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.
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.
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.