A Technician Mistake Shorted Out A UPS Device
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.
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.
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.
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.
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
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.
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
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.
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: