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Convert ATX PSU to Bench Supply

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.

convert 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 to convert ATX PSU to a bench power supply you can not just have the wires left open, luckily both of these issues are easily fixed.

Convert ATX PSU with 20-pin Connector

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 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.

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 (automotive) bulb into the different sockets to see if your attempt to convert ATX PSU has worked. 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.

Convert ATX PSU with 24-pin 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%.

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.

Any old PC power supply unit makes an excellent and cheap bench top power supply for the electronics constructor and the process to convert ATX PSU to a portable power supply is relatively straight forward. 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 to convert ATX PSU to 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!.

219 Comments

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  • Peter Harris

    A lab power supply typically has the capability do deliver an infinitely variable voltage from 0-30V or 0-60V, with control over both voltage and maximum current, while an ATX power supply offers a limited number of fixed level outputs without variable current limiting. If all you want is 12 V or 5V then an ATX power supply is fine, but it does not begin to compare with a proper lab power supply.

    There is also the requirement to attach a load to the 5V line in order to turn it on. The simple answer is a 5 ohm resistor which then runs hot, but a more legant solution is to attach an LED array, which at least provodes light without heat.

  • Terence

    Good project but the -12v -5v etc voltages are of limited current capacities, so will limit the current capacity of voltages derived using them.

  • Reşat

    Is it mandatory to connect 5w resistor to 5v pin?

  • 8000rpm

    Thanks a million. My old 20 pin pwr supply was almost in the trash,….now I have a good bench power supply!!!!

  • Yoshi yodin

    Hi can someone please help I’m completely new at this and wonder is standby necessary or can I continue with conversion without it my unit doesn’t have or must I put wire for standby myself
    Is probably dumb question but like I say I’m still new thanks

    • MICHAEL cardona

      IT’S NOT A DUMB QUESTION,,yes,,one of those wires is for standby purposes; however, you might not be able to use it for the purpose you intend , remember this is a power supply unit you are modifying to suit your needs for a specific power range.

      • Yoshi yodin

        Thank you for your help would one be able to just put in the standby cord if I can find the spot it was in? Would that work? And I much appreciate the understanding of newbie questions..

        • miguel cardona

          Normally, it’s not necessary, ONLY IF YOU, WANT A STANDBY SWITCH, (you want to take a break, the phone is ringing and you want to answer, you’re hungry), otherwise omit the standby. Think of it like on a computer/laptop, you’re taking a test and you NEED to go to the bathroom….Good Luck!!

  • Michael Cardona

    Thanks, I DO UNDERSTAND,,and your reply is very appreciated.

  • michael cardona

    Hello: Please excuse me for my lack of understanding; however, going back to your statement on keeping the Mole-X (20 or 24 pin); it’s about the excess wires from these pin Molex units; If I only need (1) one Black wire for the power switch and cut off the others completely and group together the individual wires keeping the same colours together, reds to reds, blacks to blacks etc, I shouldn’t have any problems; I already cut all the wires from a 20 pin connector and several other connectors and I ended up with a total of 13 Black wires, 7 Red, 5 Orange, 3 Yellow and (1) each of Green, Blue, Purple, Brown, and Grey; I want to be as certain as possible (i’m unemployed) before I burn up my project without ever using it; Please note that your coverage HAS helped me a lot, BUT, I still need that clarification; thank you very much.

    • Wayne Storr

      You can cut-off all your other coloured wires if you wish to do that and have just one red wire and one black wire (or any other colour). But your load current will be limited to the diameter, (AWG or mm2) of the single core used.

  • Johnny

    I would like to off the topic if you would allow it my question is what is your oppinion of electronic simulators

    • Wayne Storr

      Electronic, Schematic, CAD and Circuit-based simulators are very good these days but students relay too much on the results from their perfect component simulations. The real world will always give different results.

  • Johnny

    E xcellent

  • DrGhobar

    You said: “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.” But when I connect my device (which needs 7 volts) to +12 and +5 of my ATX power supply, it shuts down. What is your solution?

  • محمود صداقت

    How much current you can get with 24v (+12~-12)?

  • Stephen Dittoe

    For the out-of-the-box solution, try purchasing an 24Pin XH-M229 or 20Pin GY19513 ATX Power Adapter Board.

  • Barabasz

    Can I get 1.67v from the 5v line and 3.33v line?

    • Wayne Storr

      Texas Instruments do a DC-to-DC converter module PTH04070W which provides an adjustable output voltage of 0.9 to 3.6V from a 3.3V or 5V input voltage

  • David

    You mentioned combine -5v and +5v to get 10v out put, so do i wire them up? In series or parallel? Thanks

  • zepp

    welcome to this site, it’s about connectors & wire cable & tool & adapters & antenna. This company is directly operated by the factory, with complete goods, fast delivery, high quality and low price.emmmm,very nice this is site:

  • Mohamed

    how to get 60 volts from power supply of computer without inverter

  • Harpreet Singh

    Childish stuff.

  • Tony

    It would be great to see an article on the explanation and circuitry behind the required 5v rail load. How is that circuitry incorporated into the psu? How does the detection of a load work?

    I’ve seen a lot of articles on atx lab supplies etc but I’m not aware of any article on the required load detection circuitry.

    • Wayne Storr

      That’s because any “load detection circuitry” is specific to the circuit design of their PSU. Then please contact the manufacturer of your ATX PSU and ask them if they are willing to reveal their circuit design.

      • Tony

        I realise that I was simply saying it would be good to see an article on the load detection technique used in any atx supply not all atx supplies. Even I who has limited experience could likely reverse engineer a detection circuit on a atx supply. It could even be as simple as a comparator detecting voltage drop across a shunt resistor which would not require contacting the manufacture to work out.

        I appreciate this article anyway.

  • Asitha Pathirage

    Dear Sir/Madam,
    Please accept my congratulations on your extremely interesting and very well explained tutorials. I am a senior citizen, a retired Architect. But from my school days I had a interest in Electronics. During this lockdown my TV broke. I just wondered to see whether I can try to repair it with the help of a friend who’s field is electronics. But my knowledge of it is the one we had around 60s.
    In the mean time I came across your site, sort of accidently. It took me back to my Advanced Level in school days. Your language is superb, things are well explained in very simple but clear terms.
    Thanks very much and wish you all success in all your affairs.

    Archt. Asitha Pathirage

    • GENE

      I AM A SENIOR CITIZEN TOO. I STUDIED ELECTRONICS IN THE 60’S (USAF). I WORK ON LIMITED BUDGET. I AM NOW 79 YEARS OOOLD.
      YOU ANSWERED SEVERAL CONCERNS I HAD ABOUT THE VOLTAGE COMBINATIONS.

      THANK YOU,

      GENE MALLARD

  • Floyd Graham

    My apologies if this is too far off topic, but I hope someone following this thread is likely to have an insight or suggestion:

    Is there a reasonably simple way to configure a modern ATX PSU to provide +12VDC for the standby voltage, rather than +5VDC? I’m looking to replace a proprietary Dell PSU with one of higher wattage, but my system requires +12V standby.

    I’m sure there are any number of kludgy solutions. I’m hoping to just have one PSU, and inside the case, at that 🙂