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Caps for MMC

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Eartaker

Mon Apr 23 2007, 03:45AM

Registered Member #150 Joined: Sun Feb 12 2006, 09:43AM
Location:
Posts: 11

Would these be ok for an MMC, this is my first MMC.

GreySoul

Mon Apr 23 2007, 04:01AM

Registered Member #546 Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239

I still don't know why but I was advised against snubber caps for MMC design in favor of the CDE 942C20P15K caps of fame and legend.

Digikey and Rell can usually get some if you're patient...it took me about 40 days to get mine.

Anyone know why snubbers aren't good for MMCs, or was I told wrong?

Steve Ward

Mon Apr 23 2007, 04:19AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

I still don't know why but I was advised against snubber caps for MMC design in favor of the CDE 942C20P15K caps of fame and legend.

Those ARE snubber caps (snicker).

Its possible the person who advised against them didnt know what they were actually talking about, or they were thinking about the snubber caps often found in SCR circuits (typically paper dielectric, oil filled). Perhaps it just depends on what area of power electronics you are into, but when i think snubber, i think of low inductance, low loss PP film caps (or ceramics too). Anyway, an ideal snubber cap is an ideal tesla coil cap, and vice-versa (low inductance, low dielectric loss, and low ESR... basically an ideal capacitor).

As to the Illinois Capacitor, it *should* work, but you better watch the peak current rating, particularly because these are "metalized" and not "foil" and film caps.

If you can supply some specs on your coil (charging voltage, resonant frequency and primary inductance) then i can show you how to calculate the peak primary current, and the suitability of these capacitors.

Eartaker

Mon Apr 23 2007, 04:40AM

Registered Member #150 Joined: Sun Feb 12 2006, 09:43AM
Location:
Posts: 11

Hmm with the 942C20P15K I will need 70 caps. I want to build a 3" coil powered by a 15kv 50ma NST (well actually a 20ma and a 30ma both 15kv ran in parallel making 50 ma)

Terry Fritz

Mon Apr 23 2007, 04:47AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

"Snubber caps" are often made to order with specific terminals to go across specific big IGBTs... Thus, "EXPENSIVE"!! But all kinds of non-super specified snubber caps are around. If the price is right and the peak and RMS current is right. No problem!

Many cap manufactures now wildly state "peak" and "RMS currents". Those that "don't" should be avoided since their caps probably suck... Beware that some "lie" too

CD typically "understates" their caps in my experience. The "geek style" caps have actually gotten far better over the last 5 years... I really like CD caps and I am willing to pay more for them when needed.

Cheers,

Terry

Steve Ward

Mon Apr 23 2007, 05:29AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Forgot to mention, in the Illinois cap DS, it gives a dv/dt in v/uS. Current (I) = C*dv/dt, so you can use the dv/dt rating, multiply by the capacitance (in uF) and get the current rating. I think the .22uF 1600V cap was a 1700v/us rating, so thats 374Apk (which i will admit, im *slightly* skeptical of). But if there is truth to it, then id guess 2 parallel strings of these caps would work well for the size of your NST (making some very general assumptions about your coil design).

GreySoul

Mon Apr 23 2007, 02:15PM

Registered Member #546 Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239

Steve, that's why I'm here :) to get real advice instead of the local crackpot advice I can find here locally :)

When I built my MMC i was looking at some snubber caps that were little plastic boxes with 2 leads out the bottom.... they were .01 2kv from panasonic (I think) and was told to avoid them since they were metalized, and not foil. I think that the REAL point was "don't use THOSE snubber caps"

HV Enthusiast

Mon Apr 23 2007, 03:08PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

The GE 42L series work great for SSTC including DRSSTC use, although they aren't forgiving as the 942C series for spark gap type coils.

gentoo_daemon

Wed Apr 25 2007, 04:27PM

Registered Member #512 Joined: Sat Feb 10 2007, 05:42PM
Location: Chico, CA
Posts: 25

One word of advice, when you order the famous 942C20P15K, make sure you get the -f version, 942C20P15K-f because these are ROHS compliant. The original version is no longer produced (thus your order will never arrive if you order from rell.com).

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