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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: Survivor!
Date: Fri, 22 Mar 2024 11:52:29 +0000
Organization: Poppy Records
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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 ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk