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Re: [TCML] Magnetic saturation
Gee, it seems like an SCR should work pretty well--most of them turn on in a
microsecond or so. You would have to put a reverse diode across it to
handle the ringing. I've always wanted to try, and I have some big
hockey-puck SCRs, but I haven't gotten around to it. They have real big
pulse I^2t ratings.
You could use a hollow-cathode thyratron, a grounded-grid thyratron, or a
back-lighted thyratron. These would all work well, but they are pretty
exotic (read $$$$$$$). They have the advantage that they are more or less
easy to trigger.
You're right that a SR is only good for limiting the input AC current and is
WAY too slow for a main switch.
---Carl
-----Original Message-----
From: David Speck
Sent: Saturday, September 01, 2012 4:55 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Magnetic saturation
JF,
I have used a saturable reactor (SR) with good results to limit the
current draw of my potential transformer in a SGTC. A very small DC
control voltage applied to the control windings of the reactor can
produce large changes in the current allowed to pass through the primary
winding of the saturable reactor.
In my experience, saturable reactors are way too slow to be used in
place of the spark gap in a disruptive coil. I've noted that when I
turn off the control signal to my SR, the coil actually takes something
like half a second to stop discharging. I have no filter cap on the DC
control circuit to the SR, so there must be some intrinsic property of
the SR that keeps it from turning off instantly.
The operating parameters of a SR and a spark gap are totally different.
A spark gap goes from a near perfect insulator to being a pretty very
good very high current (hundreds or thousands of amps), low loss
conductor in a fraction of a millisecond or thereabouts. A SR would
have to be huge to handle the sorts of currents involved in a TC tank
circuit, and even then, the inductance of the SR core would be a huge
drain on the power of the circuit. The only significant inductor you
want in the tank circuit is the TC primary coil. SRs are just not the
right tool for the job.
Not sure how a gyrator would help in a TC circuit.
Solid state thyristors (SCRs, or TRIACs) in general are too slow to be
effective on a TC tank circuit, a pity, 'cause there are a lot of them
available. They are also very poor at handling reverse currents that a
spark gap doesn't care about. When the gap in a TC tank circuit fires,
the high voltage in the cap might start out at (+) voltage, but it
rapidly drops through zero to nearly the same voltage in the opposite
(-) polarity. This damped ringing waveform oscillating about the zero
voltage point is what makes a TC work. Some have successfully small
built TCs using specialized hydrogen vacuum tube thyratrons, but most
thyratron tubes are poorly suited to this duty, as they have poor
reverse voltage operating characteristics.
HTH,
Dave
On 9/1/2012 3:01 PM, vatercox@xxxxxxx wrote:
Dear List,
I'd like to hear some opinions on "magnetic saturation" only it does seem
similar in some ways to dielectric breakdown even if only in that it is
voltage related phenomena.
Has anyone used a saturating reactor in place of a spark gap? Is it
possible?
What actually happens when the inductor saturates? Doos the voltage across
the inductor continue to rise albeit at a slower rate like a zener diode
or does it even fall, analogous to negative resistance of an avalanching
spark gap or a neon lamp?
and while we are talking about it what about ther case of a gyrator- a
circuit using a capacitor and an active element that mimics th behavior of
an inductor. how about using a a thyristor as the active element to
simulate saturation?
JF Cox
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