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Re: Question about capacitors for coil



Original poster: "resonance" <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.