RE: terry filter theory

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>

Hi Jim:

There are three functional circuits within the Terry filter, each
operating separately.

1) The 3-terminal spark (safety) gap at the output end clamps the
voltage from either output terminal to ground to a voltage value
determined by the gap width.  So it's vital that the width be properly
set.

2) The resistor and capacitor form a low-pass R-C filter to attenuate
VHF transient "zero-crossing" bursts generated by the main spark gap.
See my web page on this topic for more info:
http://www.laushaus.com/tesla/protection.htm.   The R-C filter does
nothing to clamp mains-resonance overvoltage caused by too-wide main
gaps or ferro-resonance.  Only the safety gap and MOV's address this.
It cannot be said that the resistor does this and the capacitor does
that; neither one by itself would be useful.  They work as a pair to
pass low frequencies and attenuate high (much higher than the tank)
frequencies.

3) The MOV's are a last-ditch effort to stop excessive voltages from
reaching the NST.  In theory the MOV's are redundant, performing the
same voltage-clamping function as the safety gap.  But since the gap
width must be properly set, and often times people open them up too far,
the MOV's have a fixed threshold voltage that cannot be inadvertently
mis-adjusted and rendered ineffective.  The only trouble is that if you
do open the safety gap too wide and if you do operate your coil with a
too-wide main gap (resulting in overvoltage operation), the MOV's will
clamp and get hot. If operated for more than perhaps a minute (depending
on the size of your NST and the severity of the overvoltage condition),
they will smoke and fail.  But they WILL have protected the NST.

Since the resistors do get hot in normal operation, this means that
power is being wasted there.  But it's generally considered a worthwhile
tradeoff.

I can't comment on "adding more components".  Kinda like asking if more
stuff under the hood of your car will make it go faster...

A spark at the safety gap affects only the safety gap component's
operation, meaning that it's doing its job.  The other components still
operate independently.

Dr. Resonance - what are "reflected oscillatory currents from the tank
circuit"?

Regards, Gary Lau
MA, USA

> Original poster: Jim <branley1@xxxxxxxxxxx>
>
> Hi Folks:
>
> Thank you for the schematic on your filter Terry. Before I order the
> parts for this filter I would like to grasp the theory behind the
> design of your filter. Do the capacitors store excess current that
> would otherwise damage the nst? Are the movs' there to protect the
> nst from surges? Will the 1k ohm resistors reduce the input voltage?
> Does it take an arc at the safety gap to operate the terry filter in
> the first place? Is it overkill to add more components? Will extra
> components reduce the spark length?
>
> Thanks everyone for the informative discussions and helpful tips.
>
> Sincerely,
> Jim