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Re: [TCML] VTTC HP gate help Igbt?, was grid coil help, addendum
Hi Jim,
Unfortunately I also don't have experience with the lastest types of
silicon devices.
It is possible to make a two layer grid coil if you want to. I did that
on my large VTTC because I had limited space due to the pancake
type primary I used. There has to be enough insulation to prevent
flashover between layers.
To make grid staccato work, the grid voltage has to be driven
sharply negative to cut off the tube. I used a minus DC power supply of
about 400V, and applied this voltage to the grid through a
pass transistor, to shut off oscillation during staccato "off" times.
The staccato circuit board controlled this pass transistor. One
problem I noted is that the turn-off or turn-off was not as clean,
and the sound of the staccato was not as solid. I didn't look
into this issue because I switched to cathode staccato.
When the pass
transistor was "off", the staccato controller was basically out of
the VTTC circuit and had no effect. The normal grid leak bias
of the VTTC worked in normal fashion. In other words, a minus
voltage is needed to shut off the VTTC, but a plus voltage
is not needed to turn it on. It turns on itself due to the absence
of the turn off voltage. It's only necessary to apply, or not apply,
the negative voltage to the grid, to create the staccato action.
The pass transistors were susceptible to blowing out if the VTTC
output spark struck a ground. I had to install a protective choke
to prevent that. These were very early experiments as you can
imagine. I liked
the cathode staccato because it didn't require the minus DC
voltage, and gave cleaner staccato control. I only used the
grid staccato on my 16" spark TC, not on the higher power
systems.
I also tried a mechanical staccato system. This used a motorized
rotary switch which interupted the cathode circuit. I could control
the staccato rate by varying the motor speed. This created interesting
effects. I also tried using a mercury thyratron on my large VTTC, and
it worked well in place of an SCR, but is more complex to set up.
The mechanical staccato works well with a filtered DC VTTC also.
John
-----Original Message-----
From: Jim Mora [mailto:wavetuner@xxxxxxxxx]
Sent: Saturday, July 04, 2009 12:10 AM
To: 'Tesla Coil Mailing List'
Subject: RE: [TCML] VTTC HP gate help Igbt?, was grid coil help
New comments and corrections added.
-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
Behalf
Of futuret@xxxxxxx
Sent: Friday, July 03, 2009 6:01 PM
To: tesla@xxxxxxxxxx
Subject: Re: [TCML] VTTC grig coil help
Hi Jim,
You mentioned the grid coil is 11" dia, but the primary is
8" dia. Is this correct? I would think they'd be about the
same diameter.
<snip>
No, sorry John, that is the height of the primary form. The primary and
grid
coils are on it. I suspect it will be long enough. If the primary is at
the
bottom, the height is about 5-6 inches. So the grid can slid away from
the
primary. I can make it longer if needed by decommissioning a spark gap
demo
coil I made and fusing another 11" 8" acrylic form piece.
<snip>
I'm cogitating your well informed information. The voltage level = raw
is no
problem if I can get the staccato to work somehow. Is there is another
gate
type that can take that kind of power (2W) (not a triac clearly)? Much
of
new power technology is new to me since I worked with signal
transistors and
FET's and darlington power transistors. I suspect the silicon is out
there
now for this application.
To get the grid type staccato to work, does the tube need to be biased
in
the middle of the family of curves then driven into cutoff (**** and
saturation****)?
Maybe I need to learn about IGBT's sooner then expected. Someone that
has an
idea please jump in. All we are doing is trying to turn the cathode on
and
off to ground to vary the duty cycle of conduction to control the heat
on
the tube's plate and change the spark output profiles. The current per
tube
may be ~550ma **** actually the filaments are all paralleled ( acts as
the
cathodes) so current sinked when switched on is 4*550ma=~ 2.2 amps at
5kv
with a 833C *****and the floating voltage is 4-5.6KV which is then
clamped
to ground when the device is triggered for conduction. So we are
saturating
a switch and then removing the signal or go low for the off time
(cathode
opened to ground). A power mosfet is probably not a good enough
candidate do
to voltage constraints so I need some IGBT education. FYI a SCR once
triggered in the classic setup can't be shut off with a fullwave,
filtered
power supply. This is one of the reasons the MOT level shifter is liked
(not
acceptable for modulation).
The staccato circuit output is positive leading for on time
(saturation) and
goes low for cutoff. The signal level in is out of a 555 pin 3 and
probably
will need to be tweaked. How about it power folks, there should be a
simple
thus elegant solution to this with modern silicon.
This will be a interesting hybrid of WWII big triodes and modern
electronics.
Thanks Much,
Jim Mora
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