Is my power supply bad?
I just installed a new motherboard/cpu combo in a computer that was given to me. The documentaion for the board says that I should use a 24 pin ATX connector, but that a 20 pin will plug in. It states that the system could be unbootable if I only have 20 pins, due to insufficient voltage, and that the power supply should be rated at 300 watts or more. My power supply is rated at 300 watts, but it only has a 20 pin connector. The voltage at the power plugs for the hard drive, etc. read 4.96 volts. The plugs on my old pc are reading 5.05 volts. Is this difference enough to make the new board refuse to power up, or is something else wrong? By the way, the cpu fan starts running, but nothing else, my monitor is not even noticing that it's plugged in to the computer when powered up.
Answers:
Your power supply is not bad but isn't good enough for your current configuration. Always follow instructions in the motherboard's user's manual.
The four additional pins provide core voltage to your processor without which the processor will not turn on. This in turn will prevent the SMPS from giving a power good signal and your motherboard will not work. All the other components depend on the motherboard for their power supply.
So thats what is happening. So get a power supply with 24 pins.
Good Luck!
Other answers:
It may be that your Power Supply (PS) may not be up to spec for other reasons as well. A lot of newer boards also require and additional Auxillary connector (3.3V) to provide enough juice to run other functions on the mobo. So you don't only need the '20 Pin Molex ATX Power Connector' but also a '6 Pin AUX Connector' to provide an additional 3.3V to the board. The link below will take to some pictures of what I have just said.
I have a P4 3G Gigabyte motherboard that required the auxillary connection.
Basically your best option is to invest in a power supply with an 24-pin ATX connector. However, if you want to give it a shot, there are 20-pin to 24-pin converters available that might do the trick as well.
But it really does seem that your power supply, or motherboard, wasn't designed to work together. If the documentation recommends something, that's pretty much the way you have to go.
A lot depends upon the amount and type of equipment that you are trying to use with a 300 Watt power source. If you have 2 hard drives, 2 optical drives, and a floppy, you may be pushing that 300 Watter a bit too much. If you are looking toward the future, get a larger power source, so that when and if you add equipment to your computer you'll have an adequate power supply. Upgrading to a 24-pin is not a bad idea, there is an auxiliary 4-pin connector also that plugs into the mobo for the CPU power on AMD's. I have a 600 Watt XG Vortec, the mobo plug on it can be used as a 20 or 24-pin.
http://www.newegg.com/Product/Product.asp?Item=N82E16817167012