switching power supply 24 v - blows fuse

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

Roy Thomas wrote:

Don't forget the electrolytics

See the SMPS repair guide at the site below.

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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

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.

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.