RE: Best cap size for a sync gap

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Gary,

At 09:24 PM 4/30/2004, you wrote:
>To expand on my previous post:
>.....
>
>Oops, my bad!  This is the trouble with using an uncalibrated probe, not
>knowing exactly what you're seeing.  Assuming that the measured
>open-circuit NST secondary voltage -at-120V in was in fact 1.414 * 15kV, I
>have applied this calibration to calculate the bang voltage (BV), peak
>voltage when the gap doesn't fire (PDF), and 120*0.5*C*V*V Watts
>delivered to the cap (W), all at 140VAC in:
>
>Cp      BV      PDF     W
>.02uF   24.4kV  29.2kV  714W
>.03uF   21.9kV  31.8kV  863W
>.04uF   19.6kV  32.6kV  922W
>.05uF   17.1kV  ????    877W
>
>OK, the condition when the timing is advanced too far and the gap stops
>firing IS bad.  But setting the safety properly fixes it neatly.

That makes sense.  If you drive an NST into saturation with "just" a cap. 
the resulting hum is extremely load and obviously "bad".  But it is a bit 
interesting that I have not, you have not, and nobody else has not ever 
blown an NST from this ;-)


>I was surprised to see that the "PDF" voltage *increases* with
>increasing Cp.  I would have guessed that the more distant from the
>mains-resonant value (.01uF), the less resonant rise there would be?

More saturation, more current, more voltage ;-))


>Today I quickly hooked up an analog watt meter with my 140V .04uF
>configuration and noted that it was drawing 1.7kW at the "best" timing.
>Power and current increased as timing advanced up to the sweet spot, and
>when it reached the gap-doesn't-fire point, the power soared to 2.4kW.

WOW!!!

>It was gratifying to note that the power did not change as PFC caps were
>added, but the current was distinctly reduced.  20A w no PFC, 16A w/
>120uF, and 14A w/ 240uF.

That sounds right.  Mine has 200uF and draws ~8.5 amps (normal LTR).  Your 
saturating condition seems to be about where we would expect form all 
this.  About 30% higher power.


>I'll have more to report on 120V operation and power in a few days.

Cool!!

BTW - A bunch of post from this morning got miss-handled by the butter 
fingered moderator.  I just resent them now ;-)

Cheers,

         Terry


>Regards, Gary Lau
>MA, USA