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switching power supply 24 v - blows fuse

R

Roy Thomas

Jan 1, 1970
0
I have a small power supply (that would be expensive to completely
replace) which is faulty (blows its own fuse).

I understand that the high voltage (primary) part of the circuit is
likely to contain the faulty component and that the transforms/coils
are unlikely to be responsible.

That leaves the diodes - 4 in the bridge rectifier part and one other
all in the primary part of the circuit.

Any other likely causes of this power supply failure? There are no
obviously blown components other than the fuse.

Many thanks,
Roy
 
L

Leo Meyer

Jan 1, 1970
0
Suspect the switching transistor. If this Q is shorted from C to E it
will immediately blow the fuse.
It's a common failure in SMPS also.

Good luck, Leo
 
D

David C. Partridge

Jan 1, 1970
0
Don't forget the electrolytics

Dave
 
S

Sam Goldwasser

Jan 1, 1970
0
David C. Partridge said:
Don't forget the electrolytics

See the SMPS repair guide at the site below.

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D

Dave D

Jan 1, 1970
0
Roy Thomas said:
I have a small power supply (that would be expensive to completely
replace) which is faulty (blows its own fuse).

I understand that the high voltage (primary) part of the circuit is
likely to contain the faulty component and that the transforms/coils
are unlikely to be responsible.

That leaves the diodes - 4 in the bridge rectifier part and one other
all in the primary part of the circuit.

Any other likely causes of this power supply failure? There are no
obviously blown components other than the fuse.

Many thanks,
Roy

Probably the switching transistor, though failure of this is usually caused
by another problem like shorted output, shorted secondary rectifier or high
ESR electrolyics.

If the switching transistor has failed, I'd recommend replacing the
electrolytics as a matter of course,*before* powering on with the new
transistor. They are under considerable stress and are the No.1 most common
problem in switching supplies. If they are marginal, chances are the new
transistor will have a short life.

Dave
 
S

Skype_man

Jan 1, 1970
0
Possibly, you have a shorted MOV (metallic oxidide varistor),
it's usually black, but could be green, & it's right across the 120
volt A.C. input. Also, rarely, you could have a bad line choke.
Sky.
 
R

Ross Herbert

Jan 1, 1970
0
I have a small power supply (that would be expensive to completely
replace) which is faulty (blows its own fuse).

I understand that the high voltage (primary) part of the circuit is
likely to contain the faulty component and that the transforms/coils
are unlikely to be responsible.

That leaves the diodes - 4 in the bridge rectifier part and one other
all in the primary part of the circuit.

Any other likely causes of this power supply failure? There are no
obviously blown components other than the fuse.

Many thanks,
Roy

In addition to other suggestions check the bridge rectifier to make
sure no diodes in it are short circuit, - great fuse blower although
less likely than switching transistor.
 
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