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Does anyone have experience using ceramic disc caps for MMC tank cap service? I have hundreds of identical 1KV .002uF caps that definitely provide the most in the way of KV/$. However, I've only seen reference to one ceramic disc MMC anywhere (20KV with 10KV caps). I'm wondering if they can stand the instantaneous current demands in a rather small (3", OBIT) coil.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I have made a SGTC using an MMC of 2.2nF 4kVac ceramic capacitors, it worked fine.
When I changed the MMC to polypropylene capacitors the performance was much better even the spark gap was louder (for the same capacitance value / energy etc.)
If you can get 'doorknob' / 'transmitter' ceramic capacitors they are very good (N750 etc.) the small disk ceramics are normally Y or Z type dielectric which makes them cheap, small, high capacity/voltage but poor dissipation e.g. N750-type ceramic capacitors <1% dissipation, z- or y-type dielectric 1% to 2.5% dissipation.
So Y- or Z-dielectric discaps will safely (i.e. without letting out their own magic smoke) provide the energy needed, just with a bit higher losses? I can live with that, especially given the pennies-to-dollars price differential between them and polypropylene.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Normally I would not recommend polyester capacitors for TC use BUT some of the newer high current types do SEEM ok (igbt 'snubber' types) I haven't used them myself but I have seen successful use of them on some internet pages. Based on just my own experience; there's no substitute for good polypropylene foil/film capacitors.
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
I hace two experiences with ceramic MMC's.
1 My first Tesla coil with 2 inch sparks ran with about 10 x 3kv caps and still does on my mini coil. I might have lost a couple but this is with intermittent service over 20 years.
2 A bigger MMC for a bigger TC with more modern but smaller caps rapidly died. A few caps would smoke each run. MTBF was less than 1 minute. See pics below.
My advice is don't use them except for very small coils.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Those small epoxy dipped caps are actually very lossy for HF and I would strongly recommend not to use them. They will get very hot, and due to their high tempco this will drive the TC out of resonance quickly.
With poor energy transfer they get even hotter and start dying. Foil/PP caps are only way to go as I think.
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Ceramic MMC
