Re: Unknown type of HV capacitor

["John Sanderson" <john.sanderson-at-eng.monash.edu.au> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

> Original poster: "Jim Lux" <jimlux-at-jpl.nasa.gov> 
> 
> No... all you really want is to have the source be high enough impedance
> that it doesn't kill the Q of the capacitor.  And, you want the resistance
> low enough so that the RC time constant of the R and C is fairly short (so
> you get a good step into the cap.)

Thanks, Jim.  understood.
 
> You could use a capacitively coupled pulse generator, or something like a
> transmission line pulse generator... It's a bit tricky because you want
> good HF response (to put all the power into the RLC, so it rings) but you
> also don't want to load it.  Go far enough with this and you wind up with a
> RLC bridge...

First step is to see if I can get access to one of those fancy 
digital RLCs... Saves a lot of hassle if the techs in the 
workshop (a) have one and (b) will let me measure my cap with it.  

> Try it on some caps you have lying around first.... like those ceramic 0.1
> bypass caps, for instance...

Good idea.  Thanks again Jim.

John S.

> ----------
> > From: Tesla list <tesla-at-pupman-dot-com>
> > To: tesla-at-pupman-dot-com
> > Subject: Re: Unknown type of HV capacitor
> > Date: Tuesday, August 29, 2000 5:39 PM
> > 
> > Original poster: "John Sanderson" <john.sanderson-at-eng.monash.edu.au> 
> > 
> > Thanks for the test suggestion, Jim.  When I get the chance to carry 
> > out this procedure, I'll post the results.  (May take a while, 
> > though, because some travel is involved.)
> > Regarding the resonant frequency test, would it perhaps be better to 
> > drive the "suspect cct" through a small capacitor (say 1nF) than a 
> > resistor?  
> > 
> > John S.
> > 
> > > drive the "suspect circuit" through a reasonably large resistor (so the
> > > generator's low output impedance doesn't perturb the circuit).  Hook
> the
> > > scope probe across the cap.  When you look at the square wave, you'll
> see a
> > > damped sinusoid on each of the edges.  The frequency of the sinusoid
> gives
> > > you the resonant frequency  (approximately f = 1/(2*pi*sqrt(LC), but
> this
> > > doesn't allow for the R, which, if the Q is low will "pull" the
> frequency a
> > > bit) and the decrement ratio lets you calculate the Q and hence the R
> (since
> > > you know the C, and the fRes, you can calculate Xc... Q is Xc/R)
> > > 
> > > This is good for a quick and dirty evaluation, but not so hot for
> precision
> > > measurements, where some sort of LCR bridge would be a better bet.
> > > 
> > > 
> > > >
> > > ><< If you can somehow measure the L and R... A square wave generator
> of a
> > > few
> > > > kHz (say a TTL pulse generator or a 555), a series resistor and an
> > > > oscilloscope will tell you... Measure the ringing frequency and the
> > > > decrement ratio...  If the L is down in the nanohenries, and fRes is
> in
> > > the
> > > > hundreds of kHz, you might be in luck. >>
> > > >
> > > >
> > > >Would you please elaborate on this procedure for us uneducated 'scope
> > > owners.
> > > >
> > > >Thank you,
> > > >Marc S.
> >  
> > 
> > 
> > 
> 
> 
> 
> 
**********************************
John Sanderson
Fluidization Group
Department of Chemical Engineering
P.O. Box 36
Monash University
Vic.  3800
ph (03) 9905 1482
fax (03) 9905 5686
john.sanderson-at-eng.monash.edu.au
**********************************