How to Add Variable Voltage to Your ATX Based Bench Power Supply

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If you’ve built a bench power supply using an old ATX computer power supply, you may feel limited by the choice of voltages + 3.3V, + 5V, and +/- 12V DC. Suppose you’re breadboarding a circuit that’s meant to be run off a 9V battery? This is how to build an add-on variable-voltage “module” for your power supply.

Steps

1. Gather the materials required and construct the circuit from the circuit diagram. It would be a good idea to get the datasheet for the regulator from the manufacturers website.
2. Get some banana leads and connect the +12V and -12V outputs from your modded ATX supply and connect it to the input of your variable module. Use a multimeter to measure the output voltage.
3. Once you have built the circuit test it carefully and measure the output voltage. You should be able to vary the voltage from about 1.5V up to 22V by turning the variable resistor. If you are using the LM317 the output current will be restricted to 1.5A, if using the LM338K it should be slightly higher check the datasheet for exact information.

Tips

• I used a small piece of veroboard to build this little addon. You could use matrix board or design a small PCB. It wouldn’t be difficult.
• Use sensible wire lengths if creating a permanent connection to the ATX supply.
• Make sure that the +12V binding post is well isolated from ground and from the -12V binding post.
• During use the regulator may get hot. Use a heatsink if necessary.

Warnings

• If you’ve already built an ATX Power Supply based bench/lab supply, you’ve already run the risks - this project is less dangerous. Soldering irons can burn you, hand tools can cut you; don’t drink and hack.
• Use common sense. If you find the regulator gets hot use a heatsink.

Things You’ll Need

• You will need the following materials:
• 1x LM317 or LM338K Voltage regulator (With heatsink and heatsink paste)
• 1x 100nF Capacitors (ceramic or tantalum)
• 1x 1uF Capacitors Electrolytic
• 1X 1N4001 or 1N4002 Power Diode
• 1X 120 Ohm resistor
• 1x 5k Ohm variable resistor
• Optional
  • Some Veroboard or a matrix board and some suitably thick wire for connecting the circuit.
  • A small enclosure about the size of a wall wart to put the circuit in.

• Indispensable
  • An ATX power Supply already modded to provide +12V, -12V, +5V and -5V

Related wikiHows

• How to Convert a Computer ATX Power Supply to a Lab Power Supply
• How to Create a 12 Volt DC Test Bench for Bullet Cameras Using an Old ATX Computer Power Supply

Sources and Citations

• LM317 Datasheet (PDF)
• LM338K Datasheet (PDF)

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