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OLTC update



Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi All,

I drew up schematics for the timing control and IGBT gate drives.

	http://hot-streamer-dot-com/temp/OLTC-TimingControl.gif

	http://hot-streamer-dot-com/temp/OLTC-IGBTdrive.gif

Not much to it ;-))  I'll just get a $3 current transformer from DigiKey to
detect the current, run the output to a comparator to trigger a 555 timer
and it's done. ;-)  A current transformer provides isolation which is
pretty handy in this case.

Close to the IGBTs, I will have a the gate drive to push current into and
out of all their gates. The TLP250s are opto isolators too so hopefully
there will not be too much odd current running around.  Might get tricky
having electronics near a 2500 amp coil and 200,000 volt RF streamers
running about ;-))  I'll have to try and keep things perpendicular to the
big current loop.

Cheers,

	Terry



==================
Hi All,

I think I figured out a super simple, easy, and cheap way to control the
firing timing of my OLTC project.  In this diagram:

	http://hot-streamer-dot-com/temp/OLTC-08-07.gif

I simply put a 0.1 ohm resistor (Rsense) inline with the charging circuit.
When the current across the resistor goes negative it happens to be the
perfect time to fire the IGBT array.  The cap, resistors and diode (a 555
timer in real life) will control the ~130uS pulse width to give first notch
quenching and all.  

Here are the major waveforms:

	http://hot-streamer-dot-com/temp/OLTC-08-07-1.gif

The timing circuit automatically seeks the most powerful mode and is very
stable and reliable (at least on the computer ;-))  More importantly, such
a circuit reacts very well to odd faults like this:

	http://hot-streamer-dot-com/temp/OLTC-08-07-2.gif

I was thinking of all kinds of fancy things and ways to run the coil's
rather critical IGBT timing but they all had their problems, especially if
things we not going just right.  This new circuit is only a 0.1 ohm
resistor, an op-amp, a 555 timer, and a driver chip to run the TLP250 IGBT
drivers.

What I thought would be the most messy part of the project to figure out,
suddenly appears to be all done :-))  If this circuit and method work, it
is a major breakthrough!!

No more major technical stuff to figure out now.  Just have to build it and
see if it really works...

Cheers,

	Terry