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ATX PSU turns on immediately after power connected

Discussion in 'Power Electronics' started by ReWoP, Nov 11, 2020.

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  1. ReWoP

    ReWoP

    5
    0
    Nov 11, 2020
    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?
     
  2. davenn

    davenn Moderator

    13,671
    1,892
    Sep 5, 2009
    check the power switch on the PSU, it may have failed in the ON position
     
  3. ReWoP

    ReWoP

    5
    0
    Nov 11, 2020
    Its PC ATX PSU, the power switch only turns on mains voltage not the PSU. I've check the cable and there is no jumper wire on the 24 pins connector and also all the voltages are 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.

    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?
     
  4. philipshadow

    philipshadow

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    0
    Oct 29, 2020
    this picture shows you the output voltages, check if all the voltages match or missing
     

    Attached Files:

  5. philipshadow

    philipshadow

    20
    0
    Oct 29, 2020
    i know that a jumper wire from green to ground will turn the unit on. check if there is a short from green wire to ground with your multimeter on diode mode
     
  6. ReWoP

    ReWoP

    5
    0
    Nov 11, 2020
    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.

    The PSU model is EuroTech 580W model no. FPSPA580PSA and the control chip is WT7517
    https://www.techpowerup.com/articles/160/images/WT7517.pdf

    According to the datasheet the PS_ON should be 5V
    [​IMG]

    Could the capacitor be damaged and acting as a resistor to ground?
    I've measured the impedance between the green wire (PS_ON) to ground (with the PSU unplugged) and I've got 23.9MΩ

    [​IMG]

    [​IMG]
     
  7. philipshadow

    philipshadow

    20
    0
    Oct 29, 2020
    you mentioned in your previous post that you recorded 2.4v at PS_ON. Do you still get that reading?. Concerning the capacitor you can remove it and see if the problem still persists. Also check for any shorts on the transistors and diodes.
     
    Last edited: Nov 13, 2020
  8. ReWoP

    ReWoP

    5
    0
    Nov 11, 2020
    I've already tested the diodes and other semiconductors for simple failures, like shorts or opens, and found nothing obvious...

    Anyways, I will recheck the voltages for inconsistencies (following the 1st, 2nd and 3rd commandments: Thou shall always check voltages), then the capacitor to ground and afterwards other componentes around that circuit... Will post any finds.
     
  9. ReWoP

    ReWoP

    5
    0
    Nov 11, 2020
    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... upload_2020-11-16_17-40-37.gif

    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...
     
  10. philipshadow

    philipshadow

    20
    0
    Oct 29, 2020
    okay. keep me updated.
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

    10,591
    2,360
    Nov 17, 2011
    That is possibly too low. The power-on signal is TTL logic level and usually should be > 3.5 V for a definitive logic high.
     
    HellasTechn likes this.
  12. HellasTechn

    HellasTechn

    1,542
    214
    Apr 14, 2013
    I agree. Normally it's the 5V section that is used as standby. Ive never seen an ATX PSU use lower than that. I suspect that some capacitor may have dried out and causes short circuit or low resistance path.
     
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