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Re: Thyratron pair (was DC Drive)
---------- Forwarded message ----------
Date: Tue, 14 Oct 1997 10:14:43 +0000
From: Greg Leyh <lod-at-pacbell-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Thyratron pair (was DC Drive)
Tesla List wrote:
> >The pulse can only rise as fast as the LpCp surge impedance can allow
> >it to can't it?
> >Malcolm
>
> No, fast pulse rise time is basically related to switch turn on time.
> Spark Gaps and H2 Thyratrons turn on in a few nanoseconds, some newer
> fast tubes in the 10's of nS, IGBTs in the low 100s nS and older vacuum
> tubes much slower. Secondarily, low surge impedance may slow rise time
> a bit, but fast switch turn on time is most important.
> RWW
I believe that the answer to all this depends on whether you are
talking about the _current_ pulse, or the _voltage_ pulse.
When the gap fires (assume Trise = 0), the voltage across the primary
winding instantly rises to the full Cpri voltage.
However, the current in the primary winding starts at zero and rises
along a sinusoidal trajectory, reaching a peak current of Vpri/Zsurge,
where Zsurge = SQRT(LpCp).
The slope of the current rise just after the gap fires is given by:
di/dt = 2*pi*Fpri*Ipeak, where Ipeak = Vpri/Zsurge.
This all leads up to the issues regarding the application of new
and wild technologies to the task of switching the TC primary.
Any chosen technology must have the following capabilities:
HIGH holdoff voltage (both directions)
HIGH on-state current
LOW on-state resistance
HIGH di/dt rating (this kills SCR's)
HIGH repetition rate (this kills ignitrons)
HIGH i^2 * t rating (this kills thyratrons)
bidirectional current flow (this kills unipolar thyratrons)
So, you better read the manual before throwing the switch! $$$
-GL