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Thread: Checking PWR_OK and PSU_On with a DMM

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    Default Checking PWR_OK and PSU_On with a DMM

    Ok, I need a quick'n'dirty recap how to properly do that.

    With the PSU plugged into mains outlet, pins 15-16 not shorted, back PSU switch (if any, of course) set to "ON", with the DMM I should read on the 20 pin ATX connector from pin 9 (5VSB) 5V, from pin 14 (PSU On) something higher than 2.5V, and from pin 8 (PWR_OK) = ? (I don't recall)

    Right?

    Then, after shorting the pins 15-16 with the paperclip, the DMM should read from pin 9 still 5V, from pin 14 something less than 1V, and from pin 8 (PWR_OK) = ? (as above)

    Right again? Or is this all wrong?

    Well, thanks in advance for hints/help.
    Last edited by quest for silence; 04-13-2017 at 04:32 AM.
    Best, Luca

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    Jeez, in more than two hours no DMM man stumbled upon here... what a luck!

    Well, rephrasing/amending/summarizing:

    a) paperclip off: 5VSB =5V / PSU_ON > 2.0V / PWR_OK < 0.4V
    b) paperclip on: 5VSB =5V / PSU_ON < 0.8V / PWR_OK > 2.4V

    Can anyone confirm whether that's right or not? Thanks in advance!
    Best, Luca

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    Quote Originally Posted by quest for silence View Post
    Jeez, in more than two hours no DMM man stumbled upon here... what a luck!

    Well, rephrasing/amending/summarizing:

    a) paperclip off: 5VSB =5V / PSU_ON > 2.0V / PWR_OK < 0.4V
    b) paperclip on: 5VSB =5V / PSU_ON < 0.8V / PWR_OK > 2.4V

    Can anyone confirm whether that's right or not? Thanks in advance!
    Sorry, I was asleep still... Time zones...
    Anyways, you could always read the atx spec :P

    This is what I got (Tested with the CX-450M):

    Paperclip off: PSU_ON is 2.5v, PWR_OK should be low (0v).
    Paperclip on: PSU_ON is 0v, PWR_OK signal should be 5v, not 2.4v.

    5vSB will always remain the same 5v, it's a separate rail (I confirmed it as well).
    With the paperclip on, it should be zero. You're pulling it low by grounding it with a paperclip that is tied to ground, so I guess 0.8v is just an error margin?? (although close to 1v shouldn't be that high of an "error", what multimeter + probes are you using?).

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    Quote Originally Posted by ridgid13579 View Post
    Sorry, I was asleep still... Time zones...
    Thank you ridgid, BTW I know north american guys were under the moon: I was thinking to my european comrades, first and foremost our The Mask (hey mate, how many PSUs did you checked before now? Countless I guess).


    Quote Originally Posted by ridgid13579 View Post
    Anyways, you could always read the atx spec :P
    If I were not illiterate, yes, I could.

    BTW, that's where I took those values (just who knows whether I read the right pages or not!).


    Quote Originally Posted by ridgid13579 View Post
    PWR_OK signal should be 5v, not 2.4v.
    Yes, that's what I think I read in the book, >2.4V and <5V: I wrote >2.4V for the sake of brevity, sorry for misleading anyone.


    Quote Originally Posted by ridgid13579 View Post
    I guess 0.8v is just an error margin?? (although close to 1v shouldn't be that high of an "error",
    No, no, actually I didn't do anything atm: it's what's written on ATX specs (albeit maybe I took a wrong revision).


    Quote Originally Posted by ridgid13579 View Post
    what multimeter + probes are you using?).
    I'm going to use my cheap'n'trusty UNI-T UT139C with stock probes.
    Last edited by quest for silence; 04-13-2017 at 11:51 AM. Reason: grammar correction
    Best, Luca

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    Default

    Oh, I didn't even look at the spec :P. But I guess it makes sense..... Most things will consider 2.5v as high if they run on 5v or 3.3v...

    Quote Originally Posted by quest for silence
    I'm going to use my cheap'n'trusty UNI-T UT139C with stock probes.
    Oh that doesn't look like a bad meter for it's price. I thought you were using some super sketchy ebay meter, but at this low voltage it doesn't really matter does it?

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    Quote Originally Posted by ridgid13579 View Post
    Oh that doesn't look like a bad meter for it's price.
    Well, actually it's one of the more well regarded on EEVblog (among cheap ones, of course), and that's why I bought it (I thought that it were a safe buy).
    Best, Luca

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