I've just been repairing a linear PSU where the toroidal transformer had
failed due to a faulty 0.1µF disc capacitor across one of the rectifier
diodes which had gone short circuit. The insulation on one of the low volt
outputs from the transformer had melted - so it was obviously drawing
excessive current before it failed.

It's a 28 volt regulated 1 amp PS made by a long defunct pro audio
company. The regulator is an LM317T, and it has an adequate heatsink and
has survived. The transformer a 15 + 15v 30 VA. No LT fuses, only mains
(240v, 250mA standard type.) which had blown.

Now toroidal transformers have a high 'startup' current, so I'd guess an
anti-surge would be better at perhaps 125 mA?

Is there a 'rule of thumb' for mains fuses for such devices? And this
isn't the first disc ceramic I've come across which has failed short
circuit. Better type?

--
*Whatever kind of look you were going for, you missed.

Dave Plowman London SW
To e-mail, change noise into sound.