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From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Ideas please, how to generate arc discharge..
Summary: Capacitor discharge methods...
Message-ID: <4262@kitty.UUCP>
Date: 23 Dec 90 04:17:30 GMT
References: <90351.010238SX43@LIVERPOOL.AC.UK>
Organization: Recognition Research Corp., Clarence, NY
Lines: 67

In article <90351.010238SX43@LIVERPOOL.AC.UK>, SX43@LIVERPOOL.AC.UK writes:
> A friend wants a circuit to generate a discharge of the sort that comes off a
> Van de Graaff generator when you point your finger close to it.
> Basically he wants to be able to control :
>       -- The total charge that gets passed;
>       -- The peak voltage;

	The above is readily accomplished using a capacitor discharge
circuit.  Since you have mentioned a Van de Graff generator, I presume
that we are dealing with voltages of 5 kV and up.  If we were dealing
with lower voltages, some other circuit options might be available.

	For a capacitor having a given capacitance, charge (coulombs) is
directly proportional to voltage in the capacitor prior to discharge.
In order to control peak voltage and charge *independent* of each other,
it will be necessary to change capacitance, most likely by switching
some fixed capacitors in a series/parallel arrangement.

	Discharge current can be limited by insertion of a resistor.

	The best way to control charge and discharge of the capacitor in
this application as through use of a SPDT vacuum relay.  Since my guess
is that your discharge current will be very small (say uA range), the
use of a thyratron, krytron or other cold-cathode discharge tube is
difficult since there may not be enough load current to sustain conduction
across the tube once it is triggered. 

> The discharge should have to penetrate both air and some depth of
> an insulator, say a tenth of a millimetre, and I suppose it would have
> to come off a fine tip in order to be able to localise its passage.

	The above requirements are simply a function of voltage.
	
> Being able to control the voltage profile in time would presumably help
> but I suppose that would be asking for too much ?

	The use of a suitable LRC network switched at the time of discharge
can readily modify the voltage decay profile for a given load.

> What comes to my mind is something based on a solenoid, where if you
> have a steady current flowing and then the circuit is broken the
> back-emf generates a discharge.

	Controlling this approach is much more complex than capcitor
discharge, not to mention the difficulty in insulating inductors for
high voltages.  In addition, I gather the impression that the application
may require far more coulumbs of charge than may be obtained from the
magnetic field collapse of "small" inductors.

> Also I thought of photographic flash units. They seem to build up a
> charge  before discharging it.. I wonder if the principles involved
> there would be relevant to my problem. Ideas ?

	This would result in the problem where you may not have enough
load current to discharge a cold cathode tube.

In article <90353.091019SX43@LIVERPOOL.AC.UK> SX43@LIVERPOOL.AC.UK writes:
> Tim Coslet mailed me to say that I probably need  a "Tesla Coil".. any
> ideas what that is/ where to get one/ how to use it/ how much ...

	I suspect that you want a pure DC static charge, which is not
exactly the conditions offered by a Tesla coil (RF in the hundreds of
kHz to low MHz range).

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
VOICE: 716/688-1231   {boulder, rutgers, watmath}!ub!kitty!larry
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