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Its a 120Vac to 12Vdc power supply for a camper. I have it pulled apart and it works fine and outputs 12Vdc with no load, when I put a load on it, the voltage drops and makes a slight buzz sound.
are you sure it's a 1A load? describe the power supply. does it have sense terminals? if so are they connected to the output, or left open?
pix would help
I took the heatsink off.
The 8-pin chip on the bottom right look fine, its just this clear glue stuff, and I check these two diodes in the picture. Both had a .6V drop which I think is fine with the datasheet specs. View attachment 64420 View attachment 64421
These 2 big resistors do get very hot and actually made the PCB black. I pulled them out and the resistance of both were ok, so what do you think would be causing them to heat up? View attachment 64426 View attachment 64428
I think they would normally run hot even if the supply were working ok.
I can now see (bottom pic) where the spade connectors go to the input bridge rectifier on the heatsink. Have you checked its diodes are ok? If a diode has failed and you're only getting half-wave rectification then your supply will be struggling to give much output.
No, I'm talking about the black thing with a bolt through it, on the right side of the last pic in post #11, which the red, black and white wires connect to with pull-off spade connectors. Isn't that a bridge rectifier?
I applied currents and measured voltage drops and 1 of the 4 diodes was busted!
So you think that one diode would cause the power supply to output 12v then drop with load?
Well part has been ordered and after I replace it, I guess we will find out if that was the only problem.
Thanks guys!
With a failed diode the mains input voltage is only half-wave rectified, so when any appreciable current is drawn from the supply the voltage on the input reservoir capacitor drops way below where it should be. This means the switch-mode section of the supply has insufficient input voltage/power to operate properly and deliver its rated output.
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