Power Supply Too Big?
I replaced my working 350W power supply with a new 500W power supply. My computer would not boot with the new power supply. It would turn on for a few seconds and then turn off. I then tested the new 500W PS in a different computer and it runs just fine. I used a Kill-a-Watt meter and found that my computer is only using 87W of power.
Is it possible that the under-voltage or under-current protection in the PSU is causing it to shut off because my computer is not drawing enough power? I've also heard that it is very inefficient if you are drawing less than 20% of your PSU max power. In my case 87W out of 500W is only drawing 17% of max power.
Additional information:
This is a home-built computer with Gigabyte P55 based motherboard. The board uses the 24+4 pin ATX connector and a 8 pin connector for the CPU. It works just fine with the 350W which only has a 24 pin ATX connector and 4 pin CPU connector. With the newer 500W PSU I tried various combinations of 24/24+4 for the ATX connector and 4/8 pin connector for the CPU.
Other differences between the PSUs are that the old one had a 115V/230V selection switch and the new PSU auto adjusts. Also the new PSU is modular and I tried with and without any of the SATA power connectors connected.
I even tried clearing the CMOS, took out the battery and waited 15 minutes.
http://en.wikipedia.org/wiki/Power_supply_unit
Although a too-large power supply will have an extra margin of safety as far as not over-loading, a larger unit is often less efficient at lower loads (under 20% of its total capability) and therefore will waste more electricity than a more appropriately sized unit. Additionally, computer power supplies generally do not function properly if they are too lightly loaded. Under no-load conditions they may shut down or malfunction.
It's theoretically possible. Switching power supplies (like a PC PSU) all have a minimum load requirement, typically on the +5V rail. Perhaps there was a device you forgot to plug in on the +5V/+12V rail?
No such thing as a "too big" power supply. It's like a big engine...It needs to be big enough to pull the load, and if it's bigger than that, no big deal. It's not efficient, but it's not going to cause functional problems.
What I would check is to make sure you have the right kind of power supply. Are you sure the power connectors are the same between your motherboard and power supply. Lot of name brand PC manufacturers change the pin outs on their motherboards, so you have to buy a power supply from them.
Also, check and make sure that you've plugged in ALL the plugs. If the motherboard expects something, and it's not there, it'll automatically shut itself down.