N Cook wrote:

Phil Allison wrote in message
...

"N Cook"

So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added
fuses
of
what rating ?


** Be very wary of adding +/- DC rail fues to any power amp designed
without
them.

Very likely if one or other DC fuse blows or is removed, the amp's output
will swing fully to the rail with its fuse still intact.

Recipe for fried speakers.

You have been warned.




....... Phil





I'm just trying to engineer a more respectable equivalent of fuses for trace
rupture.
Eventually one DC rail ruptured as a fuse. I've now decided to do some
calculation to determine what the rupture current for that trace was.
Then as the common return almost fused and is the same dimension but in the
even worse case then carries twice the current , halving this trace rupture
current to use as the fuse rupture current and reduce that for current
carying capacity , by half again ?
If all output trannies fused short circuit all round so loading both DC
rails the combined common return would have ruptured at whatever that
rupture current is for the trace that did burn up.
Anyone know the term for laying double traces of solder along copper traces
to increase current carrying , so I can research it ?
I'm intrigued how you lay quite accurate molten solder runs and parallel
over and with existing traces.

Anyway measurements:
Copper trace 3.5 x .02mm and as 2 half approximated elipses of solder then
area of 1 elipse of tin+lead which is Pi x a x b , a and b minor and major
axes of .15mm and .8mm.
From tables of fusing currents for copper, also tin and lead (NB in the form
of circular wires ) for different diameters.

Copper fusing current of the trace = 12 amps
Lead+Tin elipse then 6 amps (not as much as I would have intuitively
thought)
Total 18 amps so 18/4 = 4.5 amps conventional fuse rating.
Presumably the circular to sheet allowance would up this 4.5 amp figure ,
but by how much ?
What sort of correction factor for thin sheet/non-circular heating then
rupture allowance?

Don't waste time.

The customer's not paying for you to redesign the amp. Fit some wire mesh or
whatever to stop things being dropped inside it in future as per safety regs.

Graham