My bro bought a new mobo with a 24 pin power connector. His PSU only had a 20 pin and he hooked it up anyway and claims that it works. What do the extra 4 pins supply power to and can he damage anything by using it without the 24 pin PSU. Of course I advised him against this setup but he never listens to me.

Many more recent PSs have the capability to connect to a 24 or a 20 position connector on the mboard. The connector from the PS is often in two pieces - a 20 position piece, and 4 position piece that clips onto the end of the 20 position one if it is needed for a 24 position connector on a mboard. The two connectors are sourced from the same wiring bundle from the PS. Almost all PSs these days except for some for oddball server mboasrds are wired the standard ATX way. The wiring of a standard ATX connector is the same for the 20 position portion - the same whether you have a 20 or a 24 position connector on the mboard. The varying shapes of the plastic in the connectors that surround the pins and in the sockets are such that the 20 position connector can only go on one way and can't be accidently placed in the wrong position on a 24 position connector on the mboard. Similarly the varying shapes of the plastic in the connectors that surround the pins and in the sockets are such that the 4 position connector will only go onto the proper place in the 24 position socket, and it cannot fit onto the 4 position socket that some mboards have for extra power elsewhere on the mboard.

http://www.formfactors.org/develope...

Also, some mboards have a 24 position main socket but can use either a 20 or 24 position connector from the PS - that info is usually clearly stated in the mboard manual. If the mboard manual says you MUST use a 24 position main connector from the PS that is another matter. In that case a 20 position connector from a PS will not be enough to have the mboard work properly. You can get wiring adapters separately to adapt the 20 position male adaspter to the 24 position female socket.

The extra 4 pins are for the ATX2 standard, as far as I know it's for providing a 'clean' 12v supply for newer CPUs - hence lot of newer PSUs have two 12v rails. If the motherboard only has 20 pins it just means the extra 4-pin sectional plug is superfluous and you can happily ignore it. "I've always been mad, I know I've been mad, like the most of us..."

Thanks guys. I'm amazed it's OK to do. orbital, great link. I guess he'll only lose some fan monitoring and fan control.

"...as far as I know it's for providing a 'clean' 12v supply for newer CPUs " That's the other 4 position connector that goes to a socket elsewhere on the mboard - 2 12v, 2 gnd. The one for the 24 position socket has 12v, 3.3v, 5v, and gnd. "I guess he'll only lose some fan monitoring and fan control." If there was a 3 pinhole female connector from the old PS that went to a 3 pin PS fan header on the mboard, there was only two tiny wires connected to it - it is only for monitoring the rpm of one of the two fans in the power supply. It is optional for it to be connected to the mboard, the power to the fan is within the power supply and the fan will work in any case, and circuits within the PS control how fast the fan spins, which is often according to the temp inside the PS if the fan has a variable speed feature.