Variable Power Supp ly withDigital Control
The most frequently used device inelectronic workshops and laboratoriesis a universal power supply provides a variable, fluctuation-free output.Here we present a variable power supplywith digital control that is simple and easyto construct.The circuit is built around an adjustable3-terminal positive-voltage regulatorIC LM317, CMOS decade counter ICCD4017, timer IC NE555 and 3-terminalfixed negative-voltage regulator LM7912.The AC mains supply is stepped downby transformer X1 to deliver a secondaryoutput of 12V-0-12V AC, 1A. Theoutput of the transformer is rectified by afull-wave rectifier comprising diodesD1 through D4. Capacitors C1 throughC4 are connected in parallel to rectifierrespectively, as shown in the figure. LED3through LED11 are used here to indicatethe voltage levels. The collectors of transistorsT2 through T10 are connected to presetsVR1 through VR9, respectively, whichare used to set the output voltage.Adjustable voltage regulator IC LM317(IC4) develops 1.25V nominal referencevoltage (VREF) between its output and theadjustable terminal. The reference voltageappears across resistor R16. When the voltageis constant, a constant current flowsthrough one of the output-setting variableresistors (VRset, VR1 through VR9), givingan output voltage at pin 2 of IC4 asfollows:VOUT=1.25(1+VRset/R16).Presets VR1 through VR9 are adjustedto get the desired output voltage. The collectorof transistor T1 is directly connectedto ADJ terminal (pin 1) of IC4, so theoutput voltage of IC4 will be the voltageacross fixed resistor R16, which is equalto 1.25V. When switch S3 is pressed, pin3 of IC2 goes high and the output voltagebecomes 1.2V.When switch S2 is pressed, the outputof IC1 goes high. As a result, the outputs ofIC2 go high one by one as a ring counter.Since presets VR1 through VR9 are connectedat the collectors of transistors T2through T10, respectively, different outputresistances appear between the adjustableand ground terminals of IC4, resultingin different output voltages. By using aproperly calibrated digital multimeter youcan easily adjust the presets to obtain 1.5Vto 12V.A fixed, negative 12V DC can beobtained by using fixed, negative-voltdiodesto bypass undesired spikes andprovide smooth and fluctuation-freepower. Capacitors C5 and C13 are used asfilters to eliminate ripple. Here bothnegative and positive half cycles are usedto obtain positive as well as negativeDC output. LED1, along with current-limitingresistor R1, is used for mains ‘on’indication.Timer IC NE555 (IC1) is wired as anastable multivibrator. It generates clockpulses when switch S2 is pressed. Theoutput of IC1 is connected, via an RCnetwork, to the clock input of counter ICCD4017 (IC2).IC CD4017 is a decade ring counter.Each of its ten outputs goes high one byone when a clock pulse is received. Theoutputs of IC CD4017 are connected tothe bases of transistors T1 through T10,c i r c u i t i d e a selectronics for you April 2004age regulator IC LM7912 (IC3). Thus thepower supply unit can be used for circuitsrequiring both negative and positive DCvoltages.When CD4017 is reset by pressingswitch S3, the output voltage becomes1.2V and all the voltage-indication LEDsturn off.Assemble the circuit on any generalpurposePCB and enclose it in a suitablecabinet. Use suitable heat-sinks forregulators IC3 and IC4. Since pin configurationsof the regulators are different,never fix both regulators on the same heatsink.For S2 and S3, using microswitcheswill enhance the beauty of the unit. LED2is used to indicate the negative 12V DCvoltage.


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