# ATX PSU turns off under heavy 12V load, unless increased gradually

Following the well known instructions I've built the PSU from a spare 500W ATX. I've used 10 Ohm resistor for artificial load on 5V line.

When I connect 1A load to 12V line, the ATX shuts off. I need to wait a bit and restart it.

This only happens if I connect the large load rapidly (i.e. turn on the LED driver with around 1A load). When gradually increasing the load, I can reach 2.5A with no issues.

Say I connect a 0.5A load and it works. Let it run for a few minutes, then load it with 1A, let it run again and eventually I can have my 2.5A LED light connected. I can turn off the ATX and turn it on again and it still works.

If I however keep it off for a few minutes and turn on again, connecting anything above ~0.8A causes immediate shutdown.

According to the sticker, 12V lines are rated 15A / 17A. I've tried increasing the load using 3.2 Ohm (10R and 4R7 in parallel) but that didn't help.

Is the ATX faulty or have I overlooked something? The only possible explanation that came to my mind is that the LEDs driver I'm using has some extreme power peak when warming up. So maybe it's not about turning off the PSU but about turning off the LED driver... will test it against some 12V motor

• Are your LEDs just LEDs or do they have an electronic power-supply built in. Add the details into your question (rather than bury them in the comments). Oct 14, 2020 at 21:27
• It appears that the converter's feedback/control network has a "response-to rapid-load-changes" problem. Rapid load change causes a short-time overload because the converter's feedback loop acts slowly. Thus the converter's overload protection kicks in. If I however keep it off for a few minutes and turn on again, connecting anything above ~0.8A causes immediate shutdown. that's because the output capacitors discharge in a few minutes. And, at turn-on, some high peak currents (due to empty capacitors - act like nearly zero-Ohm) flow. And the converter's overload protection kicks in. Oct 14, 2020 at 21:34
• What "well known" instructions, who knows if they are good or bad? ATX power supplies are not good general purpose power supplies. They do not work well with random loads in random situations. Care must be taken not to exceed rated load step, and that the step happens at the rated time. They are especially bad working properly, if all other outputs are unloaded except the 12V, and you disconnect the load so there is suddenly no load at all on any output, the voltage will overshoot and most likely cause the overvoltage protection to shut it down or at least the control loop to go haywire. Oct 14, 2020 at 21:40
• Is the ATX PSU in good condition? I've had one with leaky/burst capacitors that would work OK under no load / light load but fall over if you plugged too much into it as there was not enough capacitance to maintain good power, triggering the PSU to shut down. Oct 15, 2020 at 8:35
• In the ATX specification there is a limit to the rise rate of current. Someone mentioned the value in a comment here about two weeks ago. If your inrush is too large, you may simply be out of specification. Oct 15, 2020 at 9:42