How to use an AT PSU

Using an old Useless AT Power Supply for something Useful!

You may think that old AT PS’s are quite frankly useless, however I was able to lay my hands on a couple for nothing J So It was my job to make use of them. Several ideas went through my mind; power all drives/fans…. water-cooling... but soon as I don’t have a water-cooled rig or haven’t got round to building a bay/fanbus yet those were pretty much out of the question. I came up with this idea when I was wanting to test my Uptime counter (not really wanting to plug it into a running ATX PS, risking shorting the thing!) I guess your wondering what “this idea” was then…. Well unfortunately I don’t have the privilege of having a bench PS. So My goal; To convert an old style AT PS into a bench PS. You may either be thinking Why? Or How? Well it was fairly simple really, and quite fun (as long as you don’t mind the odd bit of smoke and fireworks! J) As I expect you know by now every PS (whether AT or ATX) has outputs of the following voltages; Ground (needed if you don’t already know to make up most common voltages)
5V
12V
You can make the following voltage just be connecting your jump leads in the appropriate way; 5V7V
12V

I suggest you get hold of some banana clips from your local electronic store which you can use for your bench PS by piggy back the clips. Only simple soldering required to attach these, and this way you can make your leads as long or shorta as you like. Crocodile clips would come in handy too. In case you didn’t already know, 7V is achieved by using 5V as ground along with 12V as your +V. As voltage is obtained using potential difference we end up with +7V.You can make practically any voltage you like by creating a simple potential divider with a Variable resistor (or pot as I think some ppl call them) and a fixed resistor. The following diagram should help to show how this is done.

Here the voltage across Vout is varied as the value of the variable resistor is changed. This is only really useful if you have a multimeter or alternatively you could set up a series of fixed potential dividers to give you various voltages say at 1V intervals.

The voltage at Vout is worked out by using ratios; for example if the top resistor is 1Kohm and the bottom 1Kohm there will be an even split. The formula is

Vout = Value of top resistor / Value of bottom resistor * Vsupply

Vout = ½ * Vsupply

So if Vsupply was 12V as I showed above you would get 6V out at Vout