RE: [TCML] Capacitor choice and voltage question

["Lau, Gary" <Gary.Lau@xxxxxx>]

Hi Scott,

I'm glad you asked.

The capacitor you proposed is the CD 940 family.  For a low powered coil this may do, but in the application with your NST, this family is a poor choice.  The gold standard for TC caps is the CD 942 family.  But unfortunately I don't think that Digikey doesn't sell them.  Dr Resonance does, Richardson Electronics (rell.com) does, several ebay vendors do.

The total capacitance you proposed, .013uF, is also much too low for a 120BPS sync RSG.  Using too low a capacitance would result in cap (and NST) voltages far in excess of 21kV.  With 60 Hz power, a value of .028uF is often suggested.  I also use a 120BPS RSG with a 15/60, and have found that a cap size of .04uF worked best, though this appears to be unusual.

I would plan on sizing the total cap voltage rating AT LEAST 25-30kV.

Regards, Gary Lau
MA, USA


> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Scott
> Sent: Saturday, May 17, 2008 3:15 PM
> To: tesla@xxxxxxxxxx
> Subject: [TCML] Capacitor choice and voltage question
>
> Hello Everyone!
>
> So after getting an awesome 15000v, 60ma NST, I have decided it's time
> for my second coil and a venture into rotary spark gaps.   Before I go
> and dump a small fortune into the various ingredients for a RST
> system, I wanted some advice/approval on a few of my choices.
>
> The biggest part is the capacitor bank.
> I have decided to use Cornell Dubilier axial film caps.  I decided the
> 0.10uF @ 3000v unit would be the best here (digikey part #
> 338-1187-ND).
>
> For a 1800 rpm, SRSG there would be 4 electrodes, so 120BPS (ok most
> know that...)
>
> And for the capacitance of the MMC array would be:
> (resonant NST capacitance * 150) / BPS
> or for my system
> (0.0106 uF * 150) / 120 = 0.01325 uF
>
> Ok, so 0.01325 uF, does that sound right?  It seems less than a static
> gap at a larger than resonance value.  Should I account for any other
> factors? (I got this formula off Richie's site:
> http://www.richieburnett.co.uk/tesla.shtml ) seems like a very good
> source.
>
> Anyway, onto the voltage of the tank cap.  The "peak voltage" of the
> system is 21,000 volts, so 7 of my cap units in series seems like it
> would be perfect, but I know that's not gonna cut it.
>
> So what kind of voltage should my MMC caps be able to handle?  I have
> seen estimates of up to 50,000 volts for this kind of NST, is that
> what I should expect?  Is the capacitor mentioned above a good choice?
>
> Thanks for the responses,
> Scott Leonard

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