Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>
Hi Greg,
At 04:35 PM 9/24/2005, you wrote:
Hi Jimmy, Steve, Terry,
Part of the problem I have had in grasping the OLTC and DRSSTC
concepts is that I haven't found a concise one-sentence description
for either of them. For instance, I had thought that OLTC
addressed the techniques for 'Off-Line' operation of a Tesla
Coil. Does the term also refer to the general case where a single
solid-state device replaces the spark-gap in a classic impulse-type Tesla Coil?
When originally made, the OLTC had these distinctions:
1. The primary firing voltage was just the rectified AC line
voltage. The AC line and a rectifier charged the primary capacitor
"directly" to say 340 volts. That "was" the firing
voltage!! Later, a resonant inductor helped to almost double that
voltage. But there is NO high voltage transformer used to charge
the primary cap. The primary voltage "never goes higher" than the
primary cap voltage. It works right "off the line" voltage, but can
still produce "big" coil" outputs. Unlike say a tube or transistor
coil, the OLTC was still a full powered disruptive coil.
2. Since the primary voltage is low, "enormous" primary currents
are used to impart the power on the secondary. Mine was designed
for 7000 peak primary amps!! Thus, a very large primary capacitance
and a very low primary inductance was needed. The OLTC operates in
the many thousands of primary amps range. The "Key" that started it
all was modeling with Mark's MandK program that showed a "single
turn" (low inductance) primary would still have "enough"
coupling. That meant the high currents at low voltage could be used
to duplicate a standard high voltage primary coil's magnetic field
and deliver it to the secondary.
3. IGBTs allowed for relatively low loss switching of low voltages
but at super high currents. Now that the voltage was "low",
standard big IGBTS could be used. With the primary voltage in the
700V peak range, just "off-the-self" power electronics could now be
used. By over driving the gate voltage to the 20 to 25 volt range,
the IGBTS could be hit with up to three times their rated peak
current (and they liked it ;-)). A reverse diode allowed for
reverse current flow and the IGBT could hold the system back until
the time was just right to begin (also a major idea!!)). The low
duty cycle kept device heating low. The "spark gap" was now silent and dark...
4. The OLTC was the first big coil that had very high power with
precise timing and quenching "electronically" controlled. Without a
spark gap, it could fire time after time with almost perfect repeatability.
From afar, the OLTC stood out as a "big coil" that had an
"electronic spark gap" and "no high voltage transformers". The
"whole thing" weighed 40 pounds... In England, where HV
transformers are much harder to get, they really went to town!!!
Steve Conner took the OLTC to it finest form in his OLTC-II:
http://www.scopeboy.com/tesla/ol2derby/pics2.html
Derek has also done great work on them!
http://www.roffesoft.co.uk/tesla/oltc/oltc4.htm
Finn just made one, but I don't have the link handy.
The OLTC is nice where not too giant sparks are needed but a
reliable "big" "solid state" coil is...
Then came "Jimmy and the DRSSTC" :-))) Coiling will just never be
the same... Jimmy took all the tricks of the OLTC, and used them to
"change everything!!" He gets the current "Nobel prize in Coiling" ;-))
1. The line voltage now charges a big buss capacitor for virtually
"unlimited available energy" "during a bang". Still an "off-line"
machine with no transformer, but now we can draw as much power as we
want inside the firing cycle. All the old rules about "limited"
bang energy get thrown out...
2. The primary coil and capacitor voltage is now "resonated" to far
higher voltages by an "H-bridge" of IGBTs. The push-pull H-bridge
forces current into the primary circuit. That current builds up
high voltages on the primary through resonance, but those voltage
are still safely far away from the IGBTs and other
"electronics"! Suddenly, we have our high voltage primary back with
far lower losses than the OLTC primary. We no longer need giant
primary currents eating vast amounts of power in losses. Of course,
we "still can" run giant primary currents to get like "12 foot arcs"
from a "solid state coil" :-)))) BTW - Steve still is not using
his "BIG" IGBTs :o))))
3. The DRSSTC actually reuses un-needed power buy restoring energy
back into the buss caps when the coil is quenched. We usually don't
give that much thought, but you should (as you have!) for a giant machine...
The OLTC broke a lot of ground and introduced a lot of new
technology and options. But, it never was a "perfect"
machine. They are frightening monsters that pack a lot of power and
electronics (no one goes near them ;-)), but they could not "beat" a
conventional coil. The low primary voltage and high currents just
are too lossy in the primary... They are remarkably
reliable. Aside from fried secondaries, I am not sure if the
primary drive side has "ever" been reported to fail???
BUT, the DRSSTC can beat any machine out there!!! It directly
solved the two problems of the OLTC!! The technology is much like
any high power switching power supply and so are the
electronics. For many power electronics engineering types, coiling
just got easy ;-)) There is an ever growing number of them. They
even have their own website :O)):
www.drsstc.com
Right now, DRSSTCs are where all the action is at!!!
But, there is "still a problem" with DRSSTCs. They are not "simple
and easy"... PC boards, ICs, digital scopes, current probes, EE
degrees (or in the process of getting them), and MONEY... DRSSTCs
ARE a complex business!! And not cheap... I spent about $1600 on
mine (http://drsstc.com/~terrell/) but I went all out... Maybe I
have the record of most dollars spent on one :O)) But mine has
microprocessors, fiber optics, optical gate drive, multiple
selectable software modes... Dan wrote "The Book!!". Steve has "the
big one". Jimmy is the "god"...
I think Steve is the driver master ;-)
http://www.scopeboy.com/tesla/drsstc/dr3_1.gif
And the other two sheets!!
http://www.scopeboy.com/tesla/drsstc/dr3_2.gif
http://www.scopeboy.com/tesla/drsstc/dr3_3.gif
:-))))
So, these days, "I" am trying to make them "dirt simple"... COTS
parts... No ICs... VERY cheap ;-))) Maybe "any noobie kid" will
soon be able to make one ;-))
Cheers,
Terry