DC on Tesla coil

To: "'Tesla List'" <tesla-at-pupman-dot-com>
Subject: DC on Tesla coil
From: Tesla List <tesla-at-stic-dot-net>
Date: Wed, 25 Mar 1998 00:04:57 -0600
Approved: tesla-at-stic-dot-net

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From:  Vivian [SMTP:V.C.Watts-at-btinternet-dot-com]
Sent:  Tuesday, March 24, 1998 2:46 PM
To:  Tesla List
Subject:  DC on Tesla coil

Hi All,
    On Saturday I was demonstrating my TC to a good friend when due to a
pull on the ground cable unbeknown to me my secondary became disconnected
from earth. When firing it up sparks ensued from around the base and from
Primary to Secondary.  So I shut it down very quickly.  Now when I
re-connected the secondary to the ground I got a large spark just before
contact.  So the secondary had stored charge.  Question is was it a static
charge in the sense of formed by the insulation of the coil as proposed on
the list or is their a tendency for a level of DC to be built up but
naturally bleed away by normal connection to ground.  If a spark ensues to
an object and the connection is terminated at the peak of the Oscillation
then will a charge be retained?

Before the idea is dismissed outright one might like to consider another
experience I have had.  I use to work for a company making automatic test
equipment to test bare circuit boards (no components fitted) for shorts and
opens.  We charged up a track on the circuit board to 500V or so and
measured how long it took at a known source impedance.  If it took longer
than expected it was either directly shorted to something else or was
leaking via high resistance to another trace.  Times measured were in nano
seconds and detection of shorts to another trace measuring only 0.02 pf
could be resolved or resistance's up to 1000MOhms.

One of the power planes on the board would be grounded.  The problem we
experienced was that if the circuit board had multiple power planes, testing
one plane i.e. charging it up to 500v would induce a voltage on another.  OK
so far, but the machine always discharged the plane being tested, before
proceeding to test another.  But now here's the difficult thing, the other
power planes retained their charge.  The consequence was that when that
charged plane came to be tested it blew the electronics away.  Even a Spice
model showed a charge build up with no rectification and yet all that was
happening was another part of the board was first charged up then
discharged.

The consequence of all this was that after discussing this experience with
my friend I now know less than I thought I did.

Here's a few more questions some kind sole might like to put my thoughts
back on track.

1/    When the gap fires the current in the gap will be from the transformer
and from the capacitor.  What current is hardest to quench? For the period
of one half cycle the transformer current could be considered like DC but
the
capacitor current will be AC.  Is one harder to quench than another.


2/    I have read descriptions like ... "When the Gap is quenched the energy
goes into the secondary".  Is it really breaking the tank circuit and thus
removing the lossy gap from the circuit, increasing Q and allowing the
secondary to ring true that gets the sparks going?

3/    Are the oscillations, when the gap fires, like a carrier with two
sidebands until the gap quenches and then become a single carrier.

4/    Why does the conventional TC have to be loosely coupled.  Why can't we
get more bang by increasing coupling if the insulation could stand it.


With Brain Ache

Viv Watts UK.