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I am working on converting an old ATX PSU to a DIY lab bench power supply. Everything is going well, but I can't seem to find out the correct dummy resistor load values.

The 3.3v rail has 22A and the 5v rail has 21A, and usually the dummy load should be on the rail with most power. However, I read that it's very rarely it's put on the 3.3v rail, so I will put it on the 5v rail.

But I need some help to find the correct value to put on the dummy load. And should the resistance be in parallel or series?

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    \$\begingroup\$ In series or parallel with what? Normally, you would connect the dummy load directly across the power supply output terminals. \$\endgroup\$ – Peter Bennett May 11 '17 at 16:55
  • \$\begingroup\$ In general, especially for slightly older power supplies the 5 V rail had a strict minimum load requirement and I would still start with adding load there. \$\endgroup\$ – winny May 11 '17 at 17:33
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The dummy load would typically be on a supply that did not have the minimum current load applied. Typically you would place a load on either/both the +5 or +12 V supplies to ensure minimum load.
Since in most supplies they use co-regulation for +5/+12 or +5/+3.3 you may only need a load on one of them. Here is a typical PC power supply cross loading graph.

enter image description here

Look at your power supply specs to find the minimum loads.
In the case above you could use a 25 Ohm load on 5 V, a 20 Ohm on 12 V and a 330 Ohm on the 3.3 V to meet all requirements.

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