Re: Survivor!

John R Walliker <jrwalliker@gmail.com> wrote:

On 20/03/2024 15:22, Bertrand Sindri wrote:

Cursitor Doom <cd@notformail.com> wrote:

Yesterday I had 2 minutes to waste so I blew up another electrolytic
capacitor - or rather I *tried* to.  A 10uF 10V cap across the output
of a variac with Vo set to 240VAC.  There was a considerable *pop*
but no bang and it turned out the T3.15 Amp fuse in the variac had
blown spectacularly - but the cap had survived unscathed!  Tested
fine for capacitance and ESR!  I never would have believed it.  Just
wondering how the hell it didn't get destroyed...

 
Obviously it was able to survive the overcurrent situation for long
enough to blow the fuse in the variac.  Since, as usual, you've left
off all useful information (i.e., make and model of cap) we can't
comment any further.
 
The fuse also did it's job, which is to protect downstream components
from overcurrent situations by blowing before the downstream items
themselves blow up.
 
 

More importantly the fuse protected the variac.  They are
very intolerant of even quite modest overloads.

I have successfully protected one with a thermal cutout which has low
thermal inertia and has always operated before any damage can occur.  It
is in a bench power supply that is often used to test faulty equipment,
so fuses would not give a quick and easy reset facility.

It suffered a lot of nuisance tripping on loads well below the variac's
rating until I realised that the isolation transformer, which it fed,
was drawing a large magnetising current.  I have corrected the power
factor of the transformer with a capacitor and the nuisance tripping has
now ceased.


--
~ Liz Tuddenham ~
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www.poppyrecords.co.uk