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A Technician Mistake Shorted Out A UPS Device

By on August 17, 2019
how to repair and fix ups












A computer technician brought this UPS for repair. It was completely dead. He only brought me the board and the batteries and did not include the case with it.

ups repair

The device was working fine. Suddenly it went dead so the customer opened up the device and checked the fuses and found one open. What he did was really crazy and stupid at the same time; He disconnected the UPS from the main power but left the charged batteries inside the device connected.

He used a screw driver to take the fuse out of its holder and guess what happened? He accidentally touched something on that board (he did not know what) that caused loud noise along with bright spark of light.

how to repair ups

These are 12 volts each and if you look to the positive side of the batteries, you could see a dark burnt area there. So it was clear that with the screw driver he touched positive of the connection of the batteries along with ground or a component nearby that caused this damage.

fda70n20 specification

You may click on the above picture to access the specification

After checking for faults I came across these two Mosfets that were completely shorted out.

There was also continuity between the negative and positive terminals of the batteries indicating a nasty short there. The part number of these Mosfets is DA70N20. It is 200 volts with 70 amp ratings.

fda70n20 fet test

Findings this kind of Mosfets was not easy; in fact it took me about two months to get them and when finally I got them and installed them, the short was gone but there was no power to the negative and the positive terminals of the batteries.


The customer talked about smoke and sparks so I started looking for burnt areas on that board and soon enough, I found this. Take a look

burnt circuit board

If you look closely, you would see a burnt wire that came loose from the board. This small board was located on the other side of the big board which I thought it was fine but it was not.

burnt circuit track

Once this burnt link was fixed and installed back on the big board and powered up the device, I could see some reading being registered on my multimeter screen. Take a look

multimeter reading fluke repair ups

That was done with the batteries not installed in place assuming all is well with the device.

The customer came and got his device and put everything back together and called me later on that all was well and the UPS was working the way it should be.

Mission Accomplished.

This article was prepared for you by Waleed Rishmawi, one of our ‘Master Authors’ and currently working in the Bethlehem area of Palestine repairing electrical and electronic equipment.

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Note: You can check his previous repair articles in the link below:




  1. Albert van Bemmelen

    August 17, 2019 at 3:12 pm

    Thanks for the info on those Mosfets. Most of these, like in your Welding machines, are of the powerful very high current types. I take it that this was a 110VAC UPS device of which you assumed that is was repaired measuring 82.9V DC without the batteries installed?

    • Waleed Rishmawi

      August 17, 2019 at 3:57 pm

      no, it was 220 volts machine and yes this is what I got then the customer called me and confirmed it was working fine.

  2. Parasuraman

    August 17, 2019 at 4:34 pm

    Well done! You make almost impossible looking tasks also easily solved, and also share very valuable information in every article! Many thanks!

  3. Yogesh Panchal

    August 17, 2019 at 8:47 pm

    Well done! waleed

    • Waleed Rishmawi

      August 19, 2019 at 3:35 pm

      Yogesh: thank and many blessings to you

  4. Justice

    August 19, 2019 at 4:04 pm

    Thank you Walleed, a job well-done.

    • Waleed Rishmawi

      August 20, 2019 at 3:30 pm

      Justice: you are most welcome. have a blessed day


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