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UPS shuts down after few seconds on battery backup.

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Moises Zagala

PostPosted: Wed Jan 19, 2011 4:00 pm    Post subject: UPS shuts down after few seconds on battery backup. Reply with quote

Hello to all the participitants of this interesting forum.

I write this note asking and hoping for your expertise on a repair I am working on.

The equipment is a HP500VA UPS. Model SL-501. 500 VA. 300 Watts. Input 110 VAC Current 5.5 Amps. Output 100 vac.

Symptoms: I got the UPS totally dead with fuse blown. Replaced the fuse and it go open circuit instantly after turning on. The original battery is lost. All I know is that it is a 12 volts battery. Current specifications are unknown to me. I am using a 12 volts 7 amp/hr UPS replacement battery for testing purposes considering other 500 VA UPS's use similar batteries with no problem.

the equipment uses 8 MOSFET transistors separated in two heat sinks (4 in each one). 2 of this 8 were shorted. I replaced them and the equipment turns on with very little current comsuption and the battery charges allright (it receives 13 volts constant). However when I cut the power, battery backup kicks in and I get around 100 volts at the UPS output for few seconds then the UPS shuts off automatically. I checked current consuption during battery backup and its over 5 amps drawn from the battery with no load wich makes me think there's a short or leak in the output. There are two NTC close to the output I guess to limit current inrush to the load, when I remove them, the UPS doesn't shut off on battery backup although I don't get any output and the output outlets (obiously) but I can measure 100 Volts at some points in the circuit which made me believe the inverter is oscilating but its dragged down by some short.

I have checked all the other mosfet, all electrolytes (ESR), all none polar, most of the resistances, most of the diodes and all seem to be ok.

I would really appreciate any ideas, service manual, specifications or schematics of this device. Thanks and regards Very Happy .
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Joined: 30 Sep 2010
Posts: 1

PostPosted: Mon Jan 24, 2011 3:35 pm    Post subject: Reply with quote

I didn't have the service manual so it was difficult to track the output in this double layer PCB. The culprit of the fault was a high power output diode located right before the output transformers. The thermistors were located right before the diodes, that is why when removed the backup battery mode worked. The diode in question has a part number written as ER504. The difficulty in this repair was rtelated to the fact that this diode was only mildly leaky and hard to detect with the DMM.

Here you have an image of the UPS back to life.

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