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?.

 

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.

 

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.

 

Definitely the problem was caused by the Mosffet, it is completely shorted

How do you know if that's the cause, or a result of the real problem?

 

How do you know if that's the cause, or a result of the real problem?

I got 0 ohms between any pin on the mosffet, unsoldered it and still got the same result.

 

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?

 

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

 

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

 

Your multimeter will be able to tell if they are less than 10ohms .

 

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

 

I wouldn't be worried about the resistors, but what has caused the Mosfet to blow up..

 

I wouldn't be worried about the resistors, but what has caused the Mosfet to blow up..

I understand that "blow by itself" is not an option


I will try to find another cause then.

 

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.