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Re: MMC Cap up in Smoke
Original poster: "resonance" <resonance@xxxxxxxxxxxx>
You need to use Erms x 3, so 6 kV x 3 = 18 kV DC pulse rating for the
capacitor. 10.5 kV is not enough. I also suspect you may have used
a polyesther dielectric cap and they die rapidly as you
discovered. Check with Sprague to determine the caps dielectric and
You need to use polypropylene or polyethylene diectric caps with
metallized foil to handle the current. Otherwise they heat up
rapidly wih the RF currents and the dielectric begins to melt. I
trust you were running your coil. If you didn't have a sec coil in
place the rise in Q factor will blow caps or transformer (sort of
like holding you gas pedal to the floor in a car in neutral --- the
energy has no where to go, ie, no load). In RF circuits without
sufficient load the potentials and currents can rise to very high
levels, typically the Q factor times the applied potential.
Most experimenters use Cornell-Dublier capacitors, typically 0.15 uF
2 kV, with typical of 16 pcs in series at 12 kV Erms. With your
smaller xmfr potential you could use 12 or 14 in series.
Are you guys using any special type capacitors?
I built my first MMC capacitor today it lasted a whole 2 seconds
then went up in smoke. My power supply is a 6K 20ma
transformer. The capacitors are .1uf 1500VDC Sprague 430P
227084-43F. I soldered 7 capacitors in series to get 10.5K volt
rating. My TC is a tiny little thing I built 5 years ago it had an
oil filled poly cap.
Here is a picture on the TC.
Notice the small TC in the bottom left with a 1.5" diameter 5" tall
secondary with a door knob top load.