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Q: Hard-Wire Power Supply to Bypass Motherboard Power-Up Signal ( Answered 5 out of 5 stars,   3 Comments )
Subject: Hard-Wire Power Supply to Bypass Motherboard Power-Up Signal
Category: Computers > Hardware
Asked by: nhjones-ga
List Price: $10.00
Posted: 08 Aug 2002 05:24 PDT
Expires: 07 Sep 2002 05:24 PDT
Question ID: 52126
I would like to rig up a Dell Optiplex GX-1 power supply (Dell
PS-5141-1D power supply) to turn on automatically when plugged in
rather than getting a power-up signal from the motherboard.

Background:  I have a Dell Optiplex GX1 that recently will not power
up.  A light on the ISA/PCI riser card lights up when it's plugged in,
and the power supply works fine in another identical Optiplex, so the
power supply is ok.  I hooked up the power-up switch from another
functioning Optiplex to the motherboard and it did not work.  The last
few times I've turned the (now-bad) Optiplex on, it worked fine.  I am
hoping that if I can force the power supply to turn on, the computer
will work.  Is there any way to force (hard-wire) the power supply to
turn on when plugged in (i.e., bypass the motherboard power-up
function)?  Dell has a schematic of the power supply at and there is
additional technical information on the system at

This is an old Pentium II-350 MHz system, so I'm willing to experiment
with it.  There's no risk of data loss.


Nathan Jones
NHJones (at) Bigfoot (dot) com
Subject: Re: Hard-Wire Power Supply to Bypass Motherboard Power-Up Signal
Answered By: lazerfx-ga on 08 Aug 2002 06:50 PDT
Rated:5 out of 5 stars
Hello Nathan, and thanks for the question.

This is a very interesting question, and one I have enjoyed
researching.  Fortunately, the ATX power supply unit is a pretty
standard unit.  There are several sources which will tell you where
the PSON wire is (Typically coloured green on most PSU's, although
Dell appears to colour it differently - see below).

The first article I came across is:
CK42's CPU Over-Temp Detector / PSU Shutdown How 2 Instructions -
This page is dealing with inserting a thermal temperature sensor under
the CPU, and then using the PSON signal to shutdown the PSU if that
goes above a certain point - in other words, to cut the wire using a
relay.  It deals with finding the PSON wire, the voltages used, and
similar things you would need to know.  Broadly related to your
requirements, it includes quite a bit of background information on the
PSU that is helpful when working out exactly which wire to attach to

The second article I came across is:
Technical Overview: Dell Dimension XPS Txxx and Txxxr Systems
Aproximately 3/4 of the way through this document, there is a section
entitled, "DC Power Connector Pin Assignments" which informs us that
this particular Dell PSU has a brown PSON# wire, on pin 11.  It also

"Pin 11 - PSON# is activated by pressing and releasing the power
button while the power supply is in standby state. Activating PSON#
connects the power supply’s PSON# input to ground, thereby switching
the power supply to full-on condition."

This suggests that the PSON needs to be connected to ground to turn
the power supply on, either through a switch or by hard-wiring the
wires together.

Search methodology:
PSU Power On Signal

Dell PSON color

Personal comments:
After reading about power supplies and motherboards, I would like to
interject a small not of warning here.  Since you say the system is
expendable, it is not such a major thing, but many motherboards
include a self-test system in the BIOS and elsewhere, and it could be
that the motherboard is not sending the PSON signal to the PSU because
it is somehow failing the self-test.  Having said this, it could be
that a ground-link or similar has come loose, or that a relay has
seized in the off position.

I hope you manage to sort your system out, and that these articles and
comments have been of help to you.


Request for Answer Clarification by nhjones-ga on 08 Aug 2002 09:43 PDT
Thanks for your very insightful answer.  It seems to have helped.

As the site
you recommended noted, Pin 11 is the power on pin (it is gray rather
than brown on my Dell PS-5141-1D power supply, but still Pin 11). 
When I connected Pin 11 to ground (Pin 12), the power supply does
comes on and it gives power to all the other connectors.  At first, my
motherboard didn't boot when power was applied, but after fiddling
with it, the system kinda works!  The regular power switch now works,
so I don't have to use the Pin 11 power signal.  The system seems to
dislike having he floppy, CD, and hard drive all hooked up at the same
time, so I'll just run it with the hard drive for now.

Thanks again for your help on this -- I learned about power supplies,
got the old computer working well enough, and had a successful morning
science project!

Nathan Jones

Clarification of Answer by lazerfx-ga on 08 Aug 2002 09:53 PDT
I'm glad to be of service, and I'm glad to hear that your system is
now working semi-reasonably.  You might find it interesting to pop the
lid on the PSU (after ensuring it's been turned off for an appreciable
length of time to allow residual charge to drain from the capacitors,
of course) and check the capacitors - if they are not working at full
power that could be causing the apparent lack of voltage/amperes you
are experiencing.

Thanks for an interesting question, and I hope to see you around again

nhjones-ga rated this answer:5 out of 5 stars
Answered my question completely, and even added a few ideas beyond the
specific question I asked.

Subject: Re: Hard-Wire Power Supply to Bypass Motherboard Power-Up Signal
From: simplicit-ga on 08 Aug 2002 06:57 PDT
It looks like a standard ATX Connector 
Why not try the suggestion on this page

Of course it might blow up the PSU - but I would imagine it would work as expected.

Subject: Re: Hard-Wire Power Supply to Bypass Motherboard Power-Up Signal
From: bit_bucket-ga on 08 Aug 2002 11:21 PDT
I've actually done this myself to fix a bad motherboard. The circuit
in the motherboard that told the power supply to turn the computer on
when you press the power button failed.

I just clipped the wire to pin 11 (if I remember correctly) and
screwed it to the case (ground). This made it turn on when it was
connected to wall power.

Subject: Re: Hard-Wire Power Supply to Bypass Motherboard Power-Up Signal
From: duc0n-ga on 28 Jul 2004 10:13 PDT
Very informative... I like it =-)

This is also good for case modders like myself, who may want to be
able to test the case mods before installing them, or before acquiring
a motherboard.

I plan to wire up a plug that will fit into the PSON pin and the
adjacent ground so that I'll have the ability to run my case mods
(currently some cold cathode lighting, a lit temp/fan speed/voltage
display, etc..).

Thanks again!

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