Re: terry filter theory

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Hi Gerry, Todd,

The safety gap is typically a single electrode to electrode gap. The 
main gap should be built for temperature stability. It is the main 
gap that must be set correctly (first, before the safety gap). Once 
the main gap is set according to the NST voltage, then set the safety 
gap to "just not fire".

Main Gap Adjustment = Set so that it fires consistently with only the 
NST and main gap.
Safety Gap Adjustment = Set so that it just doesn't' fire with only 
the NST and safety gap.

If during running, the safety gap is firing too often, "don't adjust 
the main gap". Instead, check to see if the main gap is getting hot 
(lowering the firing voltage) which is usually the case. If so, then 
increase the air flow and stabilize the main gap temperature. If 
after doing so the main gap remains cool and the safety gap is still 
firing too often, then the safety gap should be "slightly" increased 
(but only slightly, about 500 volts). If still the safety gap is 
firing too often, the main gap cannot dissipate the heat fast enough 
and a rebuild of the main gap and cooling mechanism (or both) may be required.

Here's a common scenario:
1) the main gap gets warm.
2) the firing voltage lowers.
3) the safety gap begins to fire.
4) the safety gap gets warm from the increased firing.
5) because the safety gap electrodes heat up, they now fire at a 
lower voltage, more and more frequently.
6) the safety gap just turned into the main gap.

The coil starts out firing with the main gap and after a little time 
the safety gap takes over. The main gap is typically the problem, not 
the safety gap (unless you mistakenly created a needle gap for a 
safety gap). The safety gap should be able to either have the same 
cooling mechanism as the main gap or be slow to temperature 
variation. The later is usually the case by using a similar or 
slightly larger radius as the main gap electrodes but of a solid 
material so that it's mass is greater and it's rise to temperature is slower.

If you really wanted to make the temperature stable in both gaps 
(ideal situation), build the static gap with a couple extra 
electrodes to be used as the safety gap so the air used for the main 
gap also cools the safety gap. The safety gap doesn't need to be "on" 
the Terry filter. You can place it anywhere, and within the air 
cooling mechanism would be a much better position.

Take care,
Bart

Tesla list wrote:

> Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
>
> Hi Todd,
>
> Yes.  There is a simple procedure for setting the safety gap length.
> Some will advocate a setting of a fixed gap length, but I don't 
> recommend this as the actual breakdown voltage is affected by the 
> electrode geometry and the altitude you are operating at (among 
> other environmental factors).  Many would recommend disconnecting 
> the TC primary, tank cap, and main spark gap from your NST so the 
> only load the NST sees is the filter and the safety spark gap.  You 
> would then set each gap (one for each NST bushing) so it would just 
> barely not fire with this almost "no load" condition.  Start with a 
> small gap size and power up the NST and see it arc across the safety 
> gap. Power down, unplug from the power source, and then increase the 
> gap a little. Repeat this procedure until the gap width is just 
> barely too wide to arc.  This will be your setting (one for each 
> bushing).  At this point the safety gap will arc if the voltage 
> across the gap ever gets larger than the "no load" output voltage of 
> the NST.  If you are using a variac to drive the NST, set the safety 
> gap with the variac at the maximum output voltage that you would 
> want to operate at.  Most would do the setting with a maximum 140Vac 
> output from a 0-140Vac variac when 120Vac is feeding the variac. 
> Once set, leave it alone.
> If during operation the safety gap fires too often, this would 
> indicate that your main static spark gap is set too wide. Reduce the 
> main gap setting until the safety doesn't fire or fires only occasionally.
>
> Gerry R.
>
>
>
>
>
> > Original poster: "Todd Reeve" <todd.reeve@xxxxxxxxxxxx>
> >
> > I am putting together a Terry filter for my coil as we speak.  I 
> > realize how important the gap width is but I am unsure how to 
> > determine the proper gap width.
> >
> > I am using a 15kV/60mA nst.
> >
> > Can anyone suggest the proper method to determine the gap width?