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Back-UPS 650 Keeps Blowing Internal Fuse F3

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Hi All,

I have an APC Back-UPS 650 (400W) UPS that works great as an inverter, but has stopped functioning as a UPS. The fact that it functions as an inverter leads me to believe that all the 12V circuits, and the inverter circuitry, is fine and functioning well and (hopefully) not part of the problem.

However, unit cannot be plugged into the wall power without blowing fuse F3.

I've traced this as far as finding a 1-ohm load placed across the neutral and hot of the power cord when F3 is in place. The primary winding of both the inverter transformer and battery charging transformer, coincidentally, are both 1-ohm loads. I *think* that somehow this load is being transferred to the hot & neutral on the power cord, but cannot see how.

Nothing looks bad or blown on the board.

I consider myself at hobbyist/student level, some formal training, some work experience, but not familiar enough with UPS to understand why the 1-ohm load is present across the hot & neutral of the utility power cord.

I've checked the main relay that swaps power over, but it seems to be working fine.

Does anybody have any ideas? Anybody familiar enough with UPS design to maybe bump me in the right direction?

All help/guidance appreciated.

Thank you for being here,


New Member
Make a light bulb limiter and use a 40 watt bulb, it should cut the voltage down to about 60 vac, and the fuse won't blow out.
Then you can go through it with a VOM and check voltages. You can use a larger bulb. If it's drawing excessive current the bulb will light brightly.
I do this all the time for repairing guitar amps. Keeps from wasting fuses and damaging other components.
Do you have a schematic?


Alright folks, I'm experimenting here with trying to revive this year-and-a-half old thread. While waiting on parts for another project, I took down this UPS which had been in a box in the attic all these months. Gearhead's suggestion of a light bulb limiter is a sound one, and good practice for anyone doing this type of work. In this case, limiting the supply to 60VAC would only trip the undervoltage circuitry and put the lamp in inverter mode.

Here are my findings: The UPS works as an inverter if I leave F3 out, leave the unit unplugged, and trip the inverter circuitry with the battery in place.

Failure mode is when when I put F3 in place, plug the unit in and power it on. In this case F3 blows instantly.

I'm somewhat of a "dabbler" in electronics (if the 18-month hiatus didn't give that away) so my troubleshooting experience is limited.

My thought is that the output circuitry must be fine, transformer, FETs, etc. Since all that works without issue. So I'm thinking the fault is at the input somewhere (I did upload schematics yesterday). Thing is, I don't even know what to look for beyond checking diodes and transistors and caps for shorts (none found). Where would be a good place to take a measurement? Inputs and outputs seem shorted, but I guess they would since they're connected to transformer windings? But I'm not even sure on that.

The fuse blows so quickly that I think something must be shorted. But I don't find anything.


Thanks all,

Nigel Goodwin

Super Moderator
Most Helpful Member
F3 is the mains fuse in the (very simple) charger section - most likely components to have failed are the bridge rectifier, D4/5/6/7 - failing that the mains transformer would be most likely.

If the rectifiers are S/C you should be able to easily test them in-circuit, one (or more) of them will read low resistance. Failing that remove all four of the rectifiers, and see if F3 still blows (if it does, then it's the transformer). Once the rectifiers are out, check them out of circuit as well, as it's 'possible' (but unlikely) that they could be leaky - it's also quite possible that the faulty diodes have killed the transformer.


F3 is the mains fuse in the (very simple) charger section - most likely components to have failed are the bridge rectifier, D4/5/6/7 - failing that the mains transformer would be most likely.
Thank you, Nigel, for your help!

All the rectifier diodes read perfectly, so I'm thinking it's the transformer. Looking up the part number on eBay, I see a dozen of them saying "removed from APC UPS" and they all have leads. Mine is mounted directly to the PCB right next to the two 5W resistors with about 1mm to spare. My unit is an older one, so I'm guessing the original design had many problems with the transformers failing from heat. Looks like they've re-designed, moving the transformer off the PCB.

Still, I'm doing this mainly to learn, so I will de-solder the diodes and run your suggested test. I'm looking at a de-soldering station on Amazon, so I might wait till I get that before proceeding. I've had a really hard time de-soldering things on this PCB so far -- everything is mounted in through-plated holes.

I'll post back once I have more findings.

Thanks for answering. I really appreciate it!

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