Re: HV Capacitors - PTFE (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 23 Sep 2004 09:26:03 -0700
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: HV Capacitors - PTFE (fwd)

At 09:33 AM 9/23/2004 -0600, you wrote:
>Original poster: <sroys@xxxxxxxxxxxxxxxxx>
>
>
>
>---------- Forwarded message ----------
>Date: Thu, 23 Sep 2004 08:47:28 -0500
>From: Carl Litton <Carl_Litton@xxxxxxxxxx>
>To: High Voltage list <hvlist@xxxxxxxxxx>
>Subject: HV Capacitors - PTFE
>
>
>Our new local HV group is finding the securing or producing reliable,
>high capacitance capacitors at low cost for working in the 10's of kV
>range a serious challenge.


How much capacitance?


>Commercial products found so far are completely out of the question in
>cost.  So much negative prose on the net about plate capacitors makes
>them unattractive.

Surplus HV caps turn up all the time.  The price is typically fairly 
constant in a $/Joule basis. (This makes sense, since it's mostly physical 
size that drives the cost, for big capacitors)

This is especially true for DC filter type capacitors, or lowish frequency 
(think Medium Voltage distribution PFC caps).

For pulse caps, you have a life and provenance thing to worry about with 
surplus.


>The obvious use of 20 or 30 free MOT caps in a parallel/series array
>doesn't appear to be used so I'm assuming there is a dielectric
>breakdown problem associated with that.

Probably more a fabrication issue. You've got to mount the cases so they're 
isolated (although they should be tied to one of the terminals of the cap, 
so you don't accumulate charge on the case without a path to bleed it off.

There are companies that sell these caps (new) for a buck or two. This 
would have the advantage that they are all basically the same value and 
physical size, which eases the packaging issue.


Why not do what the tesla coilers do?  String many .2 uF, 2kV polypropylene 
caps in series parallel. You can run them at voltages somewhat higher than 
the rated voltage without breakdown.