Re: [TCML] Capacitor dielectric examples and Tesla coil equations

["Paul Brodie" <pbbrodie@xxxxxxxxxxxxx>]

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
  ----- Original Message ----- 
  From: Bert Hickman 
  To: Tesla Coil Mailing List 
  Sent: Wednesday, December 02, 2009 17:18
  Subject: Re: [TCML] Capacitor dielectric examples and Tesla coil equations


  Paul,

  Capacitors can be lossy in various ways. The dielectric itself can heat 
  up when subjected to RF voltages, such as with Mylar. However the 
  capacitor may also have excessive equivalent series resistance (ESR) for 
  other reasons. High ESR can occur when the metalization layers are very 
  thin (and resistive), the capacitor leads are thin and flimsy (as in 
  many of the eBay Chinese HV caps!), or the interconnection between the 
  flame-sprayed ends to the capacitor plates are poorly made. All of these 
  other problems will contribute to excessive heating, especially when 
  these caps are subjected to the high currents seen in a TC primary circuit.

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