Convert ATX PSU to Bench Supply

The standard computer power supply unit (PSU) turns the incoming 110V or 220VAC (alternating current) into various DC (direct current) output voltages suitable for powering the computer’s internal components and with a little bit of imagination it is possible to convert ATX PSU to a bench power supply.

Most computer PSU’s range from about 150W up to 500W so there is plenty of power. The original ATX standard connector used for powering the motherboard was a single 20-pin Molex that has all the required +12VDC and +5VDC voltages with huge output currents and short circuit protection as well as a Power-ON wire that allows the PC’s software to turn “OFF” the PSU on shut down.

Firstly and more importantly before you start to convert ATX PSU, make sure that the PSU is unplugged from the mains supply and discharged by letting it sit unconnected for several minutes before you start. This is important! as it could result in a potentially dangerous or even lethal situation due to the high voltages inside the PSU if you decide to dismantle it. Also make sure that the metal box of the PSU is correctly earthed or grounded. You are responsible for your own safety!.

We can not just simply plug the PSU into the mains supply and expect to get the required 5 or 12 volts output. The standard PC power supply unit has two safety mechanisms that prevent it from being switched “ON” without the motherboard attached.

• Number 1, the PSU requires a “Power-ON” zero voltage signal to start up similar to the “ON-OFF” switch on the front of a PC.
• Number 2, for the PSU to correctly regulate the +5V output voltage it needs to have some sort of load attached, at least 5W to trick the PSU into thinking its attached to the motherboard

Unfortunately you can not just have the wires left open, luckily both of these issues are easily fixed.

There are several different coloured wires attached to the 20-pin ATX connector providing several different voltage outputs such as +3.3V, +5V, +12V, -12V, -5V as well as a number of black ground wires and a couple signal wires as shown in the following image along with their colour-code and description.

20-pin Molex ATX Connector

Pin outs of the 20-pin connector with the colours of the wires used in a standard ATX PSU connector.

There are a number of ways to convert a standard computer ATX power supply unit into a usable bench top power supply. You can keep the 20-pin Molex connector attached and connect directly into it or cut it off completely and group together the individual wires keeping the same colours together, reds to reds, blacks to blacks etc.

I cut off the connector to have access to the individual wires and connected them into a screw connector strip to give me a higher amperage output for both the +5V and +12V supplies. You can connect the same coloured wires together using crimp connectors or posts, is the same thing. Some of the other individual coloured wires we need to keep separate as detailed below.

To start up a stand alone PSU for either testing purposes or as a bench power supply, we need to short together pin 14 – Green (Power-ON) to one of the common black wires (ground) which is how the motherboard tells the power supply to turn “ON”. Luckily, pin 15 – Black is next to it so I connected a switch between the Pwr_On signal (pin 14) and Ground (pin 15). When pin 14 is momentarily or permanently connected to ground via a switch, the power supply will turn-ON.

Next we need to provide a small load on the +5V (red wires) output to trick the PSU into thinking its attached to the motherboard and to keep the power supply in the “ON” mode. To do this we have to connect a large resistor of 10 Ohms or less, with a standard power rating of 5W to 10W across the +5V output using just one set of the red and black wires, pins 3 and 4 will do.

Remembering from Ohms Law that the power (P), developed in a resistor is given by the equation of: P = I² × R or P = V² / R, where: P = power developed in the resistor in watts (W), I = current through the resistor in amps (A), R = resistance of the resistor in ohms (ohm) and V = voltage across the resistor in volts (V). The voltage will be +5V and the power required is 5W or above. Then any standard power resistor below 5 Ohms will do. Remember though that this resistor will get HOT! so make sure its out of the way.

One other option we have is to use pin 8 – Grey (Pwr_Ok) as a visual indication that the PSU has started up correctly and is ready to operate. The Pwr_Ok signal goes high (+5V) when the power supply has settled down after its initial start up, and all the voltages are within their proper tolerance ranges. I used a red LED in series with a 220 Ohm current limiting resistor connected between pins 8 and pin 7, (ground) for this power ready light but anything similar will do, its only indication.

