Introduction: Variable Low Voltage Load Tester

I have designed, built, and repaired computers for a number of years and within that time, I have my share of misdiagnosed power supply problems.

Some of the trickiest power supply problems crop up only under certain circumstances, like being under a certain amount of load or a load over a duration of time or even a certain temperature. While running stress tests within the operating system can be helpful, the ability to test individual outputs with a variable load is most useful.

One problem with continuous load testing is that the dummy load dissipates lots of heat. This makes building the load tester in any enclosure that is not metal rather unsafe. Even with a metal enclosure, heat builds up fast so active cooling is necessary.

This leads us to the important question: what type of preexisting metal enclosure already comes with active cooling?

Step 1: Preparing a Case

The Answer: A dead computer power supply!

You can use any brand of ATX power supply though for internal packaging reasons, one with a small rear-mounted exhaust fan works perfectly. In my case, I was able to wrangle up a dead HP power supply that fit the bill.

Once you have selected your sacrificial power supply, remove everything inside except the fan. You can safely discard the power supply board but save the wire as you can use it for this project!

Step 2: Parts Needed

Now that you have a case, you will need some parts to create the dummy load! Fortunately, I was able to do this on the cheap by purchasing all of these components on Ebay. Since most of these came from China, shipping was around 2.5 weeks.

  1. 200 watt 5 Ohm Potentiometer / Rotary Rheostat
  2. Large rotary knob for rheostat
  3. (2) 1 Ohm, 100 watt resistors
  4. 0-15 Volt DC analogue meter (I went with 0-30 by accident)
  5. Shunted 0-20 Amp ammeter
  6. 20 Amp circuit breaker (protection for whole load tester)
  7. 15 Amp circuit breaker (protection for Rheostat)
  8. Beefy Dual Pole, Dual Throw (DPDT) switch with center off position
  9. A small piece of scrap metal
  10. Small machine and sheet metal screws
  11. Wire from gutted power supply
  12. A red and a black alligator clip (optional)

I included the schematic here just as a parts reference. I built mine with a 10 Ohm rheostat but after testing, I found that a 5 Ohm one was best.

Also, it may seem overly complicated to have the load tester switchable between a series and a parallel dummy load but I have found both modes helpful, especially for testing 3V and 5V sections of a power supply. The 2 100W resistors also protect the power supply being tested from experiencing a dead short if you crank the load up to max.

Step 3: Cut Up the Case!

While it is a tight fit, everything does fit in a standard ATX power supply case with some creativity.

You will need a dremel tool with a metal cutting wheel, a drill with metal bits, and a bi-metal hole saw. The circuit breakers and rheostat are easy to mount as they just require regular holes. The DPDT switch requires some dremeling if you get a rectangular model. The hole saw is for the ammeter and volt meter cutouts.

While cutting up the case, I removed the fan avoid damaging it.

You can use your scrap metal to make a small bracket for the resistors. I attached the resistors to the bracket with sheet metal screws and the bracket to the case with machine screws. It is good to keep the resistors suspended in the case as they get Very hot under load.

Step 4: Assemble, Solder, and Test

Now that you have the case cut up, you can put everything together. I had to put the volt meter on the side that has the rheostat as the rheostat partially invades the meter's space, thought the volt meter fits perfectly underneath (see previous pics). The ammeter has a big overhang from the shunt so it won't fit in the same spot.

Once you have all of the parts screwed and popped in, you can solder everything together. The rheostat uses crimp connectors so a hand crimper will come in handy but you can solder wires into the crimp connectors if needed.

I found it easiest to follow the schematic for the wiring process as there are lots of wires in a small space, making the process potentially confusing. Luckily, I caught my mistakes before testing and there was plenty of spare wire for my oopses. I tried to keep the positive wires a color and the negative wires black but this was just for ease of assembly and testing. The fan is not on the schematic but it connects directly to the tested power source via the test leads. This is so the fan will continue to run and cool down the tester even after the load is switched off.

One thing to note about the schematic is that I used a battery symbol in place of the power source being tested. You will want to put a 12" set of red and black wires in place of that "battery" as test leads. You can put the alligator clips on the ends or test probes if you feel inclined.

As long is everything is put together in the right order, you can try out your load tester. I find that it has a useful testing range from about 3-14.5 volts; below that and you won't be able to crank the amperage up enough and above that will cause parts to burn out or breakers to pop.

Good luck with this project and please let me know if you have any questions in the comments.