Add Variable Voltage to Your ATX Based Bench Power Supply

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.

  • The hand drawn circuit is the same circuit drawn in a bit more pcb board friendly way using a LM317 regulator. There is one caveat of using +12V and -12V to achieve 24V as we will be doing: the +12V can typically supply lots of current - 6A minimum for a really small supply, often double or triple that. The -12V line, however, can often only drain a fraction of that. My supply is rated for .3A on the -12V line, for example. Before you add this module, you'll have to make sure that your -12V line is rated for 1.5A at a MINIMUM. If you're drawing for your project much less than the 1.5A max of the regulator, you may be fine, but you could very easily run into problems later.

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.
  • Voltage regulators are under constant improvements; for instance, LM338T has a 5A max current rating.
  • During use the regulator may get hot. Use a heatsink if necessary.
  • If you use the LM317 the pins are labeled differently on the data sheet pin-out diagram: Common = Adj, Vreg = Vout, Line Voltage = Vin. So the pins from left to right are; Common, Vreg, Line Voltage.
  • Make sure that the +12V binding post is well isolated from ground and from the -12V binding post.
  • Use sensible wire lengths if creating a permanent connection to the ATX supply.

Warnings

  • Use common sense. If you find the regulator gets hot use a heatsink.
  • 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.
  • Since any part on your circuit isn't really 0v(gnd) when you put it into your ATX case don't let any part touch anything, even the case.

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

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Sources and Citations

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