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RE: [TCML] Spark gaps, Solid state switches and diodes

Gary:

I still have boards available.  Contact me off list if you are interested.

In the original SISG design Terry has a 4700 ohm gate shutdown resistor.
This gave a shutdown time of 901 uSec which was plenty of time for full
decay.  I ran a series of tests with progressively smaller resistors (1/2
value at each step) and stopped when I reached 680 ohms which gave a
shutdown time of 130 uSec.  My test coil has a resonant frequency of 168K Hz
and rings down in about 80 to 100 uSeconds.  I did not go lower because I
was afraid of destroying IGBT's if I shutdown before full decay.

Waveforms can be found here:
http://www.teslaboys.com/SISG/SISGWaveforms2/index.html

I have never destroyed an IGBT on any of my SISG based Tesla Coils.

BUT....

Recently, my boys and I built a Gauss Canon.  We used the SISG to trigger
it.  I misjudged the time it would take to discharge the cap because of the
very low resonant frequency of this device (only 212 Hz!! for our unit).  So
we blew IGBT's until we increased the shutdown resistor value to 100K ohms
(shutdown time 19 milliseconds).  With the coil magnetic field wound up with
energy when the IGBT shutdown the voltage would skyrocket and blow the IGBT.
As soon as we changed the resistor, the canon began firing 1/2" steel balls
30 to 40 feet.  We are using one SISG circuit and firing at 900 volts.
Current is approximately 300 amps and each bang is about 180 Joules.

Mark Dunn




>-----Original Message-----
>From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
>Of Gary Peterson
>Sent: Monday, February 09, 2009 10:26 AM
>To: 'Tesla Coil Mailing List'
>Subject: RE: [TCML] Spark gaps, Solid state switches and diodes

>Thanks Bart.

>I understand the SISG switch is usually left closed for two or more few
>cycles in order to wring more energy out of the primary capacitor, and also
>had thought it could not be switched off any faster than that due to
>switching-speed constraints imposed by the IGBT itself.  It appears if the
>operating frequency of the Tesla coil is sufficiently low, then the IGBT
>can
>be switched off at the "first notch."  Attempting to switch it off any
>sooner than that would place a strain on the device itself, correct?

>Here is another question.  I understand that Mark Dunn designed a printed
>circuit board a while back that embodies Terry's SISG circuit design (see
>http://www.teslaboys.com/SISG/SISG4PCB.pdf ).  Was this PCB ever mass
>produced, and if so is it still available?

>G.P. 

>Snip

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