DIYbanter - View Single Post

On Monday 10 April 2006 15:18, wrote:

Possibility: original fuse was 1A slow-blow. It died because of
"normal" fatigue. Or of some temporary overload that cleared.

Then it was replaced with a 1A fast-blow. This seems consistent with
your description of the problem (especially as you leave out the very
important facts of slow-blow vs fast-blow).

Slow-blow fuses for capacitor-input DC filters are the norm, not the
exception.

Look carefully with the factory fuse or if not available the markings
saying what sort of fuse is supposed to go there.

I have been thinking along these lines. But because we tried slow blows, the
only thing I can imagine is that the original fuse is extra slow blow, but I
don't know if those things even exist. I'll ask him if the transformer has any
info written on it about special kinds of fuses.

Also, even though the average primary current may be 20mA, what
actually matters to the fuse is something closer to the RMS current,
and the current waveform is often wildly non-sinusoidal. A factor of 50
to 100 between average and RMS seems implausable, though with no load
(capacitor charging only on the very tip of waveform) it may actually
be the ratio.

Perhaps, but there were no filtercaps connected. Going with the "extra slow
blow" theory, perhaps this transformer has some kind of very deformed current
waveform, which the original fuse was designed for.

He doesn't have a scope, but I do. If we can't find the problem, we could bring
it here, and put it on my scope. But I don't think I'm gonna measure the
primary current with it, because I don't know enough about putting scopes on
the mains.