20 pin PSU to 24 pin
 

Hey, my weekend for PC repairs, I guess. My parents really old PC stopped working. I grabbed a cheap 400watt PSU and pulled the old one out (200w). The new one came with an adapter harness to allow the 24 pin connector to fit into the 20 pin plug on the MB. And the old PSU also has a tiny 3 pin connector, separate from the main one, that plugged into the MB. The new PSU did not have that connector, so I skipped it, plugged in the drives, and it works fine. Question, what is the small 3 pin plug for? And does the 24 pin harness do away with it?

The FCC only allows so much power to run through the main mobo power connector. Any other mobo power connectors are a way past this rule.

I would not skip it as it severely limits how much the mobo can pull.

That might be a connection to control fan speed on the cpu by the motherboard, if its small thin wire then it is almost certainly some form of communication connection (power is almost always 4 pins other then in a few exceptions which have more pins).

With out the motherboard's manual no way to be sure what the connection was.

4 pin can be the on/off button. Not sure though, it has been awhile since I seen some of the old stuff.

Ya though that hasn't been used since the old AT power supplies. The one Neal replaced was an ATX PSU if it was 20 pins, so it shouldn't have that (and it wouldn't be hooked up to the board anyhow)

I'm almost certain its a fan control/monitoring cable, they are about the only tiny 3 pin cable that you might find in a system. If it is and the new one doesn't have the connection then it doesn't matter, the fan in question is the psu's fan which it will handle on its own if there isn't a connector.

Ok, that's what I was thinking, the PC works fine, the CPU fan started right up when I powered up.