AC-DC power supply repair.

Thread Starter

Davizuco

Joined Jul 12, 2018
27
I am trying to repair an AC-DC switching power supply with my basic knowledge of electronics. (220 v AC to 34v and 12v DC)

The problem seems to be the MOSFFET (10N60c), It shorts and then the fuse, a capacitor and some resistors blew.

It seems like I would be able to fix it, but the SMD resistors are so bad that I can't figure out their value.

There were 5 resistors between the Mosffet source pin and ground, all of them melted and I can only see the first and last digit (1?0 , may be 120, 180, 100 or whatever) I guess they all have the same value.

The power supply uses a LD7575PS to drive the transistor.

can anyone help me find the correct resistors?.

resistors.png
 
Last edited:

dl324

Joined Mar 30, 2015
16,943
can anyone help me find the correct resistors?.
There are 5 resistors in parallel. My guess is that they're 1R0, but could also be something like 180.
I am trying to repair an AC-DC switching power supply with my basic knowledge of electronics. (220 v AC to 34v and 12v DC)
You're playing with fire. Lethal voltages are present.
The problem seems to be the MOSFFET, It shorts and then the fuse, a capacitor and some resistors blew.
The actual problem could be downstream from the MOSFET.
 

Thread Starter

Davizuco

Joined Jul 12, 2018
27
You're playing with fire. Lethal voltages are present.
Yes I know, it is around 300v in that part of the circuit, I took precautions by discharging the capacitors and watching where I put my hands. Thanks for the warning.

Definitely the problem was caused by the Mosffet, it is completely shorted, I already have a replacement.
 

Dodgydave

Joined Jun 22, 2012
11,304
Switch-mode psu, current sense resistors in series with the Source pin of the Mosfet, probably 1R0 , there must be something else that's causing the over current situation.

Can you check them with a multimeter?
 

Thread Starter

Davizuco

Joined Jul 12, 2018
27
How do you know if that's the cause, or a result of the real problem?
ahh okkk sorry, I didn't understand what you mean. I don't really "Know" 100%.. but everithing arround looks ok (diodes, resistors, etc.. ) but mosffet and it's source resistors are melted, just guessig
 

Thread Starter

Davizuco

Joined Jul 12, 2018
27
Switch-mode psu, current sense resistors in series with the Source pin of the Mosfet, probably 1R0 , there must be something else that's causing the over current situation.

Can you check them with a multimeter?
My multimeter can't check 1 ohm resistors, to small for it... almost I think it can´t
 

Thread Starter

Davizuco

Joined Jul 12, 2018
27
My multimeter tells that they are shorted, if that is what you are asking for. Same result as putting both probes together.
 

BobTPH

Joined Jun 5, 2013
8,998
The MOSFET is most likely to put a pulse of current into an inductor. If it remains on longer than it should, the current goes too high and it burns up. So whatever is controlling the gate, likely s SMPS controller chip might be culprit. Or any of the inputs to it. Like the output voltage feedback, which is usually through an optocoupler. Many possibilities.
 
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