Need help with ATX Power Supply

[galexyus]

I have a problem with my ATX PSUs. Actually the main problem is probably in the motherboard. The computer keeps burning the power supplies. The way it happens is the computer starts normally, it works for 15 minutes, then it shuts down and won't start again. At first I thought my PSU didnt have enough watts, so I got 500W, and the same thing happened to it. Now I have 4 power supplies burnt like that, and all of them have the same symptoms: they are not completely dead, when you press the power button, the fans will start working and the lights will come on for about half a second, then nothing. I inspected the circuit boards and I cannot find any damaged components. Also I checked the voltages on the ATX power connector, and pin 9(purple) gives me +5V. If anyone can guide me where the problem might be, I would appreciate it a lot.

[NZtech]

Hi
It sounds like you have an overload condition on your motherboard alright. The power supplies have over-current protection built in to them, so they will turn off if there is too much load.
If you want to test the power supply on it's own, take the motherboard connector on the end of the power supply, and face it to you with the little plastic tag upwards. Now bridge the fourth pin from the left (usually a green wire) to one of the black leads next to it. There are around three or four ground wires right beside it. This will turn the supply on, and the fan should run.

If the motherboard is the problem, try taking out all cards, RAm, CPU etc and then try again.
If still faulty, it may be one of the chopper transistors shorted. They are next to the CPU, little power transistors with coils next to them. You might be able to get one from another old motherboard.

Good luck
Mark

[Guest]

thanks, NZtech, actually I disconnected the powersupply from the motherboard and I only connected a CD drive to the powersupply to have some load. Then I shorted the green PS_ON with the ground to start, but it won't start, only the fans in the powersupply will jerk when i short the green and black. I also tried the same thing with a working powersupply and it will start, i am able to operate the cd drive. So I know it's something inside the power supply. Also about the load, I dont really have that much stuff inside, a hard drive, floppy, dvdrw drive, I thought 500Wt should be able to handle that. Also I tried to measure the voltage on red and yellow wires while green and black are bridged, it's 0 with a short burst of voltage just when I connect the green to black. As I said before, the symptoms are the same on 4 powersupplies, so i guess they all have the same problem. I would like to figure out what it is, and fix it if it's not noo complicated. Any advice would be appreciated.

[galexyus]

The last post was mine