If the power supplies are vintage Dell, the "power on", wire is grey instead of green.
Also there is a 6 pin connector with blue & blue/white wires (+3.3 vdc). Note that the pins on these have two wires each on them. If you cut off that connector, make the wires are tied together as thy were on the connector. You may also need to place a 10 ohm 10 watt resistor from +5 vdc (red wire) to ground (black wire)

I think the later Dell supllies are standard ATX