ci-05_sept07

CIRCUIT

IDEAS

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Fig. 1: Circuit of desktop power supply

Fig. 2: Pinconfiguration ofLM317

Fig. 3: Suggested powersupply box

� T.K. HAREENDRAN

DESKTOP POWER SUPPLYS.C.

DWIVEDI

U seful for electronics hobbyists,this linear workbench powersupply converts a high input

voltage (12V) from the SMPS of a PCinto low output voltage (1.25 to 9volts). An adjustable three-pin voltageregulator chip LM317T (IC1) is usedhere to provide the required voltages.The LM317T regulator, in TO-220 pack,can handle current of up to 1 amp inpractice.

Fig. 1 shows the circuit of the desk-

top power supply.Regulator IC LM317Tis arranged in its stan-dard application. Di-ode D1 guards againstpolarity reversal andcapacitor C1 is an ad-ditional buffer. Thegreen LED (LED1) in-dicates the status of thepower input. Diode D2prevents the output

voltage from rising above the inputvoltage when a capacitive or induc-

tive load isconnected atthe output.Similarly, ca-pacitor C3 sup-presses any re-sidual ripple.

Connect astandard digi-tal voltmeter inparallel with the output leads to accu-rately set the desired voltage with thehelp of variable resistor VR1. You can

also use your digital multimeterif the digital voltmeter is notavailable. Switch on S1 and setthe required voltage throughpreset VR1 and read it on thedigital voltmeter. Now thepower supply is ready for use.

The circuit can be wired ona common PCB. Refer Fig. 2 forpin configuration of LM317 be-fore soldering it on the PCB. Af-ter fabrication, enclose the cir-cuit in a metallic cover asshown in Fig. 3. Then open thecabinet of your PC and connectthe input line of the gadget to afree (hanging) four-pin drivepower connector of the SMPScarefully. �