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Re: [TCML] Using Triggered Gaps, was Designing a Coil Around a SRSG

On 3/23/13 9:03 AM, Futuret wrote:

I experimented with two methods that used an automotive
ignition coil to trigger the cap using a centrally located
tungsten rod placed halfway between the main static gap.
I've tried the disk with a hole in it ( a washer) between two electrodes. It worked ok, but eventually, I found the axial trigger electrode seemed to work better. You can adjust where the tip of the tungsten electrode is: I used it roughly even with the surface of the pipe cap with a hole in it. I ran compressed air along side the tungsten, so when it fires, it blows ions into the gap (and cools the electrode, too) 


One method supplied voltage to the ignition coil to create
the HV pulse, the other method removed voltage from
the coil to create the pulse (inductive kick).  Both methods
worked and I controlled the timing using a lamp-dimmer
type triac/diac phase controller to time the spark.  (I used
phase angle to trigger time-wise, not voltage.)  Both
methods were a little touchy to adjust and were at the
edge of their ranges for best results.
Touchy is right.. I had a tough time getting it consistent. Probably because as you run, things get warm and change the properties.
That's why I started thinking about something more like a car capacitive discharge.. I have a variety of strobe circuits (charge cap with 400V DC, then fire using a SCR) that I started to use, but I blew a couple up, and then I ran out of time to finish it.
Every once in a while I look at the autoparts catalogs and think there must be some inexpensive ($20 ish) piece that could be repurposed here. Electronic points replacements, even. Somewhere in a junk box, I have a bunch of HV transistors designed to work with the (obsolete) motorola electronic ignition driver (very high BVceo, darlington pair, good avalanche properties so that a breakdown doesn't destroy the part).
Maybe something like a filament transformer run backwards as a trigger pulse transformer going to the car coil would work. You need a fair amount of energy (100 mJ to a joule) to get a healthy spark, so it's not a "low power" device (a joule * 120 BPS is 120 Watts... ) 



 This was based

on work that Mark Metlica did years ago, and John
Tebbs also did some designing on the inductive kick
method if I remember correctly.  This work should all
be in the Pupman archives.  I had to set the main gap
about 5/8" to 3/4" wide to prevent premature firing
using a 12/30 NST.  I did these triggered static gap
tests using my TT-42 coil which normally ran with a
120 bps synchronous rotary gap.

    Jim wrote:

"On this basis, one would expect a single triggered gap with long path to be
better than a multigap."


I agree.

Cheers,
John Freau









-----Original Message-----
From: Scott Moseley <scott@xxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Sat, Mar 23, 2013 11:34 am
Subject: RE: [TCML] Designing a Coil Around a SRSG


Has anyone come up with a  good electrically triggered gap yet?  Maybe something
that uses magnetic quenching?  If so, then you could sense the voltage across
the cap, and then set a firing voltage.

-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of
Jim Lux
Sent: Saturday, March 23, 2013 11:31 AM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Designing a Coil Around a SRSG

On 3/23/13 8:12 AM, Futuret wrote:

contributes to longer sparks.  It is possible to achieve just as good
results using a static gap, but it has to be a triggered static gap.
Interestingly, in my tests, the triggered gap was very loud and bright
due to the wide gap spacing which had to be used to prevent premature
firing.


   All this noise and light didn't noticeably reduce the spark length

however, as I obtained the same spark length in both cases.


I think this makes sense.  The voltage drop (loss) across a gap is a combination
of the 50-100V cathode drop and then the ohmic loss in the spark itself. For a
reasonably high current arc, I'll bet the ohmic doesn't change all that much
between, say, 3 and 10 mm.

On this basis, one would expect a single triggered gap with long path to be
better than a multigap.

Somewhere back in the archives, I know this has been discussed..
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