12V Power Supply

In this article we will take brief look at how a ATX PSU can be utilized for various projects.

Bestec ATX PSU

In this article we will take a brief look at how a ATX PSU can be utilized for powering various projects.

WARNING: Before attempting to work on any electrical components it is essential that you are competent, isolate all live conductors and prove this with a reliable instrument. This is imperative when mains voltage is involved, the PSU must be disconnected (UNPLUGGED). Also note there may be undischarged capacitors that also pose a shock risk, allow these to discharged before handling. Carry out the following at your own risk!

ATX (Advanced Technology eXtended) power supply units are used in computers. As a result they are readily available and cheap.

The primary purpose of a PSU is to turn the incoming 230v AC (alternating current) into DC (Direct Current), in the case of a ATX PSU, various outputs for the computers components.

They come with different specifications which are stated on the units label, an example of this can be seen below.

First step

Confirm that the PSU is suitable for the desired use. For example a reprap style FFF printer: controls, motors and a single hotend using approximately 5 Amps and a heatedbed at 8 Amps. A total of 13Amps at 12 Volts gives us an estimated max 156W.


We need to make a few changes to replicate a motherboard connection to tell the PSU to turn on and deliver power. This is achieved by locating the PS _ON Green wire and connecting to ground, in this case i used 1oo ohm resistor connected to any of the ground wires (black).

ATX PSU Molex 24pin Motherboard connector

At this stage the PSU should turn on and output power. Now is a good time to verify the outputs with a volt meter. Yellow (+12Va) and Red (+5V) to ground +/-5%.


We need to apply a dummy load to the +5V rail. A resistor or load such as a light can be used here. See the example below using a 12V 50w halogen light, at 5V the additional load is 10 Amps. Be aware that the light or resistor used will get hot!

50W 12V halogen connected to +5V rail

At this stage the PSU should be providing a stable usable 12V supply.

The +3.3V does not have a use for this application however joining one of the +3.3V wires(Orange) to +3.3VS (Brown Sense wire) should remove any issues the psu has with detecting voltage drop of the 3.3V rail.

Last but not least is to check the current carrying capacity of the cables you have selected and ensure that this is greater than the maximum current drawn.


We can tidy up all the wiring and connect to suitable terminal blocks. Braiding of the cables can help reduce tangles and keep things neat.

As this is supplying a ramps 1.4 board with a heated bed i decided to use the +12Va (Yellow) for the 11A connection and +12Vb for the 5A connection.

One last suggestion is to verify the voltage and polarity before making the final connection to sensitive circuits to avoid damage.

If you feel i have left something out and would like to add to the above please leave a comment below.


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