[TCML] Mylar caps & Marx generators

["Lau, Gary" <Gary.Lau@xxxxxx>]

Changing the subject line...

Hi Paul,

I see several problems with your plan and analysis.

You're right to be concerned about the contact between copper wires and Al foil plates.  In any pulse cap (TC or Marx), the lead to plate connection must be extremely robust and deliberate.  Casual pressure is not going to cut it and may fail when pulse discharge currents vaporize the contact resistance.  

Beyond that, there's a likely problem with using DIY plate caps in a Marx generator.  The spark gaps need to be in close physical and optical proximity to each other, as the UV from each gap ionizes and triggers the next gap.  I don't know the physical dimensions of your caps, but DIY plate caps are typically not small, and I'm guessing that your stages might be too far apart to support reliable triggering.

Your analysis of the MG frequency is incorrect on several fronts.  The resistors you cited - 10mOhhm - are these the resistors between the stages?  Usually one uses high voltage inductors or MUCH higher-Ohm (many KOhm?) high voltage resistors between stages.   The charging rate is limited by the power supply impedance plus any interstage isolation resistors and not just by a tiny inserted resistance.  

But more to the point of your post (whether mylar caps can be used), the charging rate isn't the issue.  In a TC, the cap charges at the mains frequency, and Mylar caps are just fine with 60 Hz charging.  It's the discharge side of the ride that kills Mylar caps in a TC.  In a MJ, I don't know how a mylar cap will fare, as the bang rate will be much lower than in a TC.  The cap discharge current in a MJ is actually at a higher frequency than in a TC, as it's not limited by the primary inductance.  But it may do just fine given the much lower bang rate (duty cycle) if other issues (contact resistance, spark gap proximity, and proper interstage resistance) are dealt with.

Regards, Gary Lau
MA, USA



> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Paul Brodie
> Sent: Thursday, December 03, 2009 4:06 AM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Capacitor dielectric examples and Tesla coil equations
> 
> That certainly explains quite a bit of things. Keeping what you just explained in
> mind, I would like to know about using my Mylar caps in another high voltage
> application that I believe runs at a much lower frequency, a Marx Generator, if I'm
> not going to far off topic.
> 
> Before I forget, that was a very good and easily understood explanation, Bert.
> Thanks a lot.
> 
> Before I go on about my question, though, I would like to say that I was worried
> about the equivalent series resistance problem with my Mylar caps, especially
> because I am using copper leads with aluminum plates. I am using the heavy duty
> thick aluminum foil and I did go to the trouble of taking the copper wires and
> flattening them on my anvil to spread out the contact area with the aluminum but
> I'm concerned about the aluminum to copper interface and the resistance that will
> develop there. I also flattened the wire because I was also worried that the thick
> copper wire would press on the Mylar when I tightly wound the caps and that it
> might thin the plastic at that point. Anyway, I'm wondering if I should put a little bit of
> anti-ox paste on the copper wires before I put them on the aluminum or is this really
> necessary?
> 
> Now, as I understand the operation of the Marx Generator and specifically the one I
> am considering duplicating, they charge the caps in parallel until they reach a
> voltage sufficient to jump the first spark gap which initiates a series discharge of all
> of the caps through their individual spark gaps and out the top of the MG and then
> the process repeats itself.
> 
> The MG I am looking to build uses 10 mohm resistors in series with 4 nanofarad
> caps. This gives a time constant of .000000004 farads times 10,000,000 ohms or
> .04 seconds. Five time constants to charge the caps is .2 seconds making the
> frequency of operation of 5 Hz. If I have done this correctly, this should be the
> frequency at which my Mylar caps will operate and they should work okay in this
> application.
> 
> Will someone who has built and fully understands Marx Generators please let me
> know if I am going about this correctly and if my Mylar caps will work okay?
> Paul
> Think Positive

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