ATX PSU turns on immediately after power connected

Hi everyone, need help to diagnose an ATX PSU!

The PSU works fine but it turns on immediately after i connect the power plug, even without having the PSU connect to the motherboard

Any ideas where should i start testing?

Does it turn on as in, a light and fan turn on? Or have you measured a voltage from yellow to black or red to black?

You likely have a bad supply if you are measuring a voltage when plugged into wall but not plugged into mobo.

One thing to check, look for a test clip (jumper wire) between pins. Either on the front side (as shown) or on the back side of the terminal block. Connecting terminals 4 and 5 will start the PSU if the supply doesn't have a minimum load requirement.

If you don't see a metal connection, use an ohm meter to check resistance from pin 4 to 5 (when supply is unplugged from wall). If resistance is low, there may be a solder blob connecting the pins on the PCB or a stray piece of metal from the sheet metal stamping for the housing making the connection. Call the manufacturer and return the unit if under warranty.

I've check the cable and there is no jumper wire on the 24 pins connector and also the voltages are all ok. Even went and test the PSU with a motherboard and it boots and works fine.

I've already removed the PCB (discharged the high voltage capacitors) and did a small inspection (mostly visual), remove some dust but didn't find any damaged components, at least nothing too obvious.

There is no warranty on the PSU, fix is the only option.

I had the idea that measuring the green wire to ground i would have something about 5v but I'm getting around half that (2.4v). Could be a damaged resistor or something?

If the issue persists, I would advise you to get a new PSU.

This is the Repairing Electronics thread not the Replace Electronics

Can't really repair something without a make and model number. Even then, one should have a schematic before digging into the circuit.
On that note, it may be that the green sense portion of the circuit is "blown" somehow and is detecting the "power on" signal (grounded line). Trace the green wire to the PCB and see if there is a solid state device such as a transistor or opto-coupler being driven by that line. My suspicion falls on that circuit. If that does not work out, then there is likely something else wrong with the PSU, and it would be wise to replace it. A faulty PSU risks destroying your motherboard shouyld something else go wrong with it.
There is a point where repair makes sense, and a point where replace is the proper answer based on our "repair experience".

ReWoP said:

I've check the cable and there is no jumper wire on the 24 pins connector and also the voltages are all ok. Even went and test the PSU with a motherboard and it boots and works fine.

I've already removed the PCB (discharged the high voltage capacitors) and did a small inspection (mostly visual), remove some dust but didn't find any damaged components, at least nothing too obvious.

There is no warranty on the PSU, fix is the only option.

I had the idea that measuring the green wire to ground i would have something about 5v but I'm getting around half that (2.4v). Could be a damaged resistor or something?

Click to expand...

So, the power supply works with a motherboard, turns on and off ok but you claim it is fully powered when not inside a computer?

1) Measure the 5v to ground (red to black) with a volt meter when plugged into the wall and green wire is floating.
2) Then measure red to black voltage again when you connect green wire to ground. Report voltages here.
3) Also, add a 10 ohm, 10w resistor to a red wire and ground (green wire not connected to ground). Power on and measure voltage from red to black. Report here.

It is possible that a high impedance voltage source may be present on the red bus and measure 5v with a 10M ohm meter load but will drop near zero as soon as a reasonable load is applied.

Hi all,

I've done more testing, having in consideration all the suggestions above and this is what I came up:

• Effectively all voltages where being correctly outputted as soo as the power supply was connected to mains
• The green wire (PS_ON - power supply on) to ground was about 2.5v
• The gray wire (PG - power good) to ground was 0v
• All voltages where correctly reported to the WT7517 chip and the PGI (power good input) was about 2.5V and the PGO (power good output) was 0v
• When i short the green wire (PS_ON) the PG output starts reporting the correct 5v - the WT7517 chip validades all power rails
• Traced the optocoupler controlled by the chip, to indicate the primary side to start the main power rails, and it was good and correctly flipping the power on/off states when shorting/opening the PS_ON line
• On the primary side, tracked the power state circuit and found a PNP Epitaxial Transistor (A928A) that was shorted on all terminals

Having found that, I did what we should do in this situations. Having not found a direct equivalent transistor on my transistors stash, impatiently, tried to use a "indirect" equivalent transistor that also did not worked (wasn't Epitaxial and didn't noticed that was EBC instead of ECB) and maybe broken something else on the primary because even putting back the old transistor the PSU did not start again...

Anyways... I've ordered the correct transistor and when it arrives ill be diagnosing the primary side for any more faults since i can probably assume the secondary side is completely fine.
Lesson learned...