Re: [TCML] Terry Filter Caps

[FIFTYGUY@xxxxxxx]

In a message dated 8/24/08 5:22:23 P.M. Eastern Daylight Time,  
Gary.Lau@xxxxxx writes:

>I don't believe that caps used in a Terry filter need to follow the  same 
criteria for being "good" as 
>caps used for a tank capacitor.  
 
    Certainly the RMS current rating doesn't need  to be as high - unless you 
are running the coil unattended for long periods of  time. If something goes 
wrong where the Teryy Filter starts to conduct  continuously, weaker caps 
might get hot quickly. But most of us turn off  the TC when the streamers suddenly 
disappear...
 
>The current in Terry filter caps is limited by those big 1K  resistors, and 
is very small compared with >what a tank cap  sees.  
 
    Yes, true. In fact I had several arcs at my  safety gap last night with 
my pig, but the sparks were comparatively thin  (like off a stun gun). I've got 
my safety gap closer to the pig than the Terry  Filter resistors, unlike the 
usual configuration. No problem with the safety  gap extinguishing.
    But consider that through those same resistors we  intentionally push a 
few kVA of high voltage with only a small percentage  lost. The transients we 
are trying to suppress might be of even higher  voltage. So no problem for them 
to get back through those resistors, even if  they are 1K. In fact, if the 
transients couldn't get through the resistors,  there would be no point in 
having the Terry caps! 
 
>Typical values for ESR and ESL in any kind 
>of cap should present no problem in a Terry filter  application. 
 
    Once the transients get through the resistors, we  want to shunt that 
current to ground, That's where the low ESR and inductance  of the Terry caps 
becomes important. 
    That's why our MMC caps are commonly used in  "normal" electronics in 
snubber duty. It's also well-known that for transient  suppression, keeping the 
suppressor wiring low impedance is very  important.
 
> I have used ceramic doorknob caps, and even ceramic disc  caps, provided 
they have a suitable 
>voltage rating, are fine.


    Well, if they work for you, I guess they're "fine"  for you! But 
everybody's setup is going to be different. Some people here have  done a *lot* of 
coiling without ever using a filter, and have had little or no  problems that 
they are aware of. Others have destroyed ancillary equipment the  first time 
they've powered up their TC! I'm personally pretty rough on  equipment, so I try 
to make up for my shortcomings with as much protection as I  can afford. And to 
me the price difference between "ok" Terry Filter caps and  "pretty darn 
good" ones is very cheap insurance. I'd also rather blow all of  those pretty 
little caps to smithereens than have to replace my neighbor's PC,  for example. 
    I'll certainly agree that almost *any* cap even  close in ratings will 
work to *some* degree for Terry Filters. But using the  good stuff is cheap 
insurance... isn't that what the Terry Filter is about in  the first place?

    
-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities



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