[TCML] gaps

[Bert Hickman]

Hi Mike,

As with life, many Tesla Coiling questions have no simple black or white 
answers... :^)

A well-designed static spark gap can provide excellent performance. The 
most efficient static gap appears to be a single gap with high velocity 
air flow (a "sucker gap" or air blast type). Because of individual 
voltage drops within the gaps, multiple static gaps can have higher 
overall voltage drops (thus being lossier), but they can have superior 
quenching capability at less than heroic air velocity. Under similar 
quenching conditions, the single gap may slightly outperform a multiple 
gap that has a similar breakdown voltage. Static gaps are also 
recommended for first-time coilers for reasons discussed below.

Static gaps can't wring out maximum performance in NST systems. Static 
gaps tend to fire chaotically when used with an inductively 
current-limited transformer (such as an NST or ballast-limited pig). 
This causes multiple bangs during each AC mains half cycle that are of 
varying (and suboptimal) size. In order to get maximum spark length 
versus input power, best results are obtained using a synchronous rotary 
spark gap (SRSG), or a synchronously triggered static gap, combined with 
a larger size tank capacitance (see below). A properly adjusted SRSG 
consistently forces the gap to fire so that each bang is of the same, 
optimal, size - once on every incoming half cycle of the power mains.

In earlier days of coiling, many folks destroyed their NST's when they 
converted from static to rotary gaps. These were also the days when the 
accepted practice was to design coils where the tank capacitor was 
"tuned" to resonate (at mains frequency) with the NST's leakage 
inductance, a practice known as "mains resonant charging". Indeed, many 
TC design tools defined this as the optimal tank capacitor size for a 
given NST voltage and current. For a 15 kV NST, this was about 0.01 uF 
for every 30 mA of output current for a 60 Hz supply. Unfortunately, if 
the main gap was set too wide, the rotary gap was improperly adjusted, 
or the RSG was merely running too slowly, the voltage cross the NST 
could rapidly grow to ridiculously high voltages. This usually resulted 
in overvoltage failures of NST's, or for pig-driven systems, tank 
capacitors.

Over the years, it was learned that, by using a larger sized capacitor 
(called Larger Than Resonant or LTR), mains-resonant overvolting could 
be avoided. Adding a properly adjusted safety gap placed directly across 
the NST output terminals protects the NST from overvolting under 
other/abnormal circumstances. Terry Fritz's "Terry Filter" adds an 
additional layer of protection, especially against high speed transients 
that can cause inter-turn corona damage within the outermost turns of 
the NST windings. It even protects against an improper safety gap setting.

The bottom line line:
A system using a synchronous rotary gap, an LTR tank cap, a properly set 
safety gap, and a Terry Filter is every bit as reliable as a system 
using a static gap. The SRSG system also provides higher performance 
than a similar static gap system.

Bert
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Mike Doyle wrote:
> a rotary spark gap,isnt as effective,as a stationary 4 gap,setup? i use neon sign transformers,and its safer,to use them,with a stationary gap0. true,or false?
> mdoyleufo at gmail.com thanks mike.
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