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Converting ATX Power Supply to Lab Bench Power Supply F.A.Q.

Since I’m receiving at least a couple of questions each day about my tutorial video on Converting Computer ATX Power Supply to Lab Bench Power Supply and I can’t answer them all, I thought it’s time to answer to the most frequently asked questions. If you think that I forgot to highlight some important question(s), you can write that in the comments below.

  1. What are the values of current limiting resistors for LEDs?
  2. Why would you need two switches?
  3. How can I know where to connect dummy load resistors(to +5V rail or to +12V rail)?
  4. Is it nessesary to keep all the wires?
  5. Why would you need those dummy load resistors?
  6. What if I can’t find these dummy load resistor values?
  7. I can’t find brown (3.3V sense) wire in my power supply – what should I do?
  8. After switching on, my PSU don’t want to switch off from a small(between POWER_ON and GND) switch.
  9. Why would anyone need -5V and -12V rails?
  10. Where to connect POWER_GOOD (gray wire)?
  11. I can’t get rated current on 12V rail. Why?
  12. Why waste all that heatshrink?

What are the values of current limiting resistors for LEDs?
Short answer: small 470Ω-1kΩ (whichever you can find) resistor.
Long answer: LEDs should be fed with constant current(NOT constant voltage) and ATX power supply gives us only constant voltage. So to limit the current through our LEDs, we need to put a small resistor in series with each LED. Most of the indicator LEDs will be bright enough with about 3mA to 10mA of current and will have about 1.5V voltage drop. Since we are going to power our LEDs from 5V rail, and LED voltage drop would be about 1.5V, then we will get following formula to calculate resistor value:

R = (Vpsu – Vled_drop)/5mA
R = (5V – 1.5V)/5mA = 700 Ohms

The smaller resistor value – the higher LED brightness will be.

Why would you need two switches?
First mains switch switches mains power on and off. The same as if you would plug in or pull out PSU in and from power outlet. This switch is not mandatory. ATX power supplies are designed to be plugged in to mains indefinitely. Only exception if you walk out for a long time, you’d better pull the plug.

The second low voltage and low current switch usually is mandatory. You see, ATX power supplies are designed to be switched on by POWER_ON signal from computer’s main board after it already plugged in to mains. Now main board needs to get some power to operate in standby mode and to send POWER_ON signal to power supply, to swith it on. Because of that all ATX PSUs have not one but TWO SEPARATE power supplies inside. One is the main power supply, which can give you all those huge currents and different voltages, and the other is low-power 5V only stand-by power supply which can give a couple of amps to main board while main power supply is off.
So, to switch it on correctly we need to plug it in to mains first, and only then use small switch to connect POWER_ON to GND.

How can I know where to connect dummy load resistors(to +5V rail or to +12V rail)?
If your PSU was made before year 2000 then most likely you need to connect resistors between +5V rail and GND.
If PSU was made in year 2000 or later then resistors should go between +12V rail and GND.
There might be some exceptions to those rules. You should pay close attention to power supplies that were made between 1999 and 2001. There’s two standards: old one – ATX, and a new one ATX12V. Read carefully sticker on your power supply to determine which standard it conforms to.

Is it nessesary to keep all the wires?
No, it is not nessesary to keep all those wires. As a rule of thumb each wire that goes to Molex or SATA power connector can carry about 3A. you can calculate how many wires you need to keep for each rail by dividing each rail’s maximum current by 3A.
And for GND take combined number of wires for all the rails (it’s in case that you might be using all the output rails in parallel).

Why would you need those dummy load resistors?
As I said in the video, ATX power supply is a Switch-Mode Power Supply. And, unlike linear, switch-mode PSU require some output load to operate properly (or to operate at all).
Some ATX power supplies already have a small dummy load resistors inside, so they can work without load simply by connecting POWER_ON wire to GND.
But there’s a catch! Even if power supply has dummy load resistor built-in, it might not give you a stable output voltages. Without external high-power dummy load resistor, PSU’s output voltage without load can substantially differ from output voltage with some load applied. In some extreme cases you can get up to 2V difference.
You need to decide whether you need a stable output voltage or not, depending on what you will be using this PSU for. For example light bulb, or car stereo don’t need stable output voltage. But if to use it as a lab bench power supply, it is better to have a nice steady output.
For more information google for “Power Supply Load Regulation”.

What if I can’t find these dummy load resistor values?
First read answer to previous question. You can take a bit higher resistor values(but not lower). Let’s say if you don’t have 10 Ohms resistor, you can take 20 or 30 Ohms. Or combine some resistors in series or parallel(if you don’t know how to do that, google is your friend!).

But always take into account how much power will be dissipated on each resistor! It should be about two or three times less than resistor is rated for.

To calculate how much power will be dissipated use simple Ohm’s Law:

P = V2/R, where:
V – voltage across resistor (5V or 12V – depends on which rail the resistor will go across),
R – resistance.

I can’t find brown (3.3V sense) wire in my power supply – what should I do?
Nothing. If your PSU doesn’t have that wire, then you don’t need it.

After switching on, my PSU don’t want to switch off from a small(between POWER_ON and GND) switch.
In that case connect small 10k resistor across POWER_ON and +5V_STANDBY.

Why would anyone need -5V and -12V rails?
-5V and -12V rails is actually might be very useful in many cases. For example you can get 24V output between -12V and +12V rails or 8.3V between -5V and +3.3V rails, etc. Or supply dual (-5V)-GND-(+5V) rails to power an operational amplifier (or (-12V)-GND-(+12V)).

But remember that the maximum output current will be limited to the current that -5V or -12V can supply(usually about a couple of amps).

Where to connect POWER_GOOD (gray wire)?
You can cut POWER_GOOD wire off. You don’t need that wire for a bench power supply.
Since after turning on a power supply, voltage across outputs cannot rise momentarily, it’ll take some time(like 200 milliseconds, or so) to settle down at predetermined level. And because of that POWER_GOOD signal is used by computer mainboards to determine whether voltages are OK an if CPU and other peripherals can start to draw power.

I can’t get rated current on 12V rail. Why?
This could be because of cross-regulation issues or maybe you have a cheap and crappy power supply. If it’s not latter, than you might try to put some dummy load on both +5V AND +12V rails. For example, you can’t get nothing even close to rated current from +12V rail. In this case you need to put some load across +5V rail. And vice-versa, if you can’t get current you need from +5V rail, then put some load across +12V rail. I can’t tell you how much load you might need, it depends on concrete power supply. You might want to start with small load and work your way up.

Why waste all that heatshrink?
Heat shrink around mains wires is mandatory, because you can’t mix high-voltage wires with low-voltage ones. It looks good on video. If PSU would be subjected to vibration, heat shrink would protect wires from sharp heatsink edges. Have you seen heat-shrunk salami? No? ;-)

53 comments

3 pings

  1. John Sw says:

    Hi Phil. Came across this info which I thought was rather inventive and allows for different interpretations of the ATX supply:

    Video: http://www.youtube.com/watch?v=-zRFwJdTBdw

    Author’s website: http://www.dansworkshop.com/2012/01/benchtop-power-supply/

    Cheers ~ John

    1. JumperOne says:

      Hi John,
      Thanks for the link! As he said in the video, it’s not a new way of converting computer PSUs. Many online DIY stores, like SparkFun and others, carry breakout boards which do exactly the same but without dummy load built-in.
      I think whether to use external binding posts rack with ATX connector or drilling holes in PSU case and installing binding posts – it depends on needs and preferences.
      In this case I prefer to have very small portable power supply without all those looms of wires, because I carry it around a lot.

      Take care,
      Phil

      1. Richard says:

        Did u draft an electrical wiring diagram ? # it would be great if u could post that as well

  2. Michael Stephens says:

    Instead of using a load resistor I used 1 of 50 spare hard drives I’ve collected over the many years. Chances are if you have a spare power supply.. you probably have a junker hard drive as well. Not sure what that does for power efficiency etc. Its just simple and straightforward.

    1. JumperOne says:

      Why not! I think it’s pretty cool to have a spinning hard drive as a dummy load!

  3. electronicsprototypes says:

    Great FAQ!
    this FAQ has answered many of my questions
    thank a lot

  4. VIEIRA says:

    Olá, moro no brasil e suas explicações foram uteis, voce é fera, obrigado!!!

  5. VIEIRA says:

    Hello, I live in Brazil and his explanations were useful, you’re beast, thank you!

  6. Brett says:

    Hi I have a old PSU 300 watt. I have all wires you do in the video but I have one additional wire that is pink. I followed it back to the board it says +5. Is it safe to join it to the other my other +5′s or should I just cut it and seal it off?

    Thanks.

  7. Andres says:

    so dose the brown wire connect to the orange

  8. Andres says:

    also how can you attach a fuse

  9. Colin says:

    I was about to start making my own lab power supply and while looking for some tips I came across your YouTube clip.
    I must say, this is absolutely the best produced tutorial, both video quality and information value video clip I have ever seen.

    Congratulations on a job well done!

    I’m off to start my power supply modifications.

    Cheers, Colin.

  10. giorgio says:

    hello, I wanted to know one thing …
    these wires striped what should I connect them?
    http://img849.imageshack.us/img849/310/dscf0434n.jpg

  11. billib says:

    Great tutorial, I’m getting ready to buy an ATX
    power supply to make a bench top power supply.
    Your video has given me the confidence to give it
    a try, THANK YOU!
    So, should I buy a 350 or a 750 ATX and why?
    Thanks!

  12. Lucien says:

    I have a 50 ohm 10 watt ceramic resistor hanging around, can I use this for a dummy load?

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  16. Kevin says:

    hi, ive got a new ATX i want to convert, & ive got a Question.

    how to determine whether the power resistor goes onto the 5V or 12V? & what range of resistance/wattage values will work?

    my atx has no white wire. its a 500W(20A@12V & 32A@5V) i will primarily be using 12V, but at high Amps for long durations.

    thanks

  17. Bert says:

    Hello
    I have converted a ATX to bench power supply and works well. I just wanted to convert one to power supply for a waeco 12volt freezer box. I tried it before modification and it worked fine. I cut of all the excess wires fitted a lighter socket and a led to show power on and a switch between grey and earth for power on. After doing all this, it powers up good but as soon as the fridge/freezer comes on I can hear a very faint click in the power supply and the power goes off. Any idea where to start looking for this problem?
    I realy like your videos, very informative.
    I don’t know who to go to with this question so I thought you would propably know.
    Thanks in advance.
    Bert

  18. nj55 says:

    hi,
    i’ve got 2 other wires.
    the purple one and a green one, what can i do with those wires?
    and what voltages are those rated?

