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Specifications:
This MMC was designed to be used for the tank capacitor of of a small/medium size Tesla Coil system running 1kVA. The charging transformer will be 15kV 60mA, it is understood that these values will result in 60Hz resonant charging of the capacitor.
I used a MMC of similar design once before, utilizing the same Philips capacitors, with 17 strings of only 10 capacitors per string. This resulted in a final rating of .0106uF @ 20kVDC. (capacitance measured with Aron's ultra-cool Sencore LCR meter). This MMC was used on one of our first coils...with mains resonant charging made very evident by a 120Hz scream when the coil was in operation, and also the spark gap would fire at a very high setting (0.3" - 0.4") with only 10-15VAC going to the neon transformer.
This MMC capacitor, with a total voltage rating of 20kVDC, survived about 3 hours of highly stressful operation. The spark gap's setting was sometimes set at 0.3", but was usually kept around .4". Amazingly, the Philips capacitors withstood 3.5 hours of operation before the first sign of problems.
Over time, I noticed that the primary coil inductance would have to be decreased every now and then, to keep the system in tune. After several days worth of pondering, it was concluded that individual capacitors were shorting out: reducing the number of capacitors per string, therefore increasing the MMC's value. The tank needed less inductance to stay in tune with the resonator, because there was added capacitance. So as more and more capacitors shorted out, the MMC value would go up, and a subsequent decrease in tank inductance would keep everything tune. At the end of the MMC's life, the primary coil was tapped approximately 2 complete turns less than it had been with the brand-new capacitor.
Upon inspection of the innards of each capacitor, it was found that they were RIDDLED with black-outlined holes where there had been HV punch-through. However, the manufacturers claim of "self-healing" was strikingly evident as the metallization was blown away from the puncture. Self healing indeed, I am impressed.
The new MMC has not 10 like the original capacitor, but 12 capacitors per string. This increases its voltage rating from 20kV to 24kV, and the resultant increase in lifetime will be very worth the effort spent in construction.
The lifetime expectancy of a MMC capacitor is not linear, in other words, it's not as if each capacitor you add to the string gives another 1 hour of lifetime. I don't know what the function is, I want to say it's exponential but I'm not sure. The life expectancy formula would tell us, however.
