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Re: [TCML] Terry Filter Caps Suffering From Early Death

Scott -
   
  This is an unusual failure, not previously reported by any of the hundreds of people who have built and successfully run "Terry filters".
   
  It's clear that there is something "different" about your setup that is causing the caps to fail.
  Regardless, it should be relatively easy to determine the root cause of your failures.
   
  1. First, can you describe the appearance of a "failed" capacitor? Is the epoxy coating blistered, discolored, or cracked? Are there any pinholes in the epoxy coating anywhere on the capacitor? Is the body swollen, or in any way visibly "different" from a brand new capacitor?
   
  2. Verify the voltage rating printed on each capacitor. It should be 1600 VDC. Digikey very rarely screws up, but it never hurts to verify the ratings actually printed on the failed caps.
   
  3. How many capacitors are you using, in total? Terry's original design used 12 capacitors, six in each "leg", for a rating of 9600 VDC per leg (19,200 VDC across the entire string).
   
  4. Are you using 10M Ohm bleeder resistors across each capacitor? If so, what is the spacing between the resistor and the body of the capacitor?
   
  5. If you are using bleeder resistors, what type (carbon composition, metal film, wire-wound, etc) are they? What are the wattage and voltage ratings? If one or more bleeder resistors "shorts" or suffers an "arc-over", it will increase the voltage across the remaining capacitors in the string and may trigger a capacitor failure due to over-voltage.
   
  Regards,
  Herr Zapp
   
   
   
   
  

Scott <doxiescott@xxxxxxxxx> wrote:
  Hello Everyone,

As I work on my second coil, it has come time to install the NST
protection. One very odd problem I noticed with my last Tesla Coil, a
9/30, is that the caps on the Terry Filter would periodically blow out
for no apparent reason. Even with a RF ground directly in the Earth
and no sings of arcing through the filter (with the exception of the
safety gap), the caps at the end of the strings would die for no
reason.

The filter is grounded with a single 10awg wire 1 foot long going into
a 2 foot long 6awg car battery cable going directly to my grounding
post. It's positioned 2 feet away from the NST. When a cap blows
out, it seems to have little effect on the coil performance, but as
soon as one goes, the ones behind it blow up very quickly.

Now the second coil is a 15/60, 3 times the power of first, and I
would like to get this issue resolved. Does anyone have a possible
answer to why this is happening? Is there a more robust alternative
to the caps recommended in the Terry Filter recipe:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=P10501-ND

Thanks for any help with this problem. I'm sure the Filter has done a
great job protecting the coil, its just annoying replacing these
little 60 cent caps every 5 minutes of run time or so.
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