[sdiy] RE: Tube amp discharge tool (important safety information)

[Bob Weigel]

Harry...a hand made square wave for instance generated by a switch 
attached to a capacitor >> value of capacitor in question.....does the 
inverse thing to the dielectric material AND conductive material in the 
capacitor in questoin fairly precisely I would think.  I mean..I've 
never made a study of it but..then neither has anyone else here I'm 
willing to bet and it seems conceptually that it would unless there is 
some dielectric hysteresis effect or something like that :-).  I wasn't 
talking about the NORMAL charge experienced by a capacitor in a circuit 
with a transformer.  Remember you are talking to an engineer...I'm not 
going to forget that transformers have internal impedences that throw 
typically in a circuit where there's HV tube voltage involved, something 
in the neighborhood of a 10msec time constant modulated by the 120hz 
full wave rectified voltage 'start point'.   Anyway...I think we both 
understand power supplies fairly well...
    However..while we're nit picking...the esr of the cap is not the 
only limit.  :-).  I don't want to assume you are unaware of what 
happens when two pieces of metal touch in an oxygen environment and 
large amounts of current begin to move...I'm just going to figure you 
didn't thing of it before writing.  IF the esr of the cap was the only 
significant impedence in this picture Harry......you wouldn't see a 
spark.  That spark represents a lot of energy being dissipated by a very 
real impedence that got left out of the equation (and often is by us 
humans because when we think of metal to metal we think of 'super low 
impedence'. But there is a particular area of physics that delves into 
the dynamics of such interactions and there we find that everything 
isn't quite so ideal when you start trying to spoon out that much 
current across a tiny connection.  This is the reason I said "LET 'ER 
RIP".  :-)   When I see the energy being dissipated in the air, I know 
it's being taken off the internals of the capacitor which is a good 
thing.   Right?
   But like I say, I rest my case.  On sheer chance guesses I'm 
horrible.  But when that background kicks in I usually make decent 
choices regarding the safety of components and materials.  I don't 
seriously believe anyone will hurt any capacitors rapid discharging them 
but if there's a real study that's been done of course I'd be interested 
to see it.   -Bob


Harry Bissell Jr wrote:

>Sorry guys, you are all wet here.
>
>Discharging a cap by "arc" is NOT anthing like
>"hitting it with a square wave".  
>
>First, a square wave does not go from 0 - 400V
>in no time. Internal impedances in the transformer, as
>well as the natural rise of current in the transformer
>inductance, and the fact that you are pretty likely
>to NOT always throw the switch at the peak of the AC
>line... all conspire to slow the rate of charge into
>the
>caps.
>
>If you could throw that 'square wave" you probably
>just
>blew the rectifier diodes.
>
>Discharging the cap with an 'arc' or direct short
>circuit is going to draw current according to ohms
>law... with ESR of the cap as the only limit. The
>power
>is ALREADY stored in the cap. There is no limiting
>inductance here, its pure resistance and precious
>little of that.
>
>The arguement that 'I have done this successfully'
>does
>not hold any merit... nor does saying that you 'won an
>russian roulette'.  
>
>The ARC itself can be dangerous, it can burn you,
>leave
>chunks of metal missing from your amp. Leave burn
>marks on the chassis that would not inspire a
>customer's confidence in your repair skills.
>
>You ARE damaging the capacitor. Can you get away with
>it once, maybe... twice... maybe...
>
>Try this.  Take an electrolytic capacitor (maybe
>50uF)and charge it to some reasonably low voltage
>(maybe 24V).  Run a wire through a scope current
>probe. Discharge the cap by shorting it with the wire.
>Arc spark... what was the peak current ?   Maybe 50A
>
>Keep repeating this process.  After ten times or so,
>you'll notice that you never get that 50A peak, ever
>again.
>
>I know this becauce I needed a pulse current source
>for testing a current sensing safety system. The first
>few times it WORKED and tripped my circuit, then it
>stopped working.  what I found was that a new
>capacitor could
>pass enough current to cause the trip, but that the
>cap degraded every time after that.  Soon it was not a
>good capacitor anymore.
>
>I'm sharing this because most folks won't have the
>current probe, or notice the degradation. Why put in
>new caps and potentially ruin them ?
>
>There was a post about shorting the supply through onr
>of the plate connections of the power transformer...
>at least here you have a bit of series inductance and
>resistance (once saturation occurs). By then the cap
>is probably discharged. Still, that would not be my
>first choice.
>
>Shorting a cap with a screwdriver is just plain bad
>advice and should not be given.  Using a resistor to
>discharge the cap at a controlled rate, and checking
>it with a meter is GOOD advice that everyone should
>follow.
>
>H^) harry  (this is the ONLY smiley in this message
>:^)
>
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