  19. Markus says:

    Hi,

    I successfully converted my ATX psu. I want to use it for RC battery charging. However as soon as the charger starts charging the battery with 10A charging amperage the voltage of the psu drops below 10,5V and the charger aborts operation.

    Any ideas why this happens?

    Cheers,
    Markus

  20. Alen says:

    Can i use internal fan as dummy load? i tried and its working but is there any catch?

  21. Steve Marshall says:

    Phil: I am just curious, but instead of binding posts, would it be better to use Anderson Powerpole Panel that one can buy on Ebay, Amazon, etc? I am coming up with other ideas as well. I’m an Amateur Radio Operator and working on upgrading a few things and an ATX PSU, would also be an a great backup

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  23. Wil says:

    I see that someone else asked this same question, did you connect the brown 3.3V sense wire to the orange 3.3v wires? Its a bit hard to see that in the video but going by your tutorial on the wires at the start it sounds like thats what happens.

  24. Eugen Mihailescu says:

    How have you made your PSU label? It looks like a made-to-measure sticker, doesn’t it?

  25. czajkoz says:

    Hi
    I have a slim dell psu model nps-180bb A rev 02, it has 12A on +5V and 10A on 12V.
    What load i shuld use here 10W 22OHM, does the values must be the sam
    Thanks

  26. Dave says:

    Excellent video. Now go to the next step of voltage & current control. Quite a simple add on. You can get cheap pre-built LED displays and controllers on ebay. I look forward to your next instalment.

  27. Dave says:

    I forgot to say, I’ll supply the parts of your choice. Just let
    t me know what you want.

    Get the children interested..

  28. matt f says:

    hi im wanting to build a larger power supply at what point would they become unstable running them in a series. if i ran 10 in series for 120v would they self destruct?

  29. Kees says:

    Hello Dave, i have a Chieftech CFT-500-A12s.
    Can i use it for rebuld.?

  30. don doerner says:

    Can I do the same modification to an intel ATX 2.03. EN-8304946-MODEL HPC-300-101 REV:C0,SERIAL #00121769.
    DO YOU HAVE A TUTTORIAL FOR THIS POWER SUPPLY?
    THANKS
    DON

  31. Patrick says:

    Instead of a bunch of different adapters i hope your power supply can work for me.
    I want to connect a couple of different decives.

    4 x 12v (router and other similar devies)
    I want to conntect 2 mini pc’s and some screens to this power supply.

    The 2 mini pc’s have this adapter “90W Power Adapter (19V)”
    I think the touchscreen display’s are 15V.

    Is there a way to connect this all at one good power supply and how should i do this.

    I hope it works because it give me the perfect and most clean solution.

  32. atx says:

    can this power a 30a600w car amplifier or do you have to increase the amps if so how do you do that?

  33. Mark Adams says:

    Great video on p/s conversion.
    Not the entire ham community has accepted this as a viable option. For the experimenter and lower power level operators, it’s perfect.
    KC0CCF

  34. Daniel says:

    Excellent article.

    There is a new website that provides a search engine for replacements of integrated circuits, diodes, zener diodes, transistors and other semiconductors: http://www.icreplacements.com

  35. Nicolas Lambert says:

    Hi !
    Thanks for your tutorial. I used it for my own version of power supply hack.
    For thoses who are interested, I put some details about how to figure out the appropriate resistance values (ohms and watts) on my blog.

  36. milad says:

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  39. Ciaran Doidge says:

    Hi,

    I have made a bench top power supply from a bestec ATX-1523F. When connecting the power the PSU makes a quiet buzzing noise and there appears to be no voltage over any of the terminals. It did turn on once and was fine. What could this mean??

  40. Rick says:

    Top shelf video, Mr. Levchenko. Thanks for putting in all that time and effort so the rest of us can have a chance to do this project. There’s a lot of conflicting information online regarding the details of converting an ATX for bench use, but I think you cleared-up a lot of doubts I had before. This FAQ is a big help, too.
    I.O.U. one (1) pitcher of beer.

  41. Bubs says:

    I did this conversion on an older PSU which has the white -5v wire. The dummy load still needed to go on the 12v rail for this particular PSU. It had a “Pentium 4″ sticker on the side of it, which tells me the PSU must have been made sometime later in the year 2000 or early 2001.

    I couldn’t find any 5-ohm, 10-watt resistors like those used in the video, but there are 10-ohm, 10-watt units everywhere around here, so I just used that. It works fine. Thank you, Phil; your guide really helped me out.

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  1. Converting ATX Power Supply to Lab Bench Power Supply F.A.Q. « adafruit industries blog says:

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