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Re: [TCML] rotary gap disk



Hi Phillip,

Well the large NST current helped quite a bit. Still, it's one of the largest dwell times I've looked at (1.6ms). For 240 bps operation, this would be a problem, but 120 bps should is probably ok. If the psu was any smaller (or cap larger), the dwell time might cause problems. But with rsg's, hard to say what the problem was. This is just one of many aspects. But considering the small disc size and slightly large cap size, it simply adds to the probability. There's also the fact the cap is sized for full resonant charging, so this would have allowed the cap to charge rather quickly between bangs and could also have been part of the issue. We would have to spice this one to really look at it.

Take care,
Bart

Phillip Slawinski wrote:
Bart,

I just measured the disc, it has a 5" diameter.  My cap size is .05uF, and
when I used that gap I was running the whole setup with a 12kV 240mA NST.
The performance was nothing to write home about, but then again It was not
the worst run I've ever had.

On Sun, Nov 30, 2008 at 16:30, bartb <bartb@xxxxxxxxxxxxxxxx> wrote:

Hi Phillip,

One thing I didn't point out is that dwell time affects are psu and cap
dependent. Same problems can happen in large discs also. But more likely in
smaller disc sizes given the typical psu and cap sizes used. I have no clue
what psu, cap size, or disc size you used, so my comments are more of a
condition that is possible. Worst case is a small psu current, small disc,
and relatively large cap. I'm not sure if you were in this territory, but
thought I would throw it out there as food for thought (though you may
already realize this aspect).

Bart



bartb wrote:

What can happen in a small rotary is that the electrode mechanical dwell
time can become excessive, up to the point of the cap charging and
discharging a time or two during the mechanical dwell. This can cause a
problem with steady firing voltages and so it tends toward a chaotic voltage
at each break. Actually, in this situation, it helps to have a bit larger
gap to prevent these occurrences. But as the disc is larger in diameter, the
dwell time reduces linearly. A 10" rotational diameter would have 1/2 the
dwell time as a 5" rotational diameter. As the diameter of the disc is
increased, it doesn't take long before a tight gap is ideal (as tight as is
mechanically sound). It's simply another cool aspect to larger rotational
diameters. Of course, the stresses on the disc increase with larger
diameters, and another good reason to use G10 or G11 as an insulated disc
(at least for hobby coils).

Regards,
Bart

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