0

I am currently trying to convert my ATX PSU to lab bench PSU. But I ran into a problem. I want to attach a dummy load, but I am not sure if I should use 12V or 5V.

On label it says that

+5V .. 21A
+5V sb .. 2A

+12V cpu .. 15A
+12V a .. 10A
+12v b .. 11A

Should I add those amperages together to see where is more power? Or how does it work when they are separated like that?

I am soldering all 12V together and 5V together too, so I think it adds up, but i am not sure, so I want to ask you.

Bazilion
  • 35
  • 1
  • 1
    Just out of curiosity: what do you plan to do with this lab bench PSU? – dim Sep 23 '16 at 10:28
  • 3
    You're not the first one looking into this and I always give the same advise: don't. Why not ? Because these ATX supplies can deliver very large currents so if you make a short in your project large currents will flow and your wires will smoke and catch fire. A proper lab supply has current limiting that will prevent this from happening. – Bimpelrekkie Sep 23 '16 at 11:26
  • 2
    You're going to solder together the +5V and the +5Vsb? That should be fun, considering that the sb stands for stand by and is available when the PSU is "off"... – marcelm Sep 23 '16 at 11:46
  • 1
    Soldering the 12 and 5 volt wires together will result in flames. – MadHatter Jun 09 '19 at 14:25
  • I wouldn't recommend you to convert cheap ATX supplies, from my experience. Unless you take care of the electrical noise. – User Oct 15 '20 at 07:38
  • 2
    @madhatter are you really mad? The OP never said he was soldering 5v to 12v. It is clear he is soldering the various 5v lines together and the various 12v lines together. The only way the OP's sentance could be misunderstood is if you have no experience with using ATX power supplies for anything but a desktop PC power supply. If you do have experience, Please stop belittling the OP by feigning a misunderstanding of their question. – GT Electronics Feb 19 '22 at 14:39
  • Some boards exist to convert ATX to supply lab ( with some added fuses). Don't modify ATX and only uses these boards. Nothing to wire, just 'connect'. But as stated,it is not really a true lab supply. – Antonio51 Feb 18 '23 at 09:46
  • https://www.dennisdeal.com/products/geekcreit-xh-m229-desktop-computer-chassis-power-supply-module-atx-transfer-board-power-output-terminal-module_1418198?gclid=EAIaIQobChMI6b6Aseqe_QIVg7WyCh12cAJNEAQYAyABEgLtDfD_BwE&v=d3dcf429c679 – Antonio51 Feb 18 '23 at 10:11

2 Answers2

1

Usually, the sticker on the PSU will tell you about the maximum allowed power for the rails and combined power (two rails together) and total power.

It looks something like this:

ATX PSU Sticker

(taken from: http://www.xbitlabs.com/articles/cases/display/antec-psu-roundup_10.html)

For this PSU, the 3.3V and the 5V together are able to supply 140W whereas the single power values would add up to ~180W. The 12V rails are able to deliver 504W of power (instead of 648W).

So with the numbers you tell us, we can't say. If your PSU does not have a specification like this, you are out of luck and nobody can tell you (the manufacturer could, but if he didn't print it on the sticker, he isn't likely to tell you).

You should think about some properties you have to implement for a lab power supply. You want really low noise, settable output current limit with voltage foldback. Both of which you don't get from an ATX PSU.

Low output capacitance is another thing which is likely not implemented.

I'm not saying you plan isn't doable, but for a proper bench power supply, you have to add a lot of stuff and you aren't experienced, so in the end you might spend a lot more than just buying a new or used lab supply.

Arsenal
  • 17,674
  • 1
  • 34
  • 60
0

First thing you should check the specs of the PSU (label) as said in Arsenal's post.

If you can or cannot join in the wires it's a matter of internal design.

On cheaper PSUs, all wires (like yellow/12V) are fused together internally in the same spot. In this situation, you can join all yellow wires and get max current.

On some of the more expensive models, there are multiple 12V rails. A rail may be for CPU and 2 or 3 may be separate for Video card / other components. In this case joining all 12V wires can cause damage, because they are powered by separate circuits.

So if you want to do something like soldering them together, open up the PSU and see how they are connected. And while you're at that, you might as well replace the wires with only a pair that is thick enough to support all the current you need.

Overmind
  • 480
  • 4
  • 9
  • If you have to ask the OP's kind of question you probably should not start opening up a mains-powered, high-current PSU... – nekomatic Oct 07 '19 at 15:50