How To Make a Dummy Load to Check Computer Power Supply Voltages

Written by norm dickinson Google
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How To Make a Dummy Load to Check Computer Power Supply Voltages
Power supply units always provide standard Molex hard drive power leads. (medusa colour image by robert mobley from

Making a dummy load to check computer power supply units for the proper voltage avoids burning out the power supply. Modern power supplies require a load when turned on or they can burn out and self-destruct when power is applied to them. They also require a way to trigger the power switch, which is typically controlled by a circuit on the motherboard. Learn how to create a simple tool that will allow safe testing and also provide a convenient way to turn the power supply on.

Skill level:

Things you need

  • Molex hard drive power cable extender or splitter adaptor
  • Wire cutter
  • Wire stripper
  • 10 watt, 10 ohm wire wound load resistor
  • Helping hands soldering aid
  • Electronic soldering iron with stand
  • 60/40 tin/lead solder
  • Electrical tape
  • Multimeter

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  1. 1

    Cut off the end of the Molex hard drive power cable extender or splitter adaptor that plugs into the hard drive, keeping the end that plugs into the power supply lead. Cut the extender or adaptor about an inch from the end.

  2. 2

    Cut the red wire off flush with the connector. Cut the yellow wire and one black wire back to about 3 inches long. Leave the other black wire about 6 inches long.

  3. 3

    Strip about a quarter inch from the ends of the two black wires and the yellow wire.

  4. 4

    Mount the 10 watt, 10 ohm wire wound load resistor into the helping hands soldering aid so that the leads are exposed and easy to work with.

  5. 5

    Solder the yellow wire to one end of the resistor and solder the short black wire to the other end of the resistor.

  6. 6

    Wrap the soldered connections with electrical tape.

  7. 7

    Plug the finished dummy load into a standard Molex hard drive connector on a power supply and then plug the power cord into the power supply. Touch the long black lead to the motherboard connector with the green wire leading to it in order to power on the power supply. Test the power supply using a multimeter set to DC voltage. Touch the multimeter's black probe to a black lead on any connector, and use the red lead on the multimeter to test the voltage of each other lead, including the yellow (12 volts), red (5 volts), orange (3.3 volts) and white (minus 5 volts) if present.

  8. 8

    Unplug the power from the power supply unit to end the testing.

Tips and warnings

  • Attach the resistor to a metal base to provide stability and to absorb the heat generated from extended testing. A discarded CPU heat sink works well for this purpose.
  • The load resistor can get hot after a few minutes, so this device is best used for quickly testing voltage. Solder in a well-ventilated area and avoid the fumes. Soldering irons can cause burns or fires.

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