# Need help for approriate dummy load on ATX bench power supply

I am planning on converting an ATX PSU into a lab bench power supply, and I've seen plenty of videos and read numerous articles about how easy it is. I understand all of it, except one thing that noone describes fairly well. And that is the dummy load.

The dummy load usually is put on the wire with most power. In my case, I have the following power output:
+3.3v = 22A
+5v = 21A
+12v1 = 10A
+12v2 = 15A
-12v = 0,3A

So in my case it should be the 3.3v wire the dummy should be connected to. However, is it possible to combine the two +12v rails and then get 25A? I also wonder what the correct resistance and watt for the dummy load should be.

Mm, as far as I know supplies rarely use the 3,3V as main regulation so my bed is on the 5V line.

And if they are two separated rails then no.

I will double check if the 12V rails are connected, but it seems like I won't need to solder them together and maybe have two 12V binding posts.

Yeah, there is no real rule about that. PC PSU are designed to have a decent load on all rails so manufacturers aren't that fuzzed about having a real minimum. I just whacked on what I got and that happened to be 5Ohm at the 5V rail and 20Ohm at the 12V rail.

Thanks! How many watts are your resistors? I see alot of people using 10W

Yeah I used 10W but only because that's what I had. And I even used 2 for each 5Ohm at 5V is 5W, 20Ohm @ 12V is 7,2W.

Thank you so much for your help septillion! You've been very helpful!

ronnyandre:
Thanks! How many watts are your resistors? I see alot of people using 10W

If you don't want to do the simple calculation, you can use the "Monty Python Castle in the Swamp" method:

"First I used a 1/4 watt. It turned into a puff of smoke faster than I could watch it.

Then I used a 1/2 W. It turned red, flamed and then turned into a puff of smoke...

Finally I used a 10W and it is still here."

xD

Karma++

I used a 12V car halogen headlamp bulb Works a treat!

I've found half an amp from the 5V line is about right. A 10 ohm resistor. That's 2.5W (5V x 0.5A), but 10W is a common size so that is what I use, rather than a 5W resistor.

As pointed out, it doesn't hurt to load the 12V line a bit, too.