If you're considering a dual PSU setup, here's a simple doodad to make sure your secondary PSU starts when you press the power button on the front of your case. Note that this only covers dual ATX PSU setups.

Shopping List:

• black wire


• yellow wire


• green wire


• 12v 4-pin SPNO relay


• male 20-pin ATX connector


• male MOLEX plug


• blade connectors **


• heatshrink tubing *


• insulation tape *


• sleeving *
  
  

(* optional; ** depends on your relay)

Here's a wiring diagram for you - pretty simple really:

What's going on here:

To start, your primary PSU should be connected to your motherboard via the 20-pin ATX connector, and critical components (like the CPU fan, if it isn't connected to the mobo; and probably the hard drive{s} too) in case of PSU failure, and of course the motherboard headers need to be connected to the relevant front panel switches.

A 12v line and ground wire (yellow & black) from a MOLEX plug on the primary PSU are used to energise the coil in a 4 pin SPNO relay, and both sides of the switch part of the relay are connected to the PS_ON (green) wire and any COM wire (black) in the secondary PSU, preferably via a male 20-pin ATX connector.

When the primary PSU switches (on when the case power button is pressed), 12v flows through the relay coil, closing the switch, which fools the secondary PSU into switching on - it's a similar principle to using a switch to turn a regular ATX PSU into a desk/test psu (see here and here).

When it comes to turning off the PC, when the primary PSU has switched off, 12v is no longer supplied to the relay coil, which causes the switch to open, shutting the secondary PSU off.

There is a delay between the primary and secondary PSU turning on (or off), but although it would be measurable, given the right equipment, it's not perceptible to the human senses - we're talking a figure into the low milliseconds here.

The relay required is a simple 12v, 4-pin unit - the most common automotive type, typically used for switching the extra power required by fog & driving lights - you should be able to find one for around £/$4. Typically though, I'm using something a bit different. The relay I'll be using here is also an automotive 12v unit, but it's a cheaper unit intended for PCB mounting - it's slightly smaller, and the fact it has weeny pins rather than enormous spade, excuse me, _blade_ connectors will mean the finished unit will be easier to hide. Calling it a "4-pin relay" was evidently too plebeian for Maplin though - they sell it as an SPNO relay - "single pole, normally open". Which means exactly the same thing.

One good reason for buying the more conventional (and expensive) auto relay though is that they tend to have proper labeling, often extending to a diagram of which connector does what. Mine didn't have anything like this ... in fact, the damn thing had _five_ pins. Easily worked round through - a quick go with a multimeter identified the two pins at the each end of the coil - it was the only pair showing any resistance (300 ohms).

I connected those up to my desk/test psu via a quickly bodged test lead consisting of a male MOLEX plug, some scraps of wire saved from my baybus build, and some small crocodile clips found in the case of a 1960's "portable" potentiometer (which in turn was found in a skip {dumpster}) made the relay switch with a satisfying "clack". After that, I only had three pins to choose from to find out which were each end of the switch part of the relay, and a couple of continuity tests sorted that.

There's really not a whole lot to the build process - connect up a yellow & black wire to the coil side of the relay & pins 1 & 2 in a male MOLEX plug, and connect green & black wires to the switch side of the relay to pins 13 & 14 in a male 20-pin ATX connector. Sleeve it all up to make it look pretty if you like.

Here's my finished effort:

... and here it is in action - the male MOLEX plug gets connected to a spare connector on the primary PSU, and the 20-pin ATX connector gets plugged into the secondary PSU ... which then has anything you like plugged into it.