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Built ATX PSU to Multi-Volt power supply. Voltage drops when applying load, what may be wrong?

I converted my 500w PSU following instructions in here. The PSU boasts 480w real power on with 20amps on the 12v rail. I thought I had done everything correctly, and the Power supply gives correct voltage on all binding posts when there is no load, but when I connect a simple 5w lightbulb to the 12v the fan spins down and the light is not as bright as I would expect. Also when measuring the output voltage it now is down to 8,30 volts. Measuring the amp shows 0,33A. When trying out a 21w lightbulb the problem gets more visible, as the light flickers and there is a ticking sound from the PSU. The voltage now travels between 8,10 and 8,16 volts, the amp showing 1,31 amps.

I tried grounding directly to the PSU chassis, I cut the wires to the 5v USB ports (these did not work anyway, perhaps because I have no sandbar resistor?) and also tried to give power from the wires only related to the 24 PIN molex connector or only from the 4 pin connectors, all with the same symptoms. Anyone have any clue as to what may cause this anomaly?

My plan was to use the power supply as a substitute for a battery to run power in my caravan, converting straight from the 220v. There are only a set of lights and a small water pump running on the circuit, adding up to approximately 200 watts if everything is turned  on. I figured the PSU would be sufficient to do this, but maybe I am mistaken?

StigW (author) 1 year ago

It seems the contraption does work, even if it still makes a clicking noise after installed in my caravan. The lights and the water pump in the caravan works well without the psu getting overly hot. The noise does not bother me as the psu is hidden underneath the sofa. I'll keep testing but switch to battery when I go to bed, don't want to take any unnecessary risk in case of fire. Maybe I can get hold of a more powerful psu at a later stage, but for now it seems sufficient.

StigW (author) 1 year ago

Thanx for the answer! No, since I have not got available the recommended resistor (it would be somewhere around 10w 10Ohms right?), I did not add this. However, when connecting the same light bulb on the 5 volt rail it behaves in the same erratic way, only the light is very low. Voltage drops from >5 to <5 volts. I will try to get hold of resistor and see if that changes anything :)

Take two 12V car break lights in parallel on the 5V rail - that is sufficient.
If you still can't get a proper output than it is possible you fried the power supply by running it without the proper load connected.

Did you add a load to the 5V rails? Otherwise the PSU won't stabilize and might even get damaged.
There is plenty of good info available on how to modifyx a ATX PSU, seems you did use the right instructions ;)