What voltage on ATX Power Good Line?

I think I have a bad PC power supply. It turns on and runs but the attached motherboard will not operate.

After testing some other known-to-work power supplies, I think that the power good line on the supply in question is not working. Other working supplies put out +5VDC on this line while this supply puts out nearly nothing.

I just need to confirm that +5VDC is what should appear on the power good line. Googling around hasn't provided any good info on this.

Thanks in advance!

William

Reply to
wm_walsh
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Yes. +5v. But only after all monitored voltages are within specification.

Are they?

Reply to
UCLAN

Hi!

Yes. All the other voltages are right where they should be and are stable even when approaching the maximum rating of the PSU. I've never seen anything like it before.

The power good line isn't completely dead--it has a fraction of a volt on it all the time.

William

Reply to
William R. Walsh

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    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

My guess is that the fault is in the power good circuit, I've never really looked into how this works but I suspect it isn't very complicated.

Reply to
James Sweet

Short of a defective PG detection circuit, it's very possible you could have some weak caps that are not totally filtering correctly and the circuit see's a little switching noise.

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Reply to
Jamie

Voltage on Power Good must exceed 2.4 volts. Significant also is that number to three digits. If this does not occur, typically voltages on any orange, red, or yellow wire is defective. Green wire orders power supply on. Supply has about 2 seconds to measure orange, red, and yellow voltages. If OK, gray (Power Good) wire goes to less than 0.8 to well above 2.4 volts

Now, if power supply controller does not see power good, then power supply controller will turn off power supply. So, first see what one of orange, red, yellow, and purple wires do both before and when power switch is pressed. Purple should remain above 4.87 at all times (and also related to number on Power Good wire). Other voltages should rise from zero until power supply controller cuts them off. Any one voltage that does not increase as the power switch is pressed implies where to look for the problem.

Other possibility is that a failure exists in the power supply controller or Power Good driver is defective. With power supply disconnected from motherboard, connect the meter in DC volts, shunt a paper clip from green wire to black wire, then record that voltage. Repeat same experiment with meter in Ampere mode between that Gray (Power Good) wire and any black (ground wire). When a paper clip connects green wire to black wire, then current on Power Good should significantly exceed 0.02 amperes (20 mA). Set meter to highest current. Short the green wire (Power On#) to ground, then slowly decrease meter scale until a useful current number is read.

That Power Good wire must output more than 2.4 volts and 0.02 amperes. Those numbers will report things significant. And then post those numbers here to learn other facts. Those numbers may be reporting more than you realize.

Reply to
w_tom

Hi!

Thanks for the link. I already saw it though, and unless there is something I missed, it does not give a voltage for the Power Good pin. The only identification I saw there was "PWR_OK" with no further explanation offered.

William

Reply to
wm_walsh

Hi!

It would surprise me (only a little, so you know my expecations have been lowered) to find something brand new coming out of the box with bad caps.

I don't know what might have caused it. The supply seemed to have excellent regulation on all other outputs. All of the 3.3, 5 and 12 volt leads tested fine. In any case, I traded it off for a new supply under warranty and that one happens to work.

William

Reply to
wm_walsh

Take a look at the page at

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It gives you a sample ATX power supply schematic, plus a decent description of the operation, including the "Power_Good" circuit.

HTH,

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Dave M
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Reply to
DaveM

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