Re: Practical use of a tesla coil - lamp (fwd)

---------- Forwarded message ----------
Date: Fri, 19 Dec 1997 08:56:55 -0800
From: Jim Lux <jimlux-at-earthlink-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Practical use of a tesla coil - lamp (fwd)

> Date: Wednesday, December 17, 1997 1:35 PM
> From: Julian Green <julian-at-kbss.bt.co.uk>
> Subject: Re: Practical use of a tesla coil - lamp (fwd)
> More info on the tesla coil lamp ignighter.
> Ive traced out the circuit and continuity tested the tesla transformer.
> Tesla coil makes ideal lamp ignighter as at RF resonant frequency very
> voltages can be presented to the lamp whilst keeping the DC resistance
> low for high current delivery once the lamp is alight.
> Julian Green

This looks much like the circuit used for so-called HF arc stabilization on
a TIG welder. It has spark gap excited LC circuit with the L in series with
the welding current. Since you can't arc across free space with only 50 or
so volts (the welding voltage), you either have to strike the arc by
touching the electrode to the workpiece and drawing the arc (which requires
a bunch of practice to do well) or by putting a HV pulse across the gap
(which is what the HV/HF stabilizer does).

The plasma cutter we have does essentially the same thing, although it
works at 200 Volts or so across the gap. Same HV spark excited scheme.

In the case of the flash lamp, the scheme is called "series triggering" and
is used for large flash tubes. Small tubes are fired by putting a trigger
electrode on the outside of the tube (like in a camera strobe), but that
scheme has problems at very high flash powers, so they use series
triggering. Series triggering superimposes a hv pulse (40 kV) on the tube
voltage to break it down so that the discharge cap (charged to several kV)
can discharge.

The trick is that the trigger transformer secondary has to carry the full
load current, hundreds of amps in the welding/plasma cutting case, or
pulses of kiloAmps in the flash tube case. Some of these transformers use a
nifty trick of allowing the core to saturate when the discharge current
flows to reduce their inductance, making the flash shorter in duration.