Troubleshooting an ATX power supply - a question

[xby]

Did I get it right?

I am referring to the article "Troubleshooting a Dead/Unbootable PC" by Scott Wainner (May 19, 2001) http://sysopt.com/articles/troubleshoot/index.html where an insert from a publication by Axion Technologies explains how to test an ATX power supply. This insert depicts a drawing whereby a jumper shortens two pins. Since the pins are not numbered in this drawing, I am not sure whether or not I read the drawing correctly. My current interpretation is shortening pin 14 with pin 8.

If my interpretation is incorrect, please point out my mistake. If it is correct, I would appreciate some explanation.

Pin 14 is PS-ON. It provides an input signal to the power supply from the main board, which turns it low to request power ON, or high to request power OFF. If the troubleshooting intent is to simulate a power ON request, I would understand shortening it with one of the ground pins (3,5,7,13,15,16,17).

Pin 8, however, is a PW-OK pin. It provides an output signal from the power supply to the main board, informing it about the status of the power (for example, advance warning about power failure). I do not understand what is the point of shortening it with pin 14.

Any explanation? Please!

[This message has been edited by xby (edited 07-15-2001).]

[This message has been edited by xby (edited 07-15-2001).]

[cedar2]

The board needs a source for the power it sends back as a signal to the PS when you hit the PS on switch. You won`t get that from ground.

[Goldwingnut]

Shorting the one green pin to any black ground pin will energize the power supply.

[Fingers]

... and you have to maintain that connection (short), or the PS will turn back off.

[Bovon]

I have also been struggling with the circuits of an ATX psu. I have a schematic of one, but the schematic only gives the input and output voltages, and does not show the socket where each goes, which makes it diffucult to 'understand' the complete supply and how it works.

The ATX psu has two independent supplies. One 5 volt circuit is energised all of the time that the supply is connected to the 120 vac wall outlet. This 5 volts goes to the socket and is applied to the mainboard at all times when it is connected to the board. When the momentary switch on the computer is depressed, this sends a signal back to the psu to tell it to turn on the rest of the power. This is part of the power good circuit.

Looking at the pins of the socket, and the voltages there, I am a little lost as to which wires to short out, in order to make the psu to energise without being actually plugged into a main board. I have read several articals about how to do this...short pin ?? to pin ?? and it will power up...probably so, but I would like to understand what is going on and why myself.

Following is a link to a generic 200 watt ATX psu, and if you can read electronic circuits, you may gain something from this. Note on the far left, it shows +5 volts as the one that turns the psu off, and the absence of +5 volts (0 volts) turns it on. This is the PS-ON circuit.
http://pavouk.comp.cz/hw/en_atxps.html

Obviously the mainboard has somesort of flip-flop circuit to change the +5 volts to 0 volts as needed to turn the psu on or off. I have been unable so far to understand this circuit, and probably cannot, without a circuit from a mainboard to see what takes place there.

I had a site bookmarked that gets more into testing ATX supplies, but the site seems to be down at this time. If I can find some better confirmation on just which wires to short to turn on an ATX psu, I'll repost here for you.

[Fingers]

As Bovon said, an ATX mobo is partially powered anytime the powersupply is turned on.

When the case switch is depressed, it tells the motherboard to complete the circuit between the "Power-On" (green) wire, and the powersupply ground (black).

Placing a jumper between the green wire on the ATX connector and any one of the black ground wires will cause the powersupply to "power-up", and it will remain powered up until that jumper is removed.

[Barney]

Check the power switch on the PC's case and make sure it's properly connected to the motherboard. Try shorting the jumper pins that the switch uses to power on the PC. If that works, the switch is bad.

Using the jumper is not to test the PSU, but the power switch on the front of the case.

[Fingers]

No disagreement there, but if you need to turn on a powersupply that IS NOT connected to a motherboard, just place a jumper between the green and black wires.

[xby]

Thank you all for your responses, but I did not get an answer to my question. I asked what is the point in shorting the GREEN wire (pin 14, PS-ON) with the GRAY wire (pin 8, POK), as is my understanding of what Scott Wainner's article suggests.

By the way, Barney, what you say is true for the AT setup. The ATX is different.

Bovon, the purpose of the ATX setup is to provide the facilities to turn the computer on and off under software control via a BIOS call. The BIOS software instructs the main board to set the green wire (pin 14, PS-ON) low or high in order turn the machine on or off, respectively. The front switch does the same via separate circuitry.

[This message has been edited by xby (edited 07-16-2001).]

[Bovon]

xby, you obviously have a good understanding of an ATX system. I wish I did. I do not have a clue what the artical ment by connecting the green to the grey wire to turn it on. I have studied the schematic and cannot determine for myself why. I am used to system schematics being complete, and therefor, being able to trace the circuits from and back to the supply, ect. So far, I have been unable to 'fully' understand the complete ATX system.

Should you have any web sites where this gets into detail, please post here for me. Such as what all is powered on at all times...the bios?...is this one of the items that is always on?..for a wakeup feature?

Input--I need input <grin>

Thanks.

[Psycho Logical]

Vsb is used by WOL/WOR (Wake-on-LAN/Ring), and enables wakeup from standby.

It is ON when the PS is connected to a live outlet, and the (optional) rear switch is on.

Power Good (PWR OK) signal is used to prevent the system from starting until the PS has stabilized.

Standard ATX voltages and uses:
-12V -- some serial ports
-5V -- some ISA cards
+3.3V -- CPU, AGP, (some)RAM
+5V -- motherboard
+12V -- drives, fans, system bus slots


[This message has been edited by Psycho Logical (edited 07-16-2001).]

[xby]

Thank you, Bovon, for your compliment; but you give me more credit than I deserve in terms of understanding ATX. I am sorry, but I cannot make such a claim about myself. You see, I did not know anything whatsoever about ATX until last week. It was a black box for me until then.

What happened last week? Seemingly a catastrophe. My machine suddenly died on me while I was working on it. No warning and no indication that anything was amiss until it shut down out of the blue and would not turn on. After verifying that power was reaching the machine, I went ahead to investigate the power supply.

Searching the internet for information I found two articles suggesting different ways for testing the power supply - one suggesting to short pin 14 with the ground, and the other (the one I quoted in my original question) suggesting to short pin 14 with pin 8. Thinking that these two advices were mutually inconsistent, I continued to search the internet, and found the Intel ATX specification manual. I studied it, acquired a little bit of understanding about ATX, and concluded that shorting pin 14 to the ground is the way to go. To be safe and sure, I also posted the question in a number of forums, and received some replies offering the same opinion, none offering the other. Well, I decided that this is the way to go, and I tested the power supply that way, finding it to be faulty.

Well, I am still curious about the answer to my original question, and apparently you are too. Since the article was published in sysopt.com I was hoping that someone here might know the answer.

P.S.
Unfortunately I do not remember the URL for the ATX manual, but if you are interested, I can E-mail it to you. In response to your question what is powered when power is available, all I can say is pin 9, 5VSB, the purple wire (I guess that 5VSB means: 5 Volts StandBy). I do not know how much of the hardware uses this power and which software in the BIOS is involved. I suspect that every mainboard uses this standby power in its own manner.


[This message has been edited by xby (edited 07-16-2001).]

[xby]

Bovon, I found the URL for the ATX manual. Here it is for your information:
http://www.formfactors.org/developer/specs/atx/ATX_ATX12V_PS_1_1.pdf