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Re: [TCML] Terry Filter Caps



Hi Adam,

Did I get this right? Are you running a Terry Filter with a pig?
Just curious about that. I personally don't use such a filter with a pig transformer. Only for NST's. Not that it hurts, but I'm not sure it's necessary. NST's are voltage fragile. Transients are part of that. The filter helps some in that department.

Regardless, in this discussion, people should remember the MOV's across each cap/resistor circuit within the Terry filter. As long as the cap is of adequate voltage and the MOV's are designed for the "transformer", then there is plenty of protection across each individual RC section (beyond the cap or resistor ratings). But with a good design, those are last resort.

Best regards,
Bart

FIFTYGUY@xxxxxxx wrote:
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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