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Fnord

Sun May 22 2011, 06:49PM

Registered Member #2004 Joined: Sat Feb 28 2009, 11:43PM
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Posts: 39

Just a hypothetical question...

Most marx generators I've seen use doorknob-type capacitors, but has anyone ever tried something high-powered; made from a bunch of series electrolytics?
Stages made from 15 photocaps each would give around 100J @ 5kv, which should be high enough for reliable firing.

I don't know how the ESR of capacitors affects the operation of a marx, though I'm guessing not a terrible amount. And are photoflash caps rated for series use? Probably not. But I don't think we're known for running within component specs around here.

Obviously electrode corrosion would be a factor, but that's the engineer's problem.

Also, here is a noob question:
Why exactly are resistors necessary in a marx? It seems pretty similar to a Cockroft-Walton multiplier, which has none. The only thing I can think of is it will create a voltage gradient through the stages. Is this only to encourage sequencial firing?
(I mean come on, this is a stupid question, but it isn't even mentioned in the 4hv wiki)

Dr. Slack

Sun May 22 2011, 08:11PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Why exactly are resistors necessary in a marx?

They're not necessary, if you use either inductors, or switches instead. Something has got to connect the stages during charging, and isolate them when they fire. Resistors are cheap, a lot cheaper than inductors which are better. Switches are a sort of theoretical ideal, trying to operate them in sync while arranging 100s of kV isolation would be a mechanical nightmare compared to a string of resistors.

ESR - it depends what you want to do with your marx. If making loud bangs, then probably not a big problem, even though all the ESRs in series will add up to quite a bit. Most of the serious marx literature I've seen is for testing electrical distribution equipment, where you want a cntrolled and low output impedance.

Fnord

Sun May 22 2011, 09:36PM

Registered Member #2004 Joined: Sat Feb 28 2009, 11:43PM
Location:
Posts: 39

Hmm.

Any idea how much the value of the inductor will affect the output, or if there is any potential risk of damage to the capacitors by 'tuning' the system wrong? I don't think I've ever seen an inductor-based design that had much math or testing behind it... but then I haven't been around here that much.

And I obviously would want loud bangs, but any *practical* use I could think of wouldn't require much control on the output either.

Proud Mary

Sun May 22 2011, 10:39PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Very few industrial designs for Marx Impulse Generators employ charging inductors, but here is one that does:

Martin Sack, Christoph Schultheiss, and Hansjoachim Bluhm
Triggered Marx Generators for the Industrial-Scale Electroporation of Sugar Beets
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 41, NO. 3, MAY/JUNE 2005

Arcstarter

Mon May 23 2011, 12:45AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Fnord wrote ...

Just a hypothetical question...

Most marx generators I've seen use doorknob-type capacitors, but has anyone ever tried something high-powered; made from a bunch of series electrolytics?

Well, i made a HV CW multiplier with a bunch of series electrolytics. Sure a Marx is different, but they both do the same thing (charge in series, discharge in parallel). But, seeing that most marx generators are at one point or another 'shorted' for making sparks, electrolytics probably wouldn't work as well due to ESR and ESL for higher currents. So, i see no reason they won't work but that is disregarding performance and reliability.

Dr. ISOTOP

Mon May 23 2011, 01:04AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

'lytics have terrible ESL and ESR, as well as low voltage ratings.
e.g. using photoflash caps, you'd need hundreds to reach 100KV. The bang energy would be large, but because of the huge ESR and ESL of say 300 seriesed electrolytic capacitors, the bang would be mashed out and become really long -> less peak power -> less awesomeness.
Also, big electrolytic capacitors don't like getting shorted.
Also, reliably triggering a spark gap at 5KV will still be sort of tricky; Marxes want very reliable, low-jitter firing.

Proud Mary

Mon May 23 2011, 02:11AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Look at the output of any MIG and you'll notice a large negative swing immediately following the positive going pulse - a very good reason for not using polarised electrolytics.

GluD

Mon May 23 2011, 09:28AM

Registered Member #1221 Joined: Wed Jan 09 2008, 06:17PM
Location: Odense, Denmark
Posts: 196

What about a MMC like we use for TC's if you really want more capacitance.

Nah

Mon May 23 2011, 08:03PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

Also, marx generatos have a high voltage spike that flows through the caps. E caps are at most rated to 600 volts.

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