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Re: MMC Question.

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

Hi Glen,

In a nutshell, the .0106 C value is the capacitance at which the transformers L will resonate with. Using this value will shorten the life of your transformer (real short). To circumvent transformer deaths (mainly, NST's) the TCML found that running a value C that is larger than resonant (LTR) will help keep the voltage at the cap down to that of the NST. This was mainly developed for NST's as they are the widest used transformers for Tesla Coils and NST's cannot handle voltages to far beyond their rating.

The value to use depends on the gap type. For a static spark gap, somewhere around 1.6 x .0106uF is ideal. For rotary spark gaps, about 2.5 x .0106uF is ideal. Of course, you can vary from the ideal (for your "own" ideal). The idea is to not allow resonance between the cap and transformer. In the case of your 15/60 NST, if you were to run .0106uF, the NST would certainly die a quick death. Been there and done that.

If you even get the urge to run spice models of a resonant cap size, you can see the issue involved. Richie Burnetts website is a good place to help in this understanding and he has some Microsim screen captures to help see the issue visually.

AC Resonant Charging

Richies Front Page

It really helps when you also run the sims yourself. I took a look at my own resonant charging situation not to long ago (killed a 12/60 NST). My simulations showed exactly what Richie's had shown, that within a band of C around the resonant C of the NST, voltages can sky rocket. The NST is just not designed to handle voltages that high (as are few transformers for that matter). Almost all NST deaths are due to a secondary winding short (over-voltage).

Terry linked a chart of C values for LTR operation. It's a good chart to go by.

Take care,

Tesla list wrote:

Original poster: "Glen McGowan" <glen.mcgowan@xxxxxxxxx>

Now, one caveat to the below is that I did spill beer on my calculator. But a little barley never hurt anything.

I'm using an 15kv 60mA NST

z= 15000/60
So my impedance is 250000

c= 1 / (6.2832x250000x0.00006 )
So my NST would seem to prefer a primary cap of .0106uF

Currently my MMC is using 16 .15uF 2000v caps (.15/16 = 0.009375uF) would the .0012uF difference between .0094uF and .0106uF really cause me huge problems? The peak amps the STK's are rated at I now understand isn't that lovely but aside from that I should get some fairly decent discharges correct? (If the caps hold)

With all the concern over the caps I'm afraid to ask about the primary and secondary coils. I had originally planned to make my primary around 10-12 turns of 1/4" copper tubing and making the secondary coil winding out of #26 wire with about 20" of actual wound coil length. The wire gauge is not an option. I've got a huge spool of the crap that I really would like to use. I'm not entirely certain what the values should be. This is the trickiest part for me. If anybody could shed some light on this aspect it would help a great deal.

On 8/8/06, Tesla list <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx> wrote:
Original poster: "Gerry Reynolds" <<mailto:gerryreynolds@xxxxxxxxxxxxx>gerryreynolds@xxxxxxxxxxxxx>

Hi Matt,

The issue isn't the 60ma charging the caps. The issue is when the
spark gap fires, the only thing limiting the current is the reactance
of the TC primary inductance.  Any resistance in the cap, copper
tubing, and spark gap plasma will be insufficient to prevent very
very high currents.  The resistance of the CU tubing and caps (esr)
is in milliohms.  The SG plasma may be 2 to 5 ohms.  If this was all
limiting the current, you would have 4000-10000 amps.  It is the
inductance that really limits the current.

Ipeak = Vfire / sqrt (Lp/Cp)

Hope this helps,
Gerry R.

>Original poster: <mailto:Mddeming@xxxxxxx>Mddeming@xxxxxxx
>In a message dated 8/8/06 3:18:05 P.M. Eastern Daylight Time,
><mailto:tesla@xxxxxxxxxx> tesla@xxxxxxxxxx writes:
>Original poster: "Glen McGowan" <<mailto:glen.mcgowan@xxxxxxxxx>glen.mcgowan@xxxxxxxxx>
>Thanks for the extra effort regarding the caps.
>Regarding the peak current, both Charts show the same values for peak
>current. I must be missing something(?). I'm not building a beast by
>any means. But,this is my first coil and plan to power it with a
>15/60 NST. My MMC will use 16 of the STK's in series.
>Wouldn't the current remain at 60mA throughout the tank?
>I'm all ears, still learning so everything is educational at this
>point. As long as I don't stop my heart I consider it a learning
>experience. I would like to avoid the "power up and run for the
>hills" avenue as much as possible.
>Hi Glen,
>     I believe you are confusing the charging current with the
> discharge current. and the rms value with the peak values. Let us
> consider an idealized Tesla Coil running at 150 KHz and 120 BPS.
> True, the cap is charged 120 times per second (60Hz supply) at a
> rms current of 60 mA. However, when the gap fires, all those
> electrons that accumulated on the plates of the capacitor in 1/120
> of a second are now removed in 1/300000 sec - 2500 times faster.
> Since current is coulombs per second, The discharge current for
> that first half cycle could be 2500 times higher than the charging
> current. In this case 150 Amps if only for a few usec. It is this
> kind of slamming that eats up all but the most rugged caps.
>Matt D.