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[Help needed] trouble-shooting offline UPS using SG3524N PWM

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Aug 23, 2009
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Hi Everyone,

I hope someone here may be able to help me.

I have an offline UPS made by a local manufacturer who has since closed his business.

The circuit uses a SG3524N PWM controller, three AS324P quad op-amps, a MOC3021 opto-coupler and six P55NF06 MOSFETs to give a 220vac/50Hz (600 VA) output from a 12vdc/31AH lead-acid car battery.

I am attaching two photos of the inside of the UPS here. One is with the transformer and the pcb in low res and the other one is higher res of just the main pcb. The reson for the photos is that I am hoping someone here may be able to recognize the circuit and help me out with the trouble-shooting.

The problem is that the UPS is not switching fast enough to keep my desktop computer on during a heavy fluctuation or a power outage. The computer restarts making the UPS really useless. The load is just a single computer with a 17" CRT display - it should not be more than 350 watts total.

The battery is fairly new as it is still under warranty but I don't think that it is the guilty party here because once the computer reboots through the battery, I have used it for more than half an hour and there was no indication that the battery was running out.

Any helpful advice about tweaking or trouble-shooting this circuit would be deeply appreciated since I don't really have the budget for a new UPS. I do have a DMM but no oscilliscope so wave analysis is not possible.

So if someone recognizes this circuit, please let me know. If someone has a schematic for this, I can try to trace the circuit myself and see if I can fix the problem.

Thanks in advance for any help.

shooting offline


i think the current limiting are the problem..there are at the left sid shunt resistors..u can use litle tin solder and solder about 5mm of the each...i see there are an resistor blow up..checked them...a furter proble are the degausing of the CRT when u switch On ..the inrush current very high...and one more ur battery ok..maybe ur undervoltage limit working..

Re: [Help needed] trouble-shooting offline UPS using SG3524N

Thanks willi65,

Sorry for the late reply. I didn't think there were going to be any replies after the first three weeks, :)

I figured out the problem after loosing half my hair in frustration. The variable resistor in the top middle of the PCB controls the output frequency. Its blade had developed a gap (maybe from the heat) and was not in contact with the carbon track. Instead of getting 50 Hz output, I was getting more than 2k... lucky I didn't blow my SMPS.

Thanks for your advise but I don't think the degaussing of the monitor was a problem since it only happens when the monitor is initially switched on. When the UPS switches from power failure, the monitor is already on and degaussing does not happen.

But I am happy you replied. Thanks again.

with regards,

Re: [Help needed] trouble-shooting offline UPS using SG3524N

After looking at the photo i recognize this structure. it is basicaly inverter circuit. the manufacturer modify it for ups action but it's change-over time is not fast enough.

Re: [Help needed] trouble-shooting offline UPS using SG3524N

Thanks for replying sudarshan8. I was not able to repair the circuit for triggering the UPS action fast enough for my work.

I eventually sold this piece and bought a micro-controller based sine wave UPS. Seems to be working pretty well so far.

Thanks again for your reply.

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