RE: Question about capacitors for coil

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>

Concerning what size coil form to use for a 9/30 NST -

I recently rebuilt my mini-coil with a 4/20 NST, moving from a 1.9" x
9.5" x #32 secondary to a 4.25" x 9.75" x #30 secondary.  I've only done
a small amount of tweaking but this has increased performance from 14"
to about 16". The performance increase is significant, but dang, not
enough to top Karl Lindheimer's 18.125" record in the minicoil
competition! I attribute my performance increase to the higher
inductance, which forced me to increase the primary inductance, which
reduced gap losses.

But back to the secondary size issue, I would definitely not use
anything smaller than a 4" secondary diameter, unless small size is a
goal.  I'm personally disappointed that to date, everything I've found
with my mini coil points to bigger is better!

Regards, Gary Lau
MA, USA


> Original poster: John <guipenguin@xxxxxxxxx>
>
> Thanks Dr. Resonance.
>
> I have been debiting what size diameter secondary I should use with
> my coil before I goto the hardware store and grab some pvc. 4" by 26"
> seems a bit large for where I wanted to start. What would be a good
> size diameter for this transformer? I have read a lot about different
> people's opinions on hight/diameter ratios...but I'm not sure what
> diameter to start. I was thinking 2" diameter at less then 12 in
> hight...but this may be way to small for decent spark length....
>
> On 10/1/06, Tesla list <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
wrote:
> Original poster: "resonance"
> <<mailto:resonance@xxxxxxxxxxxx>resonance@xxxxxxxxxxxx>
>
>
>
> The strontium work better than the barium titanate, however, MMC caps
> work even better especially at these frequencies that small to medium
> size coils run at, ie, 200 to 300 KHZ range.  The MMC caps usually
> give longer sec sparks due to their higher efficiency.  Erms x 2.5
> will give you good reliability, so you would arrange a series of caps
> to equal 22.5 kV.
>
> 22.5 kV / 2 kV MMC cap rating = 11.25 units in series to meet the
> voltage requirement.
>
> To get the capacitance right, you could use 17 pcs, 0.15 uF, 2 kV, in
series.
>
> .15 uF / 17 caps  =  .00882 uF approx .009 uF with voltage rating of
> 17 x 2 kV  =  34 kV for the 17 pc. string.
>
> I made a coil similar to yours once using a 9 kV 30 mA xmfr with this
> cap bank.  I used a 4 inch dia. sec coil (PVC tube sanded inside and
> outside and sealed) and closewound with 30 AWG magnet wire x 26 inch
> winding length. Output spark was 18-20 inches long.  I used a 5 gap
> copper pipe sparkgap with a 200 CFM blower fan on one end.  Inside a
> 6 inch ID PVC tube x 8 inches long.  Each copper pipe was 4 inches
> long x 3/4 inch dia.  6 tubes, 5 sparkgaps set at 0.025 inches (25
> mils) each for total gap of 125 mils (.125 inch).
>
> I have photos if you wish to contact me off-list.
>
> Dr. Resonance
>
>
>  >Hello.
>  >
>  >For my 9kv @ 30ma AC transformer, I will need a capacitance of about
>  >0.0088 MFD. I have been looking at some 40kv 2000pf Strontium
>  >Titanate ceramic caps on ebay. These have a "DC" voltage rating of
>  >40KV.  I have been told that I can de-rate this rating to around
>  >40-50% min for use with a High voltage AC circuit.
>  >
>  >Would these be an OK choice in parallel to give as close as possible
>  >to 0.0088uF? I am fairly new to coiling and would like an opinion
>  >before I buy these caps.
>  >
>  >Thanks.
>  >
>  >
>
>
>
>
>