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help with capacitor ratings

Discussion in 'High Voltage' started by lokeycmos, Feb 22, 2012.

  1. lokeycmos

    lokeycmos New Member

    Oct 17, 2011
    so im working on the mmc bank for my tesla coil. my bank needs to be 8.6nF @ ~37kv.. these caps seem to be the ones everyone is using:

    MMC Capacitor - Cornell-Dubilier CDE 942C20P15K-F

    im a little confused, looking for some clarification. they are rated for 2000vdc and the data sheet says 500vac. do i use the ac rating? if so i need well over 60 caps. they arent cheap.

    one more question. the terry filter im working on from the attached pic, uses caps too. we are working with ac here, but the digikey part number says they are only dc rating. they dont even have a ac rating. will these still work? ty

    im using a 15kv nst. peak is 21210v. i want a higher rating so nothing burns or pops.

    Attached Files:

  2. johansen

    johansen Member

    Mar 11, 2009
    You have to use the ac ratings.
    However, you can exceed them within reason.
    Some say the life of a capacitor follows the 15th power of the voltage. (when compared to the manufacture's MTBF and ratings.) others have said the 20th power. (film capacitors intended for pulse duty)
  3. unclejed613

    unclejed613 Well-Known Member

    Apr 3, 2009
    i've probably mentioned this before, but when i was a teenager, i made my own HV caps out of plate glass and copper foil. i had to do the capacitance calculations, but many modern DMMs have a capacitance function on them. the dielectric constant of glass is between 5 and 10, and the dielectric strength is about 115kv/cm. teflon has a higher dielectric strength (450kv/cm), but it's dielectric constant is about 2.1. many of the old "wireless" texts have details on proper HV capacitor construction. if you like "old school", you could even try making Leyden jars.
    Last edited: Mar 13, 2012

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