CJT wrote:
Tim Shoppa wrote:

N Cook wrote:

400 watt RMS amp for use on 240 V mains.
Destructed due to metal dropping in and shorting the amp .
The only fuse is on the mains rated at 8 amp, maker's design rating, which
shows no sag , discolour or anything like that, ie untouched.
Shorted power trannies, burnt low power trannies and even a piece of 3mm
trace from the main bridge rectifier burnt through and that had 2 runs of
added solder over the track for current carrying, the other polarity trace
overheated but not ruptured
I'm thinking of bridging that gap with a fuse and another on the other rail
after cutting it.
The mains transformer is rated at 2x 47V,5 amp.
So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added fuses of
what rating ?


The power supply to your amp probably has massively oversize filter
capacitors.

Average current draw at 400W out might be only two amps, but because
of the rectifier-filter construction it doesn't draw that two amps
continuously. It draws it in very brief gulps 120 times a second.
Typical conduction angle might be 20 percent in a well designed
supply; with massively oversize filter caps it could be as small as 5
percent. Massively oversized filter caps have been all the rage in
consumer audio for too many decades now.

The size of fuse you have to put in depends not on simple average
current (2 amps) but the root-mean-square (RMS) current (which could
indeed be 8 amps at full load).

That shouldn't be terribly hard to calculate, assuming a conduction
angle of your choice. So how about adding some support for the
assertion?

10% conduction angle gives peak current of 20A, RMS current of 6.3A,
not too unreasonable to choose an 8A fuse in that case.

6.25% conduction angle gives 8A RMS, but that'd blow the fuse way too
often.

It takes big capacitors and toroidal transformers with really stiff
windings but many folks are building audio power supplies that way. I
personally don't like razor-thin conduction angles like 10% but that's
the style of other folks, not me!

Tim.