[Gllug] OT: Multiple rails in in ATX PSUs

Chris Bell chrisbell at chrisbell.org.uk
Thu Oct 13 07:53:27 UTC 2011

On Thu 13 Oct, general_email at technicalbloke.com wrote:
> Hi,
> I wonder if I understand this correctly...
> All new ATX power supplies must have at least two 12v rails.
> Is that how it works or am I missing something?
   I have opened several power supplies, and the layout is usually:

The main ATX connector connections looking at the socket are

Orange*  +3.3v          +3.3v   Orange*
Orange*  +3.3v          -12v    Blue
Black    Ground         Ground  Black
Red      +5v            Switch  Green (ground for main output on)
Black    Ground         Ground  Black
Red      +5v            Ground  Black
Black    Ground         Ground  Black
Grey     Power good     -5v     White if fitted
Lilac    +5v standby    +5v     Red
Yellow   +12v           +5v     Red

with an optional clip-on extension

Yellow   +12v           +5v     Red
Orange   +3.3v          Ground  Black

   An additional light brown lead used for output voltage sensing and
regulation is connected to one of the 3.3v outlets marked * 

   There are also optional leads supplied with two, three and four pairs of
Yellow   +12v           Ground  Black

   Most older units had all leads of the same colour connected together at
the PSU, but the one open in front of me has two separate +12v areas.

   I initially test a suspect power supply by connecting it to a mains
safety isolating transformer fed via a 100 watt tungsten lamp so that both I
and the local mains supply are safe. Measure the standby +5 volts, which is
also used to drive the main switch-on circuit, then short the green lead to
ground using a special tool (cut from a paper clip) to check everything else.

Chris Bell www.chrisbell.org.uk
Microsoft sells you Windows ... Linux gives you the whole house.

Gllug mailing list  -  Gllug at gllug.org.uk

More information about the GLLUG mailing list