Question about APC Back-UPS CS 500VA UPS System

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MOSFETs keeep on blowing

Hi, whenever i replace IRF3710 transistors they all blow on switching, what could be the cause of this? joseph

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Check the voltage regulator and the transistiors

Posted on Mar 24, 2009

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Chargeing circuit and inverter apc backups

Because the equipment usually protected by a UPS (computers, etc) is sensitive to all kinds of power issues, you'll really want to be able to confirm any repairs you do. If you're not set up for that, it would be much safer to either have APC test and repair it, or if it makes more sense financially, just replace it. Prices have come way down. They haven't reached the level of toasters or VCRs, as far as being "throw-aways," but they're getting close. Just save the batteries.
Assuming it's not still under warranty, you can try fixing it if you have good electronics/electrical training, and the test equipment needed to both troubleshoot AND test it after repairing.
If you have access to the schematics for your unit, that's a major plus. If not, you will have to do a really good visual check, and then have to verify every diode and transistor individually.
Problems with charging circuits usually end up just being a couple of diodes. Inverters blowing up will usually show obvious damage. A good visual inspection of the unit in general, and the transistors on the heat sinks, will typically show obvious damage (transistors burned, or missing chunks). When I was working in the R&D lab for a UPS company, I would always replace all of the transistors on a given heat sink, even if only one tested bad. Your 900va unit probably has four large heatsinks, with 3-4 transistors on each. Make sure you use the same kind of transistors as replacements, unless you have really good info on allowed alternatives from APC. Stats sheets are not good enough, and you might just have it blow again. Also, sometimes the heatsinks are shared by two or more circuits, so a damaged transistor, if unreadable due to damage, might not be the same as the others on the same heat sink. Verify, verify, verify.

May 17, 2015 | APC Back-UPSĀ® XS 900 (BX900R) UPS System

1 Answer

Powerware5115 will not switch on

A weak or dead battery pack its the main reason of a unresponsive/good health UPS, but never forget to test the electric cord, the fuse and the protective relay.first, this sounds too simple, but thats a common fault.

Battery pack must have at least 5 volts with half amp of current to power up the UPS internal relays, giving electric power to the main switch.

To rise the 12/24 DC volts to 115/120 AC volts, the UPS uses some electronic components (semiconductors) called MOSFETS , and sometimes, blown (browned) MOSFETS on the electric board are the most desastrous cause of a unresponsive (completely dead) or faulty UPS. (I remember that some time ago I see a TRIPP LITE SMART 1500LCD UPS with twelve blown MOSFETS, when happened, that kind of fault must be saw like a little volcano!).

If you know how to take all the safety electric measures, and how to use a portable voltmeter, you can test the voltage/amperage on the battery pack, the electric cord and the replaceable fuse (if has any). Basic and intermediate electronic knowledge is necessary to test MOSFETS and other electronic components on the circuit board.

Aug 09, 2014 | Powerware 9120 UPS System

1 Answer

Smart ups 1500

Thanks for the info Dmar.

If you would like pricing on replacing the UPS (entire unit) let me know and I'll be glad to help.

If you need UPS, Surge, or Power Related help contact:
Inside Service Sales
Daniel Salazar 800.260.2259
Power & Systems Innovations, Inc.

Power & Systems Innovations, Inc. is an Elite Certified Service
Sales Partner for APC/MGE brands manufactured by Schneider Electric

Visit us at

Apr 11, 2014 | Computers & Internet

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first of check the load connected to it , it should not be above 1500VA. if not, the check the out put driving mosfets of ur ups. photo of mosfet is below.


Jan 06, 2011 | APC Back-UPS RS Battery Backup (apw...

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My UPS Powerware 9120 6kva. At inverter section. what is the component type for Q213,Q214,Q215 and Q216, i means type and it part number. Can U forward to me the Service Manual for that UPS. TQ

no but I can tell you that for old ups mosfets are used and for new ups IGBT (insulated gate bipolar transistor )are used the IGBTs are not available on shops you have to take help of company

Feb 18, 2010 | Powerware 9120 UPS System

1 Answer

I have a powercom (VGD 1000 VA)online UPS.All MOSFET (IRF 3710)had been shorted and DC fuse cutted off.I mounted all 6 mosfets and DC fuse.Display has been come and it gives out puts in AC mains but not in...

It sounds to me like you have a shorted cell in one of the batteries and the current limiting resistor on the charging side is out of tolerance (too low). Though it is more likely that they fail too high, they can go low in the event of overheating. I would check the resister with an ohm meter vs what is printed on it. It should usually be within 10 to 20% you can put one in series to limit the current, but you need a high current diode in parallel with it as no to limit the current to the inverter. I don't remember, but i think your model uses 2 12vdc batteries. You should replace all of its batteries and check the resister. Also, if there was a plastic insulator on the old mosfets you took off, you must have it on the new ones. this is to isolate the collector from ground. I home this helps,


Apr 06, 2009 | Powercom (BNT1000AP) UPS System

1 Answer

9120 Battery Failure

sounds like inverter is unhappy(causing the input fuse to blow. maybe, just maybe, this Powerware 9120 has given up the ghost? The inverter can short for a variety of reasons. Try to take out the battery completely, and run it as just a power conditioner. If that works, then you have an unhappy inverter in there.

Feb 05, 2008 | Powerware 9120 UPS System

2 Answers

APC Smart-UPS 1000VA shut down and now will not power on. How do I troubleshoot? No power, no lights, nothing and I've never tried to troubleshoot a UPS before. Where do I start?? :-( Thanks Chris

Hello Chris,

I have a similar unit and there is a fuse inside that can blow. Mine did. It worked again after replacing the fuse.

Another option...check the battery connections.

Another option...are the batteries getting old? If so, when they won't reach a certain level of charge the unit will not power on.

Finally, make sure you don't have a blow fuse or popped circuit braker in your home.

Hope this helps?


Nov 21, 2007 | APC Smart-UPS 1000VA UPS System

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