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Difference between CDE940 and 942?

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LAN_403

lpfthings

Sat Mar 13 2010, 12:19AM

Registered Member #1361 Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305

Hi, I was wondering, what is the difference between the CDE 942C and 940C caps?

Farnell offeres free shipping on all online orders, and is also based in Aus. They carry the 940C version of the caps but not the 942's.

I think I remember someone mentioning they had arcover problems in TC duty?

Cheers,
Dan

Herr Zapp

Sat Mar 13 2010, 01:35AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

lpfthings -

Why not take a look at the CDE website (

,) and compare the specification sheets for the 940C and 942C series caps ("Capacitors by Series Type")? You'll learn a lot more if you look at the datasheets and see the difference in construction, and compare peak and RMS current ratings for the same value caps in both series. This will be more informative than just having someone tell you what the basic difference is.

There is an important difference in the construction that is significant if you intend to use them in a TC tank circuit or other applications where they will see high pulse current values.

Regards,
Herr Zapp

oucivileng

Mon Mar 15 2010, 01:04AM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

I used 940C caps for my 12kV/60ma SGTC coil. They were rated at 3kV and 0.15uF. I had 8 of them in series for 2400V and 0.019uF. They worked well for a while but I didn't run it very frequently. I've now blown three of them. One actually caught fire and charred the plastic board it was mounted on. Basically, if you buy them, don't expect much in terms of longevity and have a few spares handy in case one blows.

They seemed to do better if I ran the coil for about 10 seconds and then shut it off for another 10 or so.

Goodchild

Mon Mar 15 2010, 05:01AM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Ive had no problems with the 942Cs yet I use them in my DRSSTC.

The trick is to build a bigger than required MMC that way it will last a lot longer.

Steve Conner

Mon Mar 15 2010, 10:03AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

DRSSTCs have better controlled peak currents, so the metallized film capacitors work OK. Even if you don't use a current limiter circuit, the IGBTs will probably blow before the tank capacitors do.

In a spark-gap coil the peak current is basically "pick a number between 0 and infinity". You can even trash the 942 series if you're not careful about spark-gap width, safety gapping, tank circuit impedance and so on.

Luca

Mon Mar 15 2010, 04:05PM

Registered Member #2481 Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134

Steve McConner wrote ...

DRSSTCs have better controlled peak currents, so the metallized film capacitors work OK. Even if you don't use a current limiter circuit, the IGBTs will probably blow before the tank capacitors do.

You can even have a current limiter (peak) but if you exceed rms rated current you burn caps because of therml dissipation... So, I think that a good estimation of the rms current is important in order to avoid capacitor failure.

Regards,

Luca

lpfthings

Tue Mar 16 2010, 07:22AM

Registered Member #1361 Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305

I received 9 of the 2kV .033uF caps today. I connected them all in series and connected them to my 15kV 30ma NST and set a spark gap about 1cm.

And wow, you guys werent lying when you said SGTC's are loud! Even just the .033uF bank was enough to make me want to turn it off after <1 second, and bet it would have pissed the neighbours off!

If it wasnt running at 50hZ, and was just like 1 or 2 BPS, I may have run it for longer, though it was too loud for my backyard. Can't imagine what the huge cap banks are like!

Am I also understanding right that I shouldnt set the spark gap to a point where it doesn't fire? Would it cause higher voltages on the caps? Or corona breakdown or something?

Steve Conner

Tue Mar 16 2010, 10:11AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Don't run a SGTC without the secondary! That's one of the ways to blow your capacitors, as it causes huge RMS currents.

Am I also understanding right that I shouldnt set the spark gap to a point where it doesn't fire? Would it cause higher voltages on the caps?

Yup, that's another way to blow them!

lpfthings

Tue Mar 16 2010, 10:26AM

Registered Member #1361 Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305

Nah, I wasn't running it in any tesla coil setup, just connected them straight to the NST and setup a little spark gap. Wasn't brave enough to take the gap any further than 1.5cm or so. 1. It was too loud anyway. 2. I didnt wanna blow up my caps

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