DIY ATX Power Supply Load Tester Design for 400W-2000W With Adjustable Steps

I'm trying to build my own load tester for ATX power supply testing. I'd like to be able to plug all the standard ATX connectors into said load tester and be able to set loads ranging from 400W-2000W, with the ability to increase the load in steps of say 200W, 100W and 50W and 10W intervals. I want to be able to hook my TSD1002B scope to each voltage output (3.3V, 5V and 12V) to measure ripple at various power levels as well as measure actual voltage outputs at various voltage levels.

I've found a ton of stuff around the web for building dummy loads and the like but most I find are 50W or so. I was thinking maybe coils from an old hair dryer with some heavy duty relays to control everything. What are you guys' thoughts on building something like this?

Well, first, do you know the formulas:

P = IE where 'I' is current, and 'E' is the voltage

and

E = IR where, again, 'I' is current and 'R' is resistance.

With these two formulas, and a little algebra, you can come up with any formula needed for calculating the needed resistance for a particular load condition, such as P = I²R and P = E²/R

The trouble with using something like hair drier coils is the resistance changes when they heat up. But since you will be using them at such a low voltage, perhaps it would work. Just measure the cold resistance and use the appropriate formula, and then test them under power to make sure the resistance didn't change.

You might consider 12V lamps, but it may be tricky to get precise power values (in case that's important).

So, I would go with power resistors. You can get them fairly cheaply at All Electronics, but you'll probably need a lot of them. Use series/parallel combinations to get the needed Power denominations. You might be able to reduce the number of resistors by coming up with clever switching arrangements (i.e. use switches [relays/transistors/even diodes] to rearrange the resistors). If you use Power FETs for the switches (much cheaper alternative to relays) you can probably wire it all up to a single rotary switch, then have one array of resistors for each power supply line with FETs to select the power denomination (by switching resistors in/out in parallel/serial).

OR, you could get some Nichrom wire and make your own resistors --perhaps add taps to the wires for different power settings. Make sure you observe TempCo (i.e. don't allow the wires to get so hot they change resistance) You can control that with wire diameter, and also, if wound, by not winding the windings too close to each other (and perhaps the form that you wind them on (probably ceramic) can somehow serve as a heat sink [but I'm not versed in such things]).

That's just off the top of my head.

* www.allelectronics.com (good for power resistors, not good for Power FETs -- try DigiKey or Mouser for those).

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