Re: [TCML] gaps

[bartb <bartb@xxxxxxxxxxxxxxxx>]

Hi Bert,

Excellent assessment as I always expect to see in any of your postings!

Your first two paragraphs sort of stuck out to me and didn't quite match 
my experience (the note about the most efficient static gap appearing to 
be a single gap). For that particular issue, I disagree. If within a 
single or very quick time frame, then I would agree. But there are 
various types of coilers (and by various I mean how they run their 
coils). For me, I like to run "long runs" and for others, 20 seconds is 
a long run. For "those" extremely short runs, a single gap would be 
efficient, but for those of us that run the 5 minute or longer runs, a 
single gap will never do well except maybe at low power.

The gap has to find a thermal equilibrium. That's when heat 
dissipation/time becomes important for an efficient static gap. A single 
gap would only do well in this situation at low power. Higher powered 
NST coils need quick thermal stability. For myself, I've found largish 
copper tubing to be good for a static gap. It's quick to heat but quick 
to dissipate also. With the right air flow, rather high current can be 
used efficiently. I have tried the same air flow on 1/2" tubing and the 
gap was lousy! Then tried the same on solid stock. Great at first, but 
then suffered immense losses due to heat build up in the material (about 
15 seconds into the run, efficiency dropped like a rock). Surface area 
makes a big difference. But even 1.25" tubing surface area without the 
right air flow is no better than small 1/2" tubing. The air flow and 
tube size are very important for an efficient static gap that will rival 
the best SRSG.

As far as "chaotic firing" of static gap systems, I "sort of" disagree. 
It is a bit "higher" bps than one might calc, but I have not found it 
all that high. There is a pattern to the break. Because of that, it's 
not so "chaotic" in my measurements (so far). I still need to measure 
current and bps concentrically to get a handle on it. My most basic 
thoughts are current is dramatically increased (ferro resonance?), the 
gap breakdown voltage decreased for a moment (?), or the gap had a 
transient occur resulting in a hv situation that forced breakdown. 
Always, this occurrence occurs directly "after" an expected break down. 
This is the not so chaotic behavior. There is a reason for it. It's just 
a matter of determining if it is a sudden current or a voltage 
controlled issue.

Take care,
Bart

Bert Hickman wrote:
> 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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