Question on ATX Power Switch Pins and Case Switch?

I'd like to better understand.

On an ATX MOBO you have two power switch pins to which the case on/off
switch is connected. It must be connected and the on/off switch
toggled in order for the PSU to run.

I can just put a jumper on the two pins, but the PSU doesn't seem to
want to run.

So I am wondering - what is actually happening in simple terms? Is
one of the two pins 'hot', and does the case switch 'pass' current via
the connector to the second pin or what? Or does the case switch have
the current and pass it to both MOBO pins?

Be gentle please.

The case switch is a momentary short across the 2 pins
on the mobo which causes the motherboard to pull the
on pin of the power supply low and keep it there
after the switch is released. The symptom you describe
indicate a possible problem with the mobo. However, you can
test this by shorting pins 13 and 14 of the 20 pin
connector together, with only a drive as a load on
the power supply, and see if the supply turns on. If it
does run, then the mobo is the problem, if not
then the supply is the problem.

<one@two.three> wrote in message
news:t3n6315l2667pmomuo1u1r3g2qgmlkua4l@4ax.com...
> I'd like to better understand.
>
> On an ATX MOBO you have two power switch pins to which the case on/off
> switch is connected. It must be connected and the on/off switch
> toggled in order for the PSU to run.
>
> I can just put a jumper on the two pins, but the PSU doesn't seem to
> want to run.
>
> So I am wondering - what is actually happening in simple terms? Is
> one of the two pins 'hot', and does the case switch 'pass' current via
> the connector to the second pin or what? Or does the case switch have
> the current and pass it to both MOBO pins?
>
> Be gentle please.
>
>

On Sat, 12 Mar 2005 18:04:07 -0500, "Pen" <nospam@nospam.com> wrote:

>The case switch is a momentary short across the 2 pins
>on the mobo which causes the motherboard to pull the
>on pin of the power supply low and keep it there
>after the switch is released. The symptom you describe
>indicate a possible problem with the mobo. However, you can
>test this by shorting pins 13 and 14 of the 20 pin
>connector together, with only a drive as a load on
>the power supply, and see if the supply turns on. If it
>does run, then the mobo is the problem, if not
>then the supply is the problem.
>

Thanks

On Sat, 12 Mar 2005 18:04:07 -0500, "Pen" <nospam@nospam.com> wrote:

>The case switch is a momentary short across the 2 pins
>on the mobo which causes the motherboard to pull the
>on pin of the power supply low and keep it there
>after the switch is released. The symptom you describe
>indicate a possible problem with the mobo. However, you can
>test this by shorting pins 13 and 14 of the 20 pin
>connector together, with only a drive as a load on
>the power supply, and see if the supply turns on. If it
>does run, then the mobo is the problem, if not
>then the supply is the problem.
>

How would I distinguish between MOBO failure and CPU failure? Could
either failure by itself knock the PSU down?

Thanks

Yes. Either one is normally causes the
power supply to turn on briefly and then shut down.
You then pull the CPU and see if the PS turns on
or shuts down.

<one@two.three> wrote in message
news:u958311e18ld9f71l5ehd4q12ef4p466dj@4ax.com...
> On Sat, 12 Mar 2005 18:04:07 -0500, "Pen" <nospam@nospam.com> wrote:
>
>>The case switch is a momentary short across the 2 pins
>>on the mobo which causes the motherboard to pull the
>>on pin of the power supply low and keep it there
>>after the switch is released. The symptom you describe
>>indicate a possible problem with the mobo. However, you can
>>test this by shorting pins 13 and 14 of the 20 pin
>>connector together, with only a drive as a load on
>>the power supply, and see if the supply turns on. If it
>>does run, then the mobo is the problem, if not
>>then the supply is the problem.
>>
>
> How would I distinguish between MOBO failure and CPU failure? Could
> either failure by itself knock the PSU down?
>
> Thanks

>On an ATX MOBO you have two power switch pins to which the case on/off
>switch is connected. It must be connected and the on/off switch
>toggled in order for the PSU to run.

>I can just put a jumper on the two pins, but the PSU doesn't seem to
>want to run.

If you short the pins and leave the jumper on, the machines should power up for
a few seconds then shut off again (same as holding the power button in when you
turn it on). If you short the pins and remove the jumper once it starts, it
should stay running. (same as pressing and releasing the power button).

>So I am wondering - what is actually happening in simple terms? Is
>one of the two pins 'hot', and does the case switch 'pass' current via
>the connector to the second pin or what? Or does the case switch have
>the current and pass it to both MOBO pins?

The switch is just a momentary action switch.. all it does is shorts the two
pins while you're holding it, and disconnects them as soon as you let go of the
button (ie. nothing special or fancy there). One pin is ground, the other is
PS_ON, which carries a small voltage and is connected to the PSU circuitry (via
the green wire) to turn it on while it's shorted to ground (I'm not sure of
exactly how it does this).

The mainboard has a simple circuit that keeps the PS_ON wire shorted to ground
once you press the button briefly (this is so you don't need to hold the button
in).

If you want to fire up an ATX psu without connection to a motherboard, you just
need to short the green wire to any of the black wires, or the metal casing
(you'll want to put some kind of load on it first though, such as a hdd, cdrom,
etc).

Hope this helps you understand a little more..

Regards,
Chris

Skeleton Man wrote:

> >So I am wondering - what is actually happening in simple terms? Is
> >one of the two pins 'hot', and does the case switch 'pass' current via
> >the connector to the second pin or what? Or does the case switch have
> >the current and pass it to both MOBO pins?
>
> The switch is just a momentary action switch.. all it does is shorts the two
> pins while you're holding it, and disconnects them as soon as you let go of the
> button (ie. nothing special or fancy there). One pin is ground, the other is
> PS_ON, which carries a small voltage and is connected to the PSU circuitry (via
> the green wire) to turn it on while it's shorted to ground (I'm not sure of
> exactly how it does this).

Close, but not quite. The switch actually has NO connection to the PS_ON
(green wire.) The switch goes to a logic circuit on the MB. A momentary
short of two pins of this logic circuit toggles the output - which *is*
connected to the PS_ON wire - to a low state (which turns the PSU on.)
A second momentary short of the pins toggles the output high, which turns
off the PSU.

>Close, but not quite. The switch actually has NO connection to the PS_ON
>(green wire.) The switch goes to a logic circuit on the MB. A momentary
>short of two pins of this logic circuit toggles the output - which *is*
>connected to the PS_ON wire - to a low state (which turns the PSU on.)
>A second momentary short of the pins toggles the output high, which turns
>off the PSU.

Thanks, I wasn't 100% sure where the logic circuit fitted in..

Regards,
Chris