# [TCML] secondary size, and caps

bunnykiller bunnikillr at cox.net
Thu Jul 3 21:49:28 MDT 2008

```Hey Nicholas...

there may be a difference in output, possibly 1/2" worth.... but it can
be made up in properly designing the primary coil and cap along with the
spark gap...

when designing a set of MMC's to end up with the approprieate voltage
capacity, you need to consider the output voltage of the power supply (
1st ).
if you have a 9KV unit, you need to create an MMC that can handle at
least 1.4 times the 9KV rating. Why??  the voltage rating of the
transformer is set for RMS value of the total voltage output of the
transformer ( which is not the voltage shown on the plate). RMS is sort
of like an average voltage rating of the hi peak and low peak ( 0VAC) of
the AC sine wave. So in actuality, the transformer can put out 12,600 V
peak to peak but is rated for 9KV. With this in mind you should design
your MMC to handle a minimum of 13,500 - 14,000 V.

There are caps out there strictly rated for DC only ( like electrolytic
types) and there are caps rated for AC/DC duty, but these are normally
rated in DC voltage on the cap. They can be pushed to maybe an
additional 10% higher than the stated voltage rating but the life span
will be reduced.

To design an MMC, you will need to place some caps in a string ( series,
end to end like batteries in a flashlite) to build up the total voltage
rating of the caps. As you string caps in series the voltage handling
the string. For example you place 10 2000VDC caps in series, the string
should be capable of handling 20,000 V.  Now here is the down side to
this stringing of caps.   As you add caps to the string, the total
capacitance drops, the more you add the faster it drops.  Basically 2 in
series = 1/2 the original value. 3 = 1/3 original value and so on
heres the math for 3 caps

1/ (( 1/uF) +(1/uF)+(1/uF))   =  1/ ((1/.15) +(1/.15)+(1/.15))  = 1/
((6.66) + (6.66) + (6.66))  = 1/ 20 =  .05uF

since the capacitance has now dropped way to low to be effective, you
now need to add a few strings together in a parallel manner. As you join
the strings together in paralell the capacitance now increases. Lets say
you have 3 strings of 3 seriesed caps, each string rated for 6KV at
.05uF. By paralleling them you now have 9 caps total at a rating of 6KV
but now at .15uF ( 3 X .05).

For your 9KV unit you will need 7 caps in series per string and
depending on your current supply of the 9KV unit,  2 strings in paralell.

As far as getting a true 120 BPS you have to make something called a
synchronized rotory gap. This requires a motor that spins at 1800 RPM (
not 1760) but I wouldnt worry about getting a true 120BPS with the size
of your power supply you are using at this time, go for the static gap
and set the gap width to a point where you get the cleanest sounding
60Hz hum from it, or if you have good tonality 120Hz buzz. When you
start hearing a lot of popping that isnt steady, you have opened the
gaps a bit too far and you are missing gap closure ( gap conduction) .

Scot D

Nicholas J. Goble wrote:

>I have a few more questions.
>
>I'm using a 9/30 NST.  I originally wanted to have a 4" diameter, 20"
>tall secondary, but after a wire shortage, I only have a 17.5" tall
>secondary.  Will this make a difference in my overall coil performance?
> If so, how much difference will it make?  If I splice the wire and add
>on another 2.5", will that do anything?  I read that the coil was
>supposed to be continuous.
>
>I want to build my MMC and I've been told that I should just use a
>bunch of 942C20P15K-F caps.  I've asked this before (Designing an MMC),
>and I got a lot of questions answered.  I now understand how voltage
>works on the MMC, but I'm still confused as to what my capacitance
>should be.  I know I should shoot for 120BPS, but I can't find how to
>make that happen.  Could someone explain how to get my BPS to 120 or
>just tell me how many caps (.15mF 2000V) I should put in series to make
>a suitable MMC?
>
>know it says 2000VDC and I'm not sure about the "VDC".  I'm not
>building a DC coil.  Does VDC mean DC volts?  Where can I find suitable
>caps to use on an AC coil?
>
>Thanks
>
>Nicholas Goble
>_______________________________________________
>Tesla mailing list
>Tesla at www.pupman.com
>http://www.pupman.com/mailman/listinfo/tesla
>
>
>

```