SX43@LIVERPOOL.AC.UK (12/17/90)
Please excuse my ignorance if the answer is trivial, Im no electronic engineer.
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;
Being able to control the voltage profile in time would presumably help
but I suppose that would be asking for too much ?
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.
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.. I thought that setting the initial
d.c. in the solenoid should allow tuning of the total charge passed
in the discharge, but maybe it would tune the power dissipated in
the discharge ? And how would the discharge be redirected from the
switch which cuts the current to the required discharge-point ?
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 ?
Any ideas, pointers, or even circuits muchly appreciated.
I would be most grateful if copies of any related posts be emailed to me
(see header) as I may not be spending time on the net nowadays/soon.
Thanks again,
Fazal.SX43@LIVERPOOL.AC.UK (12/19/90)
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 ... Thanks for any feedback.. Fazal.
steveq@syd.dms.CSIRO.AU (Stephen Quigg) (12/21/90)
>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 ... Tesla coils are pretty old stuff, basically a high voltage RF transformer. I've got a book on how to make them, but it was printed in 1916! You probably couldn't buy one today, and if you built one, you'd probably wipe out all communications in the district unless you were in a screened room. They are VERY noisy (electrically speaking). You can get commercial units that simulate static discharge; the ones I've seen illustrated (never actually seen one) are hand held guns. I don't know how they work however. You might try one of those gas stove lighters and see if you could adapt it to what you want (I'm not sure what voltage you are after). Some of these units are electronic and some are piezoelectric (mechanical).
larry@kitty.UUCP (Larry Lippman) (12/23/90)
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 FAX: 716/741-9635 {utzoo, uunet}!/ \aerion!larry