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Big doubts about Terry's filter

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LAN_403

lhl_henrylui

Sun May 23 2010, 05:01PM

Registered Member #1498 Joined: Thu May 22 2008, 07:08AM
Location: Hong Kong
Posts: 72

Terry's filter consists of some little capacitors and two 100W resistors, what are their purposes?
I'm going to make a TC with an NST as the power supply and I should choose the right primary capacitor value so that it doesn't cause resonance in the NST, right? But what value should I choose?Two times the resonant frequency?
Can I omit the MOVs? The safety gaps seem to be enough for handling over-voltages.

Herr Zapp

Sun May 23 2010, 07:22PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

lhl -

Before dismissing the "Terryfilter", why not take a little time to gain some understanding of the stresses a NST sees in a Tesla coil circuit, and review the meticulous testing that Terry Fritz performed before settling on the final design of his R-C filter array.

Regardless of what value you select for your tank capacitor, the primary circuit will generate broad-band RF noise & high-voltage "spikes" every time the spark gap fires. This is what typically damages the insulation in the NST, and has nothing to do with selecting a "resonant" or "larger-than-resonant" capacitor value.

If you don't really understand how to select a larger-than-resonant tank cap value, please review the 4HV archives, where this topic has been covered in detail many times, and where you'll find lots of useful information on how to build and tune a spark-gap coil.

If this is your first Tesla coil, you may want to spend some time researching how a Tesla coil operates, how to select appropriate component values, how to determine resonant frequency, etc. One of the best sites for providing this necessary technical information in an easy-to-understand format is Richie Burnett's site at

Study the sections on spark-gap coils, and you will have a much better chance of building a coil that "works".

(Yes, you can omit the MOVs, if you carefully adjust your safety gaps. They are primarily a backup to protect against improperly adjusted safety gaps.)

Herr Zapp

HV Enthusiast

Sun May 23 2010, 08:50PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Also be aware that are two main camps in regard to the use of the Terry Filter. Some people swear by it, and some reject it altogether.

Personally, i've never used a Terry filter on my NST powered coils and never had an issue.

Herr Zapp

Sun May 23 2010, 09:19PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Dan wrote:

"Also be aware that are two main camps in regard to the use of the Terry Filter. Some people swear by it, and some reject it altogether."

This may be true, but many NSTs have been destroyed in Tesla coil systems, some after only a few minutes of operation. Given the fact that non-GFI equipped NSTs are getting more and more difficult to find (at least in the USA), and disabling the GFI in GFI-equipped NSTs can range from difficult to completely impossible, I think anything that can reduce the chances of destroying a NST may be worthwhile. The cost of the resistors and bypass caps in a Terry filter are minimal, and Terry's test data shows that the R-C filter does effectively snub the HV transients. Looking at the current "street price" for non-GFI 60ma NSTs, the cost of a Terry filter looks like cheap insuranece to reduce the chance of insulation breakdown within the NST.

It's kind of like using sealed internal baffles in secondary coils: some TC builders never use them, other use them religiously. There are many documented reports of secondary coils being damaged or destroyed by internal arcing, and if properly installed baffles will 100% eliminate this failure mode, then its cheap insurance to use them.

Herr Zapp

Hazmatt_(The Underdog)

Tue May 25 2010, 04:48AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I've done some work on that but the posts are so old its going to take me a long time to dig them out, so if you search for "Calculating LTR" that would bring up some answers for your transformer/cap situation.

I researched and did a simulation of the Terry filter and it does work, however my numbers were vastly different then Terry's. My simulation showed me that you want the smallest capacitence possible to short the HF transients, like 10pF, not 2000pF because at 2000pF you're not hardly catching anything at all.

Just download LTSpice and you can prove it to yourself in a little simulation, and you can do your own approximations.
In the real world we do use what's available rather then the ideal in a sim, but the 10pF doorknobs are out there, just somewhat hard to find anywhere other then Surplus Sales of Nebraska.

Matt

Herr Zapp

Tue May 25 2010, 05:34AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Hazmat -

Terry Fritz' final R-C filter design used six series-connected .0033uF film caps from each NST hot leg to ground, which is 550pF, not 2,000pF.

I've used 500pF ceramic doorknob caps for my filters, and surplus 500pF ceramics rated at 20 or 30kV are very common and inexpensive.

Would you care to share your LTspice simulation? I'd be interested in seeing the circuit response with 10pF bypass caps.

Regards,
Herr Zapp

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