Re: Why do my caps keep blowing?
Subject: Re: Why do my caps keep blowing?
From: Terry Fritz <twf-at-verinet-dot-com>
Date: Thu, 28 Jan 1999 19:51:52 -0700
At 12:24 AM 1/29/99 +1100, you wrote:
>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.
Be sure you understand what we mean by "resonant charging". This is were
we pick the capacitor value to match the transformer in such a way that
they will resonant at the line frequency. This allows far greater energy
to be "pulled" from the transformer. However, there is a BIG caution.
Without any loading or spark gap, the transformer and capacitor will
resonant and the voltage will keep rising higher and higher (within a
fraction of a second). You MUST have a spark gap that will fire at a
certain voltage to drain energy from such a system. Without this loss, the
voltage will go to around 80000++ volts!! There is a law that says the
part you care about the most (your capacitor in this case) will blow up
Simply having a static safety gap will protect against this super high
voltage rise. The gap will fire like a machine gun but nothing will fail
(the current is supper high without a primary inductior so you should
probably have the inductance in circuit too).
Running a cap and transformer without over voltage protection is probably
the most common reason they fail. The effect is instant and the damage is
total! You may even want to put more safety gaps across the transformer
and the capacitor if they are of great value. safety gaps do not affect
the system in any way but if something goes wrong, they are all that is
going to save the part from the super high voltage. Also, if the secondary
strikes the primary system, they may very well save you there too.