View Full Version : ATX Power Supply PowerOn Trigger

23-12-2003, 09:18 PM
I have an ATX computer (quite new, Celeron 300Mhz) which won't turn on. Nothing happens when you press the power button. The power supply is not the problem, I have bought a new one with no difference.

The problem is the PS-ON signal (Pin 14 on the motherboard plug) is not signaling. For the power supply to be on, the PS-ON wire must be < 0.8V. To be off, open circuit or > 2.0V.

I have cut the wire to see with a multimeter what voltages are present, and found that on the motherboard side it is ~4.5V and ~3.5V on the powersupply side. When joined as normal it is ~4.5V.

If I short the wire to ground, the power turns on, but I have heard that this is bad for the power supply.

Please help, I am close to getting it going it is the best PC I own.

23-12-2003, 10:02 PM
>(quite new, Celeron 300Mhz)

how can a 300mhz celeron be NEW ????

have you tried just hitting the reset button?

check ram, some boards won't run if ram is faulty.

what voltage is the standby wire?

23-12-2003, 10:43 PM
Is the +5Volt standby voltage, on pin9 OK?

Graham L
24-12-2003, 01:22 PM
Those voltages on the wire are probably correct. The PS side is ~3.5 V ... that's the "floating high" voltage of TTL logic. (I'm assuming you measured with a digital meter, with megohms of input resistance). The MB side should have 5V from the 5VSB supply, which is on all the time the PSu is plugged in. 4.5 is low, but since it pulls up the PSU side when connected it's probably OK.

The turn-on is accomplished by pulling that line to ground as you have discovered. A momentary pulse will do it, I think. That won't harm the supply, except by causing it to function as usual, and eventually wear out. ;-)

I suspect the switch, or its connections. It should have three wires boing to it ... +5V from the MB (using the +5VSB from PSU) on one side , the control wire (the centre connection) which eventually goes back to the PSU, and GND on the other side. The switch should normally hold the control lead at "5V", and go to "GND" when pressed.