Testing the Power Supply

Once assembled you should end up with something like this.

When you plug the PSU into the wall socket and turn the switch “ON” at the back of the power supply (if it has one), only two voltages should be present at the connector. One is pin 14 the Pwr_ON green wire which will have +5V on it. The second is pin 9 the +5V Standby (+5VSB) purple wire which should also have +5V on it.

This standby voltage, is used for the motherboard’s power control buttons, Wake on LAN feature, etc and typically provides about 500mA of current, even when the main DC outputs are “OFF”, so it can be useful as a permanent +5V supply for small power uses without the need to turn the PSU “fully-ON”.

Some newer ATX12V power supplies may have “voltage sense” wires that need to be connected to the actual voltage wires for proper operation. In the main power cables you should now have three red wires (+5V) all connected together and three black wires (0V) connected together as the others have been used for the switch and LED. Also connect together the three orange wires to give a +3.3V output if you require it to power smaller devices or micro-controller boards.

If you have only two orange wires, you may have a brown wire instead which must be connected with the orange’s, the +3.3V for the unit to be able to power up. If you only have three red wires, another wire (sometimes pink) must be connected to them. But check this first.

If everything looks ok then we are good to go and the PSU should switch “ON” giving you a very cheap bench top power supply. You can test the output voltages using a multimeter or connecting a 12V bulb into the different sockets to see if the PSU works. The voltage combinations that can be outputted by the PSU are 24v (+12, -12), 17v (+5, -12), 12v (+12, 0), 10v (+5, -5), 7v (+12, +5), 5v (+5, 0) which should be sufficient for most electronics circuits.

You could also connect a LM317 Adjustable Voltage Regulator, a 5k adjustable potentiometer, a 240 Ohm resistor for biasing and a couple of smoothing capacitors across the +12V supply to give a separate adjustable output voltage from about 2.0 to 12 volts but this is an additional feature.

The 24-pin Molex ATX Connector

In newer desktop PC’s, version 2 ATX power supplies are used called ATX12V. The old 20-pin connector has been replaced by a larger 24-pin Molex connector or even a 20+4pin connector. The four additional pins are: two additional pins numbered 11 and 12 are +12v (yellow), and +3.3v (orange) and the two additional pins numbered 23 and 24 are +5v (red), and ground (black) respectively. The newer ATX12V pin outs and colours are given in the following table for reference.

24-pin Molex ATX Connector

Pin outs of the 24-pin connector with their respective colours of the wires in the PSU cables.

The newer type ATX12V PSU’s are a little more tricky to convert as they use a ‘soft’ power switch function and require a much larger external load resistance. To get them to start-up, or switch-ON, the supply must be loaded to at least 20W or 10% of the rated power for the larger 600W+ PSU’s. Anything below this the power supply may run, but regulation will be very poor less than 50%.

Also some of the newer and larger wattage PSU’s require pin 14 – Green (Power-ON) to be connected permantly to ground using a SPST switch. Obviously each type of PSU is different from different manufacturers, so you need to find what works for you.

Again the voltages that can be output by this unit are the same as before 24v (+12, -12), 17v (+5, -12), 12v (+12, 0), 10v (+5, -5), 7v (+12, +5), 5v (+5, 0). Note that some ATX12V power supplies with a 24-pin motherboard connector may not have the -5V (pin 20) white lead. In this case use the older ATX power supplies with a 20-pin connector above if you need the additional -5V supply.

An old PC power supply unit makes an excellent and cheap bench top power supply for the electronics constructor. The power supply unit uses switching regulators to maintain a constant supply with good regulation and short circuit protection cause the unit to shutdown and be re-powered immediately if something goes wrong.

The only downside with using an ATX PSU as a bench power supply is that the cooling fan’s rpm responds to the amount of current being drawn from the PSU so can get a little noisy. Also the ATX PSU requires a certain amount of fresh air to keep it cool inside which may not be possible when laid onto a bench.

All in all, converting an ATX PSU to a bench power supply is an easy project with many uses. Not bad for something that would otherwise get thrown away but remember, unplug from the mains supply first before you start any modifications as you are the only person responsible for your own safety!.