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I know this is a commonly asked question but most of the answers I got are more than 5 years ago.

So basically, My friend found a ATX power supply and was gonna give it to me. Since I already have a power supply for my computer, I planned on turning it into a overpowered bench power supply. It has 1x 3v3, 1x 5v, 2x 12v and a -12v. It also have a writing in the corner v2.2. [that's what he said]

Also, I was gonna turn the 2nd 12v into a power source for my to-be-created dc soldering station. So it's gonna be a DIY bench power supply + soldering station hybrid.

Can anyone tell me anything about the minimum load for each rail?

I found something on the internet that it needs 10ohms on the 5v. So that's 500mA. I also found that it needs 2-4A on the 5v rail as the minimum load. Almost all threads are contradicting each other so I felt I need to recreate a new thread.

Also, I'm a beginner at this so not speaking "idiot or dumb or stupid or other understatement" is very... idk, good I guess.

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Over 20yrs ago a PC PSU would not perform as well as today and required some preload up to 10 % when good performance was expected with no load.

Today I have not had this experience and can easily operate all outputs with no load. So,there is no minimum load spec. The Intel spec is for no damage to the unit but shutdown is permitted.

Since the most common design uses a multiwinding shared transformer to define the output voltage ratios , feedback on only needed on the primary output. That used to be 5V out for current but now is 12V with options for a secondary 12V with independent regulation for less load crosstalk between peripherals and GPU.

Your performance may vary and every design may be different. I might test it then add 5% preload if it shuts down after a transient load on your project,

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ATX Power Supply: is a minimum Load required on each supply?

I would suggest that the answer is still ...YES.

However the answer is more complex if you delve into the individual supplies.
Most supplies will meet the minimum Form Factors spec that @Jim quoted in the comments. However many of the top tier ATX supplies far exceed this spec.

In particular many of the higher powered supplies (500 - 750W and above) have now separated the +12 and +5 converters so they don't interact (co-regulated +12 and +5 supplies are common on lower power supply units (250 - 400W range). For lower power units I'd always recommend you have some pre-load, but it does not matter whether it is on the 12V or 5V supply if they are co-regulated supplies.

To begin to understand what might happen if you have no load, consider this report from TomsHardware from 2011. They show testing of a larger Corsair 550W power supply and compare to other units available at that time.
Notice that they did NOT test down to zero amps but the supply stayed well within the expected ATX regulation limits of +/-5% across a broad range of load current. This unit along with many of the comparison units would benefit from some pre-load. I'd suggest that a minimum of 3-5W load would be adequate in most cases.

enter image description here

Compare the charts from the Corsair unit made in 2011 to that of a bang up to date Seasonic SSR-500SGX, newly updated in 2019.

enter image description here

Again the unit is not tested down to zero load, so even though it's regulation is amazing compared to the Corsair, I'd still suggest that a minimum load be attached.

It's also worth noting that the Seasonic appears to be using co-regulation of the 5v and 3.3V supplies (notice the little bump in the output voltage at about 40W which is a giveaway the supplies are coupled).

The biggest potential problem using an ATX power supply as a general purpose bench supply is that you may have very sudden sharp reductions in power supply current when you disconnect a load. Since the switching regulators are working at some (typically fixed) frequency, you may suddenly turn off the load just when the supply is dumping a large amount of energy into the output cap. This could result in a voltage spike, enough to trip the over voltage sensors.
The resistive load you put on the supply helps ameliorate this problem.

So to sum up: If you are using an ATX supply as a bench supply it is good practice to apply a minimum load to the supplies. You could make this a dynamic load and sense any voltage spike, but this complicates what you wanted as a simple solution. Just using a load resistor is certainly easy, and dissipating say 20W across the supplies an easily implemented solution.
Exactly how much load you need for a given supply depends on the design, but if you measure a supply with no load on it and the output voltages are above +2% of nominal, I would definitely use a pre-load resistor.

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  • \$\begingroup\$ As of now, Thanks for the answer but my PSU is like 5 years old as of now. The build was 4 years old. Also to whoever answered with the Intel guidelines for power supply was a great help. At least. Can anyone link me to the other past versions of it. \$\endgroup\$ – Khyles Gibrian Ramos Jun 26 '19 at 22:47
  • \$\begingroup\$ The Formfatrors.org website can be found in the Wayback machine: web.archive.org/web/20041012173921/http://www.formfactors.org/… ….this discusses cross regulation (3.2.3) and no load operation (3.4.3) ....but you can clearly see they NEVER expected no load on the supplies. \$\endgroup\$ – Jack Creasey Jun 26 '19 at 23:11

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