Re: Why do my caps keep blowing?
I confirm the metric/imperial confusion is correct. I am using 1 millimetre
of poly, I.e. 40mil or 40 thousandths of an inch.
I really am surprised that tesla fans still measure in inches but all other
units we use are metric!
Anyway I haven't yet built the coil as I am trying to build an
Thanks to DR.RESONANCE-at-next-wave-dot-net I will now buy some of his mica caps at
the excellent price he offers.
BUT I STILL WANT TO KNOW WHERE I WENT WRONG!
The capacitor value of 0.008uf was chosen to resonant charge from my
I used 450V/mil breakdown figure from the www.pupman/howto/MATH.TXT info
sheet & calculated breakdown at 18kv
After vacuuming caps for 4 days I applied 12kv for 10secs while applying
vacuum to remove final residual air. This process was repeated once every
60seconds for 10 minutes.
Then I increased the time the voltage was applied gradually over 10 more
minutes until all entrapped air was removed.
Transformer oil was fresh Mobilec 26 (same as used in power grid)
Finally after 5 minutes continuous 12kv application the cap perforated
leaving a hole 1/8th inch wide in the middle of the poly.
A very slight unevenness in the Al was noted at this spot but the plates had
all been finished before assembly with 400 wet & dry paper to remove surface
irregularities then the plates were washed,dryed then cleaned with 1
application of acetone and 2 applications of alcohol and assembled in as
near to clean room conditions as I could manage.
Poly IS Clear Poly 4mil (100 micrometer) thick but just obtained from local
hardware store. It is on a roll 2m wide and about 100m long. They just cut
it from this.
However the capacitor roll was slightly uneven as I just cant seem to get
the stuff to roll up with even tension on all poly layers.
Any clues how to do this please ?
The voltage was applied with no coil or load attached.
Just 12kv,30ma straight into the cap.
I would just like to know what went wrong.
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Thursday, January 28, 1999 9:30 PM
Subject: Re: Why do my caps keep blowing?
>Original Poster: Mark E Finnis <mefinnis-at-medicine.adelaide.edu.au>
>At 19:04 26/01/99 -0700, Jeff Corr wrote:
>>>I am making them from 1mm thick poly (10layers of 100um)
>>First off thats only 10 layers of 1mil thick poly, or 10 mils. Each mil
>>is good for around 300 volts, so your capacitors are only rated at
>>3000 volts. Thats why they die.
>Hang-on chaps ! We have metric/imperial confusion ......
>1mm = approx 40 mil
>Now at 300V/mil we then get 12kV, but using 10 layers I would have thought
>you would get a little more
>I use total 1.5mm or 60 mil in 6 layers. 60 mil is the absolute MINIMUM
>recommended in most of the archive stuff I have read, however my caps have
>survived no problems at 15 kV.
>Don't know why you are getting failures.
>1. Where did you get the PE from? Is it really PE, or is it some other
>plastic? How pure & what quality?
>2. Do you use safety gaps for the caps? how wide is your gap set?
>As original post stated holes in middle, I assume it is not an edge