ATX power supply minimum load

Question

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 going to give it to me. Since I already have a power supply for my computer, I planned on turning it into an 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 going to turn the 2nd 12V into a power source for my to-be-created DC soldering station. So it's going to 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 ask a new question.

Answer 1

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.

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.

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.

Answer 2

All ATX power supplies are made from a ‘flyback’ switching power converter. They derived their multiple voltages from multiple secondary windings with separate rectifier and output filter capacitors. The flyback switching regulates its output by sensing one of the multiple output with isolated feedback, usually an opto-isolator feedback path to the primary side AC input switcher. Most common is the 5v supply but since newer supplies have more power from the 12 vdc output (due to use of power hungery graphic cards) some use the 12 vdc output instead of 5v for feedback regulation control. To keep the feedback stable a minimum output load often needs to be provided. Typical minimum load is 2 to 4% of rated output current for the voltage supply line used for feedback. Again it may be 5v output or 12v output needing the minimum load resistor. Some newer supply don't need a minimim load. So how do you know if you need min load and should it be on 5v or 12v outputs? If older supply it is 99% sure to be 5v supply. Try it without a load and if output remains off or jumps around you need a minimum load. Don't let it jump around to long as it might damage supply.

The regulating switching has a minimum 'ON' time which was the original reason for minimum load requirement. Newer switching regulators have a pulse skipping mode when load is light or non-existant. These require no minimum load but the pulse skipping does not have the best regulation as the output is lightly 'kissed' periodically just to maintain the approximate output DC voltage. The minimum 'ON' time for the 'kiss' results in more output ripple voltage variance.

Newer supplies may also have power factor correcting in the primary AC side circuitry. It provides better efficiency and has no impact on how the outputs are used.

What the single output voltage feedback means is the only output voltage (5v or 12v) that has the feedback sense control has high regulation accuracy. Other voltage outputs are dependent only on flyback transformer turns ratio and may have their output voltage vary more depending on their output load current demand. Their output voltage may also be impacted by the load on the feedback sensed (5v or 12v) output voltage supply.

Answer 3

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,

Answer 4

As prevoius speaker depending of topolgy you will need some preload. Multiwinding transformer with feedback from only one line will definetely need it to avoid drift of the other power lines, while one line is loaded.

However I found at this website that modern ATX supply may not needed it or only need it on +15, +5 and on the +3V3 line.

Answer 5

i m experimenting these days with my old atx . that voltage spike one user mentioned occurs with 5 watt and 21 watt loads but not with 26 watts , when i turn off the unit .

it still works without voltage changing at the output with one 330 ohm resistance on the 5 volts as the load but i think the other problem needs at least that 26 watts as the minimum load .