Hi guys,
I am trying to repair an LED worklight that suddenly stopped working after a period of use. The unit is an LED driver but also has a Bluetooth speaker incorporated with it.
I am relatively new to electronics and have a decent understanding of Switch mode power supply theory but troubleshooting them is a whole different thing.
On opening up the unit I found that 3x resistors in parallel with the Source of the mosfet were blown (See picture – new resistors under black heatshrink bottom right). Also the main 500ma board fuse had blown. So that was 2 obvious failures, but I knew that it was likely something else had caused the resistors to blow. I removed and tested the MOSFET and it was conducting both ways and essentially shorted. I also tested the Diodes on the rectifier bridge and on the secondary side. Secondary Schottky diodes were also conducting both directions.
So I replaced all components I found faulty – Diodes (x2), MOSFET, 3x resistors (1.3 Ohms each – I used regular bead type resistors instead of smd type to test) and the 500ma fuse.
I powered up the unit and nothing happened. However I did smell a slight burning smell which I now believe to be the 500ma fuse blowing again. There is another short somewhere…
But now I am stumped. I have no idea what to look for next? I assumed it might be the PWM chip to drive the FET but the number "5673A" does not match anything and I cannot get a data sheet to test the pinouts. Could this PWM be shorting causing the fuse to blow? They usually have overcurrent protection but seeing as the resistors all burnt out could this be the culprit?
Capacitors look ok, and test fine with an ESR meter. Would a bad cap cause a dead short though?
As you can probably tell I am pretty new to this so please feel free to correct me if I am talking nonsense… but at the same time go easy. :0)
Any info you guys could give to steer me in the right direction would be much appreciated.
Stevo
I am trying to repair an LED worklight that suddenly stopped working after a period of use. The unit is an LED driver but also has a Bluetooth speaker incorporated with it.
I am relatively new to electronics and have a decent understanding of Switch mode power supply theory but troubleshooting them is a whole different thing.
On opening up the unit I found that 3x resistors in parallel with the Source of the mosfet were blown (See picture – new resistors under black heatshrink bottom right). Also the main 500ma board fuse had blown. So that was 2 obvious failures, but I knew that it was likely something else had caused the resistors to blow. I removed and tested the MOSFET and it was conducting both ways and essentially shorted. I also tested the Diodes on the rectifier bridge and on the secondary side. Secondary Schottky diodes were also conducting both directions.
So I replaced all components I found faulty – Diodes (x2), MOSFET, 3x resistors (1.3 Ohms each – I used regular bead type resistors instead of smd type to test) and the 500ma fuse.
I powered up the unit and nothing happened. However I did smell a slight burning smell which I now believe to be the 500ma fuse blowing again. There is another short somewhere…
But now I am stumped. I have no idea what to look for next? I assumed it might be the PWM chip to drive the FET but the number "5673A" does not match anything and I cannot get a data sheet to test the pinouts. Could this PWM be shorting causing the fuse to blow? They usually have overcurrent protection but seeing as the resistors all burnt out could this be the culprit?
Capacitors look ok, and test fine with an ESR meter. Would a bad cap cause a dead short though?
As you can probably tell I am pretty new to this so please feel free to correct me if I am talking nonsense… but at the same time go easy. :0)
Any info you guys could give to steer me in the right direction would be much appreciated.
Stevo
Attachments
-
182.5 KB Views: 24
-
235.2 KB Views: 22
-
143.8 KB Views: 20
-
167.3 KB Views: 22