Re: OLTC Questions

["Tesla list" <tesla-at-pupman-dot-com>]

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

Hi Steve,

At 09:06 PM 1/14/2003 +0000, you wrote:
>Hi
>
>This post is aimed at Terry Fritz and others working on the new OLTC 
>technology. I read about this just today and I was pretty impressed. I 
>would very much like to try messing around with this in a small 100-200 
>watt size but there are a couple of things I'm not sure about:

Neat!!


>1. How do you make sure your IGBT switch turns off at a primary current 
>zero? I imagine that if you turned it off when the primary current was 
>high, it would blow up. Do you have some kind of current sense circuit or 
>are you just waiting until the ringdown is definitely over?

I have it timed by a 555 timer.  I either wait until the ringdown is 
complete to shut it down or I time it to turn of at a zero 
crossing.  However, I have transorbs across the single turn primary that 
will absorb spikes beyond what the IGBTs like.  I can actually spin the 
timing knob around at any point and quench at full primary current peaks if 
I want.  The transorbs are taking a big hit at that point but they seem 
happy and don't get hot.  I try not to push my luck however ;-)


>2. Would a SCR or triac work instead of an IGBT? I don't know if SCRs can 
>be made to turn on quickly enough, but they would sure simplify things 
>because they automatically turn off at a current zero :)

IGBTs have nice low voltage across them even at hundreds of amps.  That is 
the key to keeping the power dissipation in them down.  Other devices tend 
to look very resistive and my dissipate a lot of heat.  Perhaps others 
could be used but I chose IGBTs to keep the power dissipation as low as 
possible.  At full power, I may see 5000+ amps, so any voltage drop across 
the IGBTs is a big deal.


>Any thoughts on the matter much appreciated

I do hope to get the OLTC stuff written up better soon.  Most information 
is at:

http://hot-streamer-dot-com/temp/Off-LineTeslaCoils(OLTC).txt

And in the files beginning with "OLTC" at:

http://hot-streamer-dot-com/temp/

Cheers,

         Terry


>Steve C.
>