Wiebe Cazemier wrote:
Hi,

A friend of mine is trying to repair an active woofer of a speaker set, which
started to blow out the primary fuse suddenly. The problem would appear to be
in the torroidal transformer, because even when nothing is connected, the fuse
blows. We decided to be stupid and put in a 2A instead of 1A fuse. It didn't
blow, and the primary current was only 20 mA. All secondary voltages were
normal. (two assymetric outputs, and one symmetric with center tap).

So, it would appear the inrush current is very high. But, because the device
worked normally in the past, it would appear that the inrush current suddenly
increased. Is this possible? To be more precise, is this possible when all the
secondary voltages are normal? If so, how?

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.

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.

Tim.