5, IC6), there is an identical second pair,IC7 and IC8 which is also tied to the databus. These latter latches control transistorsTl ... T8 which switch the various resistorsand potentiometers in the voltage andcurrent ranges. As T4 and T5 are connectedto relays Re4 and Re5 (which connect thecurrent-limiting resistors in parallel), andT1 ... T3 are connected to Re 1 ... Re3

(which switch the voltage ranges), there arethree unused outputs left which may beused for additional low power relays.The control signals for the two sets oflatches are binary. If the X (SELECT) andY (ENABLE) signals are both I, the outputof N1 is zero; latches IC5 and IC6 are then'transparent' and the converter is connecteddirectly to the data bus. If either the X orthe Y signal changes state, the latches blockand their outputs hold the last binary wordinput before they cut off.When X is 1 and Y is 0, the low level outputfrom N3makes latches IC7 and IC8 'trans-parent': the logic levels present on the databus are then transferred directly to thebases of switching transistors Tl ... T8.If neither of the situations outlined abovepertains, the circuit is completely isolatedfrom the system controlling it..Summarisinq, in the first situation men-tioned, the microprocessor controls theoutput voltage, while in the second situation,the voltage and current ranges are switched.

ConstructionDepending on the programming system used;the circuit described may have to be changedto meet individual requirements. The busconfiguration, the voltages corresponding tothe different logic levels and the addressdecoding needed to obtain signals X and Yare the elements which may have, to bechanged.Relays Re4 and Re5 must each be able tohandle the maximum output current;Re1 ... Re3 may be miniature DIL typesand can be mounted directly onto theprinted circuit board.The 5 V supply section for IC5 ... IC8 canalso serve as an interface between the D/Aconverter and the microprocessor system bus.Power transistor T9 must be mountedon a heat sink capable of dissipating up to60 watts, and this assembly should be wellventilated. The use of thermoconductivepaste (silicone grease) is advisable.AdjustmentA digital voltmeter and a digital command

8/3/2019 7 Nov 1983 Programmable Power Supply

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prog rammab Iepower-supplv

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N1 ... N3 = *IC9 = 4011 +ts 12 V0 +8, 58'

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Figure 1. The circuitdiagram of the program-mable power supply. Asshown here it is suitablefor CMOS input levelsbut can be made TTLcompatible by 'changingthe supply voltage toIC5 ... IC8 to 5V.

system (preferably a microprocessor) toprogram the voltage supply are required foradjusting the circuit.First, apply eight logic 0 levels to inputsBl ... B8 and a logic 1 to the SELECT (X)and ENABLE (Y) lines. Adjust PI such thatthe output voltage (Uref) from IC4 isexactly 10.000 V on the digital voltmeter.Next, set the ENABLE (Y) line to logic 0and data lines B4 and B8 to 1. Relays ReIand ReS should make ..Reset the ENABLE. line to 1 and all data lines Bl ... B8 to O .The output voltage (Uoutput} should bezero volts. If not, adjust P2 to compensatefor the offset.Set all lines Bl ... B8 to logic 1 and adjustUoutput with P3 until it is 5.000 V. Checkthe output current which should be about2A.Then, set B5 and B7 to logic 1 and all otherB lines and the ENABLE line to 0; relaysRe2 and Re4 should now make. Reset theENABLE and Bl ... B8 lines to 1 and

adjust Uoutput to 13.000 V with ~4; theoutput current should now be about500mA.Finally, set line B6 to logic 1, and all otherB lines to O . When a logic 0 is applied to theENABLE line, and this line and the B1 ... B8lines are immediately reset to 1, relay Re3makes and Re4 breaks. Adjust the outputvoltage to 30.000 V with P5; the outputcurrent should not exceed 50 mA.The programmable power supply is nowready for use. It will be very useful inapplications requiring great precision andflexibility. All that remains is to write thenecessary software to control this powerinterface: a small computer system such asthe Elektor Junior Computer is eminentlysuitable. If you havewritten any interestingprograms for using this interface and youthink others might benefit from them, weshould be pleased to hear from you. III

decodingX Y Function

D/A conversion0 range switching

'00 0

The signals for X and Yare obtained by using asuitable combination ofaddress lines of whatevercomputer is used.11-34