Re: Question about capacitors for coil

["Tesla list" <tesla@xxxxxxxxxx>]

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.
>
>