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Marx with mmc Caps??

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LAN_403

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Wed Aug 22 2007, 03:34PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

Today i got 230 Wima fkp1 4,7n 4kV dc caps really cheap. The first thing i thought was to connect them in series by 8 to build a 587pF 32kv dc cap..

Cause i have so many of them i thought it would be great to build a bipolar marx with these caps. 14 stages for each marx gen.

What do you think? I like the idea of potting each cap in wax or epoxy to make it look like a "real" pulse cap. wima capacitors are high quality here in germany and especially the fkp 1 are used as snubbers. so they have low self inductance and are film/foil.. so is it possible or is it not worth to try cause of losses in the caps or something?

Marko

Wed Aug 22 2007, 03:41PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi

I would think of it as a pity to use FKP1 caps in a marx, where most cheaps like polyesters and ceramics would do. Don't pot them, at least, they look good as MMC too.

But if you have no use for them, why not.. I actually plan to use a bunch of seriesed ceramic (10nF 3kV) caps in a marx as MMC of five in each stage.

Steve Conner

Wed Aug 22 2007, 04:51PM

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

Yeah, it should work fine. The Wima caps are probably much lower loss than the Russian ones Mike Harrison used in his 1MV Marx.

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Thu Aug 23 2007, 08:50AM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

He, nice. Thats what i wanted to hear.
And Marco, its not a pity and what other use should i have for this 4kv caps than for a marx generator? if i had bought this caps at my local electronics store for the regular pricing, i would be a poor man now, but i got em really! cheap. 230 of this caps for 50â‚¬ thats around 68$. they are too small for any other use. 4,7nf is nothing and i dont build sgtcs. as mmc for a drsstc it would be a pity, i have much better caps for this use.

Sulaiman

Thu Aug 23 2007, 09:12AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I have Philips 376-series capacitors which are very similar to Wima FKP1s
very good capacitors.
I've given up potting for various reasons.
Now I use superglue (cyanoacrilate) to glue them to acrylic plastic
a little drop of superglue on the capacitor face opposite the pins allows compact stacking
(or for TC use leave air circulation space for cooling)
It's then very easy to wire them up.
IF you get a failure it's easy to replace one or more.

e.g. mys SGTC MMC, two banks of 10-series x 3-parallel (10s3p) 68nF 1kV capacitors.
On a piece of acrylic that was a drawer faceplate from an old fridge.

Marko

Thu Aug 23 2007, 11:59AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I wouldn't glue the caps for same reason neither, there are lot of other nice ways to mount them. I would hate myself if I glued the caps and then needed them for something else.

The caps would probably make great DRSSTC MMC too, several tens of them in parallel would make great high current cap, actually much better than those we make out of few 942C20P15K caps in series-parallel.

And why not build a SGTC from them too?

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Thu Aug 23 2007, 01:11PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

mh, but for a drsstc mmc say 8kv dc and only 117nf i would need 100 caps! And these caps are huge,. 37,5mm pinout. of course a great pulse cap but would it affect performance in a bigger range then say 5% ? i already build drsstcs with big mmcs of mkp caps, and they never got hot or damaged,mh okay, i never used less than 10 caps for a drsstc mmc.

so i looked around and wanted to solder 8 caps on a pcb to form 1 capacitor for the marx. a bit expensive though cause i need 7 EU pcbs for a single marx tower. but its the easiest way. it looks good, mechanical stable and i can add screw terminals to screw each pcb on,.
another thing i wanted to know is if i should add bleeder resistors in the range of 1-10meg per cap

Sulaiman

Thu Aug 23 2007, 04:04PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If you look at the photo I posted above you'll see one 33MOhm per three 68nF 1kV capacitors.
Works fine, and VERY importantly, discharges the bank in seconds for safe handling
T = R.C = 6.73 seconds

If T is too large then you will have to wait a long time to handle the MMC
If T is too small then you will waste power.

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