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Registered Member #2553
Joined: Fri Dec 18 2009, 01:36PM
Location: St Cloud Minnesota
Posts: 97
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.
Registered Member #4396
Joined: Mon Feb 06 2012, 11:58PM
Location:
Posts: 30
Use the DC rating, thats whats used in every design that I know. I know its AC, but really you should think of it as DC that experiences hi pulsed loads at Fres. Thats what you are looking for... High voltage withstanding, and high pulse rating. In my reading and limited experience, you cant beat the CDE 942's for that. 942s use polypropylene tech, they have lower loss tangents (reactive losses) than other film cap technology. That means that you will need less parallel strings for the same power dissipation (= less money overall for your pulsed current needs, same power handling without heat).
Registered Member #4108
Joined: Sun Sept 25 2011, 11:43PM
Location:
Posts: 149
Tesla coils are an ac circuit. The reason they are rated higher for dc is because of capacitive reactance. DC current stops flowing after the cap is charged. Since ac is always changing as soon as the wave reverses it has to do twice the work to get rid of the previos charging and charge it back up the other way. So go by the ac rating
Registered Member #4396
Joined: Mon Feb 06 2012, 11:58PM
Location:
Posts: 30
I sort of meant that coilers seem to get by using the DC rating on the CDE 942 series from what I've seen. The CDE 942 specs are written for continuous duty and have some saftey margin built in. We usually just want want them to run for a few minutes, so you can save money kinda over stressing their voltage rating.
All MMCs that I've seen say, "I've got X series 2kVDC CDE 942's for an X*2kVDC voltage rating."
I know you aren't "supposed" to do it, I'm just saying people seem to get away with it, and those parts aren't cheap. Maybe because hes looking to filter the NST's output, its epically not a good idea.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Here is a useful table for designing your MMC using the CDE 942 series .15uF/2KV capacitors.
The recommendations are based on both maintaining an adquate voltage margin in a TC tank circuit and limiting internal heating of the capacitor dielectric.
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help with capacitor ratings
