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Wiring an ATX and AT PSU Together

This (simple) tutorial will help you to get an AT power supply to automatically turn on when you press the button on a computer with an ATX power supply. This enables you to use a second power supply to power things such as peltiers, and not break your budget for one large one.

DISCLAIMER. The following information involves working with wires that will carry household current, that’s 115 volts AC. Coming into contact with these wires may injure or even kill you, start a fire, blow up your computer, etc. If you have never held a soldiering iron before, you may not want to attempt this. I can not be held responsible for any problems that you may have.

OK, now for the fun stuff! You will need a soldiering iron, soldiering flux, soldier, a 6v or 12v DC coiled relay capable of handling 115VAC through it’s switch terminals, a molex connector such as that found on an old fan, and electrical tape. All of this can be had at Radioshack or All Electronics. Let’s jump right into it!



We start with a diagram of what exactly your doing. Following are pictures of how to do this in real life.



First, we must do a simple modification to the AT PSU. Locate its power switch and remove it. You will then have two wires, which when touched together, will run the PSU. Don’t try it though, you could zap yourself!



Next, tin the two wires on the AT PSU. Do this by applying a small amount of flux, and then some soldier until it fills the gaps between the wire’s strands. Do the same to the wires on your molex connector. This makes the job of actually soldiering to the relay a lot easier.



This is basically what you will be doing to the relay. At the bottom are the relay coil terminals, with two wires which go to a molex connector. Now, if you are using a 12v relay, these would be the 12v leads (the yellow/orange wire and the black next to it). If you are using a 5v relay, you want the other two wires. In this example, I am using a 6v relay which functions just fine on the 5v line. Now, take the two wires that you have prepped from the AT PSU and soldier them to the terminals that will be closed when the relay is activated. The terminals are all labeled on the back of the relay package.



Here is another view of the relay showing the molex connector. What will happen is when the ATX PSU powers on (when you push the power button on your computer), the relay will activate. When the relay activates, the terminals close and the AT PSU’s power on circuit is completed and the AT PSU turns on. Now you want to thoroughly wrap the relay and connectors in electrical tape. This is to prevent any short circuiting (or worse)…



Now, put the PSUs back in your computer and reconnect your hardware. Hook the molex connector on the relay up to a power out connector on your ATX PSU. Plug the power cords for both PSUs in, and there you have it. When you push your front panel power button, both PSUs will start up automatically.