Convert ATX PSU to Bench Supply

Is it possible to use an ATX power supply unit or PSU from an old PC as a bench top power supply to power 5V logic, but with some limitations.

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.

atx psu

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

20-pin molex connector

 

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

Pin Name Colour Description
1 3.3V   Orange +3.3 VDC
2 3.3V   Orange +3.3 VDC
3 COMMON   Black Ground
4 5V   Red +5 VDC
5 COMMON   Black Ground
6 5V   Red +5 VDC
7 COMMON   Black Ground
8 Pwr_Ok   Grey Power Ok (+5 VDC when power is Ok)
9 +5VSB   Purple +5 VDC Standby Voltage
10 12V   Yellow +12 VDC
11 3.3V   Orange +3.3 VDC
12 -12V   Blue -12 VDC
13 COMMON   Black Ground
14 Pwr_ON   Green Power Supply On (active low)
15 COMMON   Black Ground
16 COMMON   Black Ground
17 COMMON   Black Ground
18 -5V   White -5 VDC
19 5V   Red +5 VDC
20 5V   Red +5 VDC
 

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 connected to ground via the 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.

20-pin molex connections

 

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

24-pin molex connector

 

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

Pin Name Colour Description
1 3.3V   Orange +3.3 VDC
2 3.3V   Orange +3.3 VDC
3 COM   Black Ground
4 5V   Red +5 VDC
5 COM   Black Ground
6 5V   Red +5 VDC
7 COM   Black Ground
8 Pwr_Ok   Grey Power Ok (+5 VDC when power is Ok)
9 +5VSB   Purple +5 VDC Standby Voltage
10 12V   Yellow +12 VDC
11 12V   Yellow +12 VDC
12 3.3V   Orange +3.3 VDC
13 3.3V   Orange +3.3 VDC
14 -12V   Blue -12 VDC
15 COM   Black Ground
16 Pwr_ON   Green Power Supply On (active low)
17 COM   Black Ground
18 COM   Black Ground
19 COM   Black Ground
20 -5V   White -5 VDC
21 +5V   Red +5 VDC
22 +5V   Red +5 VDC
23 +5V   Red +5 VDC
24 COM   Black Ground
 

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%.

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.

78 Comments

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  • P
    Pong

    Instead of using a passive resistor as a load for 5 V. Can we use a DC fan instead? So it will work as additional ventillation for the lab bench. Or using a lamp, maybe ?

  • t
    thabo

    it really works?

  • T
    Trevor Mitson

    Hi. Looks like l have the newer desktop PC power supply, ATX12V 650watt with the 20+4pin connector,would a resistor of 10 Ohms, with a power rating of 20W be OK to use?
    Thanks

  • E
    Eric

    Hi there,

    Hope you’ll be able to answer my question.
    I did the following:
    Connect green with black. 24 pin connector.
    All worked.
    Then, when I cut all the cables, it does not work anymore.
    I only need the 12v.

    Evertime green touches black it spin for less then a second.
    If green touches grey, the same thing happens.
    There’s nothing in the cables or connectors , right. ?

    Hope you can tell me what it is missing.

    Thanks a lot.
    Eric

    • A
      ASHTR

      The problem may be due to some back emf due to its net inductance. The problem can be rectified by adding a diode between the green and black

    • MarkF

      Hi Eric,
      Did you ever solve the problem you described?

      I’m having exactly the same problem – i.e. tested the power supply by connecting the green and black wire and all was OK, but then after cutting the wires, one spin of the fan and then shutdown,

      I’ve tried putting a 10ohm resistor across the 5V to ground as some describe below and also putting a load (a set of LED strip lights) across the 12V to ground, but still no joy.

      Cheers,
      Mark

  • A
    Alex

    Hello! So I got an old PSU which I know for sure works(i powered the PC it was in on before modding it) but after connecting all the stuff you mentioned, when i plug it in it just wheezes and does nothing(not even the 5v standby works).
    The wires are just as in your diagram since they’re labeled on the PSU case, I tried both 5 and 10W dummy loads on the 5V wire, and I still get nothing but wheezing.
    What could be the issue? Thanks a lot!

    • W
      Wollowstone

      Also, check if you have a “new” PSU. You may have to connect the 12V to draw curent from it, instead of the 5V. You can know that by looking at the PSU, under the MINIMUM curent it requires (to be “stable”).

    • W
      Wollowstone

      Have you connected the green wire with a black one? It is about the same as turning the power on from the front of you PC (in addition to turn the real PSU power switch on, normally in the back of the PC).

  • D
    Daniel Mackey

    Thank you for taking the time to write this up. I noticed you didn’t mention 3.3V? We actually use something similar to this to power part of a production amusement attraction in San Diego called The Floor is Lava (https://puzzalarium.com/lava). The 3.3V rail comes in handy for small Ardiunos and their clones that I guess were expecting battery power in portable or wareable applications. You would think you could do everything off one system, but occasionally it’s easier just to stick in a little Arduino to hack something and the 3.3V is helpful.

    Thank you again for taking the time to do the write-up.

  • J
    James B

    Very nice write-up. I just ended up buying an ATX Benchtop Power Board off EBay for less than $10 instead, just plug the 20 pin harness in and it works, no mods needed.

  • P
    Premaratna . M.D.W.

    Very useful tutorial. But I found following things not included therein. Please explain.
    1. There is no -3.3V terminal; thereby 3.3V dual supply is not possible.
    2. There is no reference to other wires, intended for HD, DVD, etc.

  • s
    sharpie

    I’m about to mod an 800W ATX psu and was wondering if I could have multiple circuits instead of having all the orange/red/yellow cables bundled together ?
    The sticker on the side says the 3.3 V rail can handle 25 A, The 5 V rail can handle 30 A and the 12 V rail can handle 125 A.
    I have 4x 3.3V cables, 8x 5V cables, 12x 12V cables with enough 0V black cables to suit.
    If I understand this correctly I could potentially have :
    4 x 3.3V 6.25A outputs
    8 x 5V 3.75A outputs
    12x 12V 10A outputs
    I can’t see why this wouldn’t work……….am I missing something ? I’ve needed a multiple output psu for some time to run to run my telescope mount, cameras and other astronomy equipment. This…if it will work….seems to good to be true.

    • W
      Wollowstone

      You can’t add the amperage, they are listed for the TOTAL amperage for a given voltage. As example, even if the 12V has three isolated “banks”, the PSU label list the maximum of the total from the three banks.

  • J
    John Doe

    > … so their is plenty of power…
    … so *there* is plenty of power…

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