ATX bench power supply "dummy load" - can it be a fan?


I'm reading up all the various youtube videos, websites, etc on converting a surplus PC ATX power supply to be a benchtop supply, and all of them note the ATX requirement for a dummy load between 5V and ground or the PSU will turn off. I have a couple of pragmatic questions about this - typically they're looking for 5-10 ohms in a 10Watt resistor as the load. This seems rather a low resistance value, which in my limited understanding means surely that the design would waste a lot of power down this resistor to ground? Based on Ohm's law I see 1A wasted, or 5W of the PSU's output being turned to heat at the 5R value on the dummy load.

I was wondering if rather than wasting this power I could put it to good use - by replacing the PSU fan with a 5V one, and having that be the dummy load. Again in my limited understanding that would be putting the dummy load to good use, as the PSU has a fan anyway. Looking at the spec of one 5V 12cm cooling fan I found online, I see it draws 0.23A at 5V so through Ohm's law I arrive at 21.7R as a load. That's twice any dummy load I've seen in these designs so far, which means it will draw less current, but I'm wondering if that's a bad thing? If it would work that's only 1.15W and rather than unused heat, it would be saving me whatever the power consumption of the standard fan is.

Since I've not seen this done my initial thought is that's because it won't work. Before I invest in bits thought I'd ask the brains trust here for your advice, this idea workable or what fundamental am I missing?

Thanks ! Geoff

The circuit board is the load, all those ICs, really busy.
Big Amps, that’s what makes using a big switcher worthwhile, not milliamps.

You need a dummy load to test one, but otherwise, right, it’s a waste. You don’t want to use a board populated with expensive semiconductors as a test load when some resistance will do (lamps!).
It’s usually the 5V that requires the minimum load, it varies, but that’s around 10% of max rated.

A 12V car bulb would probably do - something like a 21W brake/indicator bulb.

Is a motor the wrong kind of load? Could I make up that by using an equivalent wattage in fans on the enclosure for example?

There's nothing "not right" using fans. They're more expensive (and noisier) than resistors or lamps, but if you need the breeze - go for it.


You got the idea right and the answer is YES. You can use a fan.

Anything that drives by the power supply is a load. It is entirely up to you to use anything to increase or decrease the load.

Thanks, BTFDev