Re: additional transformers [rolled caps]

From: 	richard hull[SMTP:rhull-at-richmond.infi-dot-net]
Sent: 	Friday, August 01, 1997 5:52 PM
To: 	Tesla List
Subject: 	Re: additional transformers [rolled caps]

>> > >
>> > >Peter,
>> > 
>> > I recommended leaving a full 2" edge separation metal to edge of dielectric
>> > in all rolled capacitors!
>> > 
>> > Richard Hull, TCBOR
>> Thanks for the info on that, I will go 2" on my future overlaps. It
>> seems strange to me that sparks seem to travel much further along the
>> surface of an insulator, no matter how clean it seems to be. Maybe this
>> is due to microscopic impurities on the surface of the plastic?
>> Another little trick that you probably have already figured out is to
>> insulate the input connections to the plates by rolling a few layers of
>> poly around them.
>> Cheers, Peter E.
>Peter, Richard, All,
>I had the same problem with a rolled poly cap I made once, and I 
>*was* using a 2 inch surround from the foil to the edge of the poly.  
>Dismantling the cap after the failure showed these burns distributed 
>all along the edge of the foil outwards into the poly.
>I had a chat with a capacitor engineer at Condesnser Products about 
>this phenomenon and was told that this is why HV capacitors are 
>generally not successful when one tries to apply more than 8-10 kV 
>across a single stage.  He explained that the energy exchange between 
>the plate and the local surface of the dielectric becomes too violent 
>and just completely overstresses the dielectric causing it to 
>break down.  Placing additional thickness of poly does not 
>significantly reduce this violent surface area exchange so you will 
>still get failures no matter what you do if you try to run a single 
>stage cap at 15 or 20 kV.

This is the very reason I have stated even in tape #3 where the big rolled
jobs are made that one should never run them at over 10KV.  Series for more
volts.  In tape #8 where we build another type of poly cap, I note that 7-8
KV is the max on the single unit.  Nonetheless,  many never headed my
warning and blew out there caps almost immediately.  Too many coilers rush
to the 15,000 60ma neons and use a single capacitor unit.  They will lose it
sooner or later.  

The two inch boarder is minumum on caps used over 5KV per unit.  Once aged
and all the air is out I have run my single, big, rolled oil caps with 12KV
neon transformers and luckily have never lost one.  When working on maggeies
#1-8 from 1990-1995 I used two of the big rolled jobs in series with a
14,400 volt potential transformer and never suffered a break down, but the
stress was reduced on each unit by series stringing them.

Since then I have used only professionally produced discharge caps.  The old
1990 built rolled caps are still kickin' and are often used in snap setups
for visitors, etc.

Concomittant with the use of homemade caps is a sense of power input
limitation in the form of break rate.  Lots of folks kill these spartan
units with 15,800 rpm 24 point rotary gaps!!!!  Even 2 KV might blow them at
that rate of delivery.  Newbies always want the 20,000 volt transformers and
turbine speed rotaries and wonder at the ease of destruction of their

There is rarely a real viable need for break rate over 360/second (maggies
can handle more).  If a rotary gap has more than 8 points, it will be a bear
to quench well with out super high speed and attendant cap destruction.

I have no way of knowing, but I suspect a lot of Good CP caps fell to the
turbine rotary effect.

Richard Hull, TCBOR