Troubleshooting faulty battery changer

Thread Starter

Dritech

Joined Sep 21, 2011
901
Hi all,

I have a battery charger which is blowing the fuse immediately when plugging the power plug (switch on).
Following the replacement of the fuse, I found that there is continuity between live and neutral. Started desoldering components close to the mains supply to check for continuity (varistor, caps, etc), but none of these were shorted.
Then the bridge rectified was desoldered and found that pins 1 and 2, 3 and 4 have continuity from both ends (polarities).
The pads marked in red below were also found to have continuity (pads of the + and - of the rectified).
Assuming that I am not missing anyting and that the conclusions above are correct, what can be the likely possible cause for these two pads to be shorted? I cannot find any components with visible damage/burns. I was going to start desoldering the TO-220 components, but this will be a headache. Anyone with some experience who can suggest the likelyhood of the fault?
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DickCappels

Joined Aug 21, 2008
9,512
It might be that your bridge is shorted, and it might be that failure further downstream from the bridge caused the bridge to short. It is probably a good idea to check all of the semiconductors in the power output section of the circuit so you don't have to replace the bridge more than one time.
 

Thread Starter

Dritech

Joined Sep 21, 2011
901
Hi DickCappels. Is it common for FETs to fail short? Also, there are some TO-220 components with the heatsink which have two leads. What can these possibly be?
 

Dodgydave

Joined Jun 22, 2012
10,591
Yes FETs can blow up, usually they take out the bridge rectifier, they can blow from over current caused by overloading,, the two pin devices on the heatsink are Diodes.
 

Thread Starter

Dritech

Joined Sep 21, 2011
901
Yes FETs can blow up, usually they take out the bridge rectifier, they can blow from over current caused by overloading,, the two pin devices on the heatsink are Diodes.
Thanks for the reply. What I meant, if FETs fail/blow up, is it normal to get shorted? Reason for asking is that the + and - pads of the recrifier as shorted. This means that the positive and ground rails have someting which is shirted.
 

Dodgydave

Joined Jun 22, 2012
10,591
Yes when fets blow they normally go short between D and S, ..
What you have is a switchmode psu, which chops the mains at high frequency and produces a lower voltage that is rectified by high speed diodes on heatsinks. ( two or three pin types) ..

What make and model is the charger?
 
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