Wednesday, October 2, 2013

How to Use An ATX Power Supply For General Purpose

Good morning everyone. It's 7 am! I can hear the harsh tweeting of the infamous house sparrow and it is really hot in here too. Still hope it will be a good morning. Apparently it is not, I am kinda dizzy so I should get a cup of coffee or something.

Anyway as I am awake I think I can write something. So, allow me to write something about using computer power supply as general purpose power supply unit.

What is an ATX power supply?

ATX is a standard. It stands for Advanced Technology eXtended. It is a worldwide standard for motherboard, casing power supply etc. So, all ATX power supplies will comply this standard. They will have almost the same output voltages, same size and so on. There are other standards available for today's computers.

Simply it is the power supply used by many computers right now. We know computer needs power to run. Though it is supplied from AC main line of 110V/220V, internal components of computers run on lower voltages like 12V, 5V, 3.3V even 1V. Different components, different voltages. So this power supply has to take an input of AC 110V-220V and convert it to DC 3.3V,5V,12V etc.

So, if you have one of this power supplies you can easily use that one to utilize that DC voltages. For example, 5V is very common in development boards and other stuffs like it and 12V is very common for amplifier applications.

So where is the problem?

Yes I can use these power supplies for any other purpose, I will just plug in the power supply in the AC main line and start using it. 

No. That's not going to happen. First of all these power supplies are not linear like we talked earlier. ATX power supplies are mainly switching power supplies ( you can find more about switched-mode power supply here ). It uses higher frequency in the transformer thus lowering the size of the whole thing. And ATX power supplies use one of it's pins to determine whether it will stay on or off. 

If you look at a ATX power supply you will see bunch of wires coming out of it (guessing it's non modular). Wires might be color coded. If it's color coded then it is then it is very easy to understand. Yellow for +12V, Blue for -12V, Red for +5V, Orange for +3.3V and Black for Ground or 0V. 

You will also see 20/24 pin header, which actually goes into the motherboard and thus when you press the power button of your computer you can see the power supply turns on. 

The mechanism is there is a Green wire (Standby Power) in that 20/24 pin header which needs to be shorted to the Ground to tell the power supply to turn on. As soon as you press the power button this wire is grounded via the motherboard and the power supply turns on, so does the computer.

So, what to do?


So, just take a small piece of wire and short out the Green Wire and Black wire. If you can't find which is the green wire use a multimeter, if he power supply is in off state and AC supply is provided to it, one of the 20/24 pins should give a +5V reading, that is the green pin. 

You can also use a switch between Green and Black wire to turn on as your require and turn it off after that. 

 I am using this small breadboard jumpers for connecting it.

This is the ratings of the power supply that I'm using. Here you can see the voltages with there corresponding current is printed on it.

Measuring the 12V Rail (Yellow)

Measuring the 5V Rail (RED)

As you can see that after shorting out the green wire, the fan has started spinning and the voltage on the Rails are good.

Sometimes these power supplies might not run properly without any load, in that case just use such a resistor that can draw some current and act as a dummy load. 

A small resistor between 5V rail and ground acting as a dummy load


Why would anyone use one of these?

1. Cheap. This power supplies are relatively cheaper than linear power supplies

2. Small. Size of these power supplies are pretty small comparing to linear power supplies

3. Quality. Power quality of these supplies are good. It has good voltage regulation and lower ripple and noise.

4. Power. These power supplies can provide high amount of current for running most of your project stuffs. Like you want to build an audio amplifier with CD drive on it. These power supplies can save you from hassle of making one linear power supply with very large transformers.

5. Compact. As these power supply units offer different voltages it can be used for multiple purpose. Also you can use a boost converter to convert the 12V to higher voltage levels, there is ample current provided in this rail so you don't have to worry much.

6. Ease of use. This power supplies can easily be modified and used.


1. High Voltage Application. Make sure to ground the power supply unit properly. Don't run the power supply without the metal cover.

2. Don't exceed the current limit labeled on it.

3. Don't Short circuit any other terminals. 

4. Don't touch the power supply while it is on.

You can visit the index page for more posts. Hope You like it.
I'm using this power supply for demonstrating purpose only.

1 comment:

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