Its funny how power supplies fail sometimes.

This one only lost the 12volts, so led’s on the motherboard and case were still lighting up. It had me fooled for a while. This is the first time that carrying a multimeter might have been useful. But since I don’t carry spare PSUs, It still would have involved a second visit.

Customer saying she once heard a bang, was the first clue that the PSU might be dead. Shining a flashlight into the PSU gave the second clue: a small cylinder (about 1 cm long & 0.5 cm in diameter) ie a blown capacitor, was resting on the bottom of the PSU casing.

Customer needed the PC quite urgently, so I replace her 420Watt PSU with my 350W for 1 day, until I get hold of some 450W units.

You might think using a 350W to replace a 420W is a bit dangerous, but I checked the volts and amps (a generic 420W seems to be only slightly more powerful than a 350W Antec PSU).

The trick with PSUs, is that Watts = voltage X current (amps). Since PCs need a combination of 5volts & 12 volts, its possible to manipulate the overall wattage.

Modern CPUs need a PSU that can supply a lot of current at 5 volts, and the need for 12 volts is generally just for hard disks, CD/DVD drives (ie not much), so you could have the following situation:

PSU1 is rated at:
12V X 20A = 240 W
5V X 40A = 200 W
total = 440 W

PSU2 is rated at:
12V X 25A = 300 W
5V X 28A = 140 W
total = 440W

so which is better for a PC?

PSU1 can handle a lot more current for 5V, and only slightly less current for 12V. Best for a modern PC with a power-hungry Pentium 4 CPU.

PSU2 has only reached 440 W by increasing the current at 12 V (at the expense of the 5V supply). This is only good if you have something that needs a lot of 12 Volts (eg a pentium3 PC with 4 hard disks and 4 CD/DVD drives).

Of course, there are also many other factors that are considered in designing a PSU, so the above description is somewhat simplistic, but it does give an idea of what to look for in a PSU.