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RE: [TCML] Spark gaps, Solid state switches and diodes
Bart:
An SISG coil fires at twice the line frequency, in my case 120 Hz. As one
increases the SISG threshold voltage (by adding SISG circuits) there comes a
point where one approaches the peak voltage of the power supply and the cap
is not sufficiently charged to fire the SISG in one cycle. As a result the
firing rate is halved because the SISG fires on every other cycle. On my
coil I reached a point where it would sometimes run at 120 Hz and other
times run at 60 Hz. It seemed somewhat random as to which frequency it
chose. I began to decrease the turn-off time to see if it would add enough
charging time to push the charging over the edge and return to consistent
120 Hz operation. It seemed to work at the time, but in hindsight the time
gained was so theoretically small that it should not have made any
difference. I encountered no difficulties, so I left the resistors at the
final value of 680 ohms. Of course, the "minimum" value depends on the ring
down time for a particular coil.
You might also appreciate that this testing was done in June/July 2006. I
believe at that time only Terry and I had working SISG coils and mine was
running with (4) and (5) boards at 14 to 18 kV. Thus what at the time
seemed like an important test protocol for at brand new technology seems a
lot less important today.
Mark
>Hi Mark,
>The 680 ohm seems "on the edge" and specific to your coil. Were you
>trying to perform solid state forced quenching? This would be ok for
>some coils to a degree, but could be very risky to say my 70kHz coil
>that I'm running my SISG on where 330us is 3rd notch primary quenching.
>The 4700 ohm provides plenty of ring time without an IGBT energy crash
>and doesn't compromise charging. I'm just curious what you actually
>gain from setting Roff so low?
>Best regards,
>Bart
>Mark R Dunn wrote:
> 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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> http://www.pupman.com/mailman/listinfo/tesla
>
>
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