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Hi Wil,
Tesla coil primary circuits are designed to "ring" over at least several
cycles in order to allow complete energy transfer from the primary to
the secondary. Most thyratrons are designed to conduct current only in
one direction. And even those thyratrons that are designed to handle
somewhat oscillatory currents cannot directly handle the high-Q ringing
seen in a Tesla Coil primary circuit. If not protected from reverse
current flow, the thyratron's cathode will be permanently damaged as ion
bombardment blows off the cathode's emmisive layer.
Thyratrons are normally used with Pulse Forming Networks (PFN's) that
provide a unipolar output pulse, and not the oscillatory currents in a
primary circuit. If you simply want to generate high voltage pulses, you
can make your own PFN (a ladder network of capacitors and inductors)
charged by a HVDC source. The PFN is connected through the primary of a
closely-coupled pulse transformer and to the anode of the thyratron.
When the thyratron is triggered, a relatively square HV pulse is
generated on the secondary side of the pulse transformer. The PFN is
often "resonantly" (re)charged through a HV choke and HV diode such that
it's stored voltage is about 2X the supply voltage. A good discussion of
DC resonant charging can be found on Richie Burnett's site:
You can use a thyratron in a TC primary circuit using a similar circuit.
In this case, you'll need to connect a string of high voltage high
current bypass diodes across the thyratron to handle reverse current
flow. You would also substitute a primary tank capacitor or MMC instead
of the PFN. A trigger pulse of sufficient width (or a series of timed
trigger pulses) are used to trigger and re-trigger the thyratron during
the entire "ring up" period of your Tesla Coil. The tank cap current is
conducted through the thyratron during one half cycle and the bypass
diodes conduct during the next half cycle, allowing the primary circuit
to ring. Once the thyratron trigger signal is removed (ideally after
secondary ring-up), the thyratron stops conducting.
Although permitting precise switching, thyratrons are actually quite
lossy (lossier than spark gaps!), so you may need to be careful about
not exceeding RMS current and overheating the tube. The ringing time in
a TC is usually much longer than the typical pulse times in PFN pulsed
power applications.
Good luck,
Bert
--
Bert Hickman
Stoneridge Engineering
http://www.capturedlightning.com
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William Howard wrote:
Dear Group,
I have a large thyratron I purchased years ago and want to build a coil
with it.
Does anyone have a schematic for a relaxation circuit around (to suit a
tesla coil)? I was think of using neon bulbs after a megaohm resistor to
create a relaxation oscillator to trigger it.
I used florescent bulb starters once after a megaomh resistor to trigger a
relay which dumped 400v from a 20000uf capacitor bank into a ignition coil
which had a capacitor in parallel to the primary. It would produce a loud
thump every 5s and a large discharge about 5cm...
The relays soon died but the discharge into the concrete was impressive :)
Hoping to pull off the same thing again but with a higher cyclic rate. I
want to produce impulse currents.
Any ideas would be welcome. I only have mots for a power supply...
Cheers!
-Wil
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