Re: Caps

To: tesla-at-pupman-dot-com
Subject: Re: Caps
From: David Sharpe <sccr4us-at-erols-dot-com> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Fri, 30 Apr 1999 21:03:42 -0600
Approved: twf-at-verinet-dot-com
Tesla List wrote:
> 
> Original Poster: Adam <psycho-at-tradewind-dot-net>
> 
> Tesla List wrote:
> 
> > Original Poster: Terry Fritz <twf-at-verinet-dot-com>
> >
> > Hi Scott,
> >
> >         I use 10 TDK HI-Q doorknobs wired in parallel with excellent
results.
> > These are supposed to be real expensive fancy caps that I lucked on for a
> > super price.  However, the are apparently lower Q versions that have a
> > habit of dying in Tesla service.  From what I hear, if they cost like 100
> > bucks "EACH" they are good.  If they cost like 10 bucks each they are
> > bad...  Some doorknobs are made for easy DC applications while others are
> > for critical high power RF stuff.  Unless you get lucky like I did, they
> > are not a reasonable choice given the high cost.  Personally, I would go
> > for the MMC primary caps that so many (me too) have found to be and
> > excellent choice at a reasonable cost.
> >
> >         Terry
> >
> > At 05:47 PM 4/26/99 -0400, you wrote:
> > >Nobody answered my questions, so I'll ask again.
> > >
> > >Doorknob caps:  How suitable are they for a NST powered Tesla coil?
> > >
> > >Do HV ceramic caps work poorly, or not at all?
> > >
> > >Does anybody know of good commercial sources for new and/or surplus HV
caps.
> > >
> > >-Scott
> 
>-----------------------------
SNIP

Doorknob capacitors (barium titanate dielectric) are good for DC and EMI
filtering and GLB capacitors for VTTC.  The active electrodes are plasma
sprayed on and will be blown off the capacitor at high di/dt levels. 
This will result in gradual reduction in capacitance value until 
capacitor value is so small to be useless in original application.

Doorknobs also have a horrible TEMPCO (-20% to +80% over 0-30 deg C).
I have a small magnifier using a 9kV 30mA NST with 7 2500pf 6kVAC rated
units in a doubled sided PCB parallel assembly, and two of these
assemblies are arranged in a Tesla Equidrive configuration. From winter
to summer use, my tune point will shift 2 TURNS  :O).  Tune point will
also shift with tank circuit current self heating of capacitor  :^C

Bottom line, conditional ok use in low power (<12kV, 30mA NST)
applications with short run times, realizing that capacitance value will
not be stable, therefore your tune point will shift.

Regards

DAVE SHARPE, TCBOR
Chesterfield VA, USA