Re: OLTC Questions

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

Original poster: "jimmy hynes by way of Terry Fritz <teslalist-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>


The IGBTs that Terry used worked at currents up to 750 amps, but were rated 
at (I think) 204 amps. you would probably be fine with 1 IGBT.

140pf is a very large topload. With 400 amps through 3.5 uh, the toroid 
voltage wont pass 65kv. If there are going to be any streamers, you better 
have a sharp breakout point. I would go with a higher turns ratio, I think 
65kv would be too low. If you have to go to really thin wire to get enough 
inductance, you might want to put multiple layers of wire on the secondary 
( in paralell, of course) to lower the wire loss. You dont want to have the 
same problem as Terry's OLTC. Terry, do you know how much of the secondary 
loss is the wire and how much is the form?

Higher break rates tend to produce shorter sparks. I think I remember John 
Freau saying it took about 20% more power at 240bps than 120bps for the 
same spark length.

  Tesla list <tesla-at-pupman-dot-com> wrote:
Original poster: "Stephen Conner by way of Terry Fritz "

At 21:57 19/01/03 -0700, you wrote:
 >Original poster: "Terry Fritz"
 >
 >Details on this are at:

(snip)

OK, thanks for the information. I put together a basic schematic of my
proposed OLTC. Note: I left out EMI line filters etc and I may need to
change the value of the charging reactor.

http://homepages.strath.ac.uk/~cnbp111/oltcschem.gif

It's basically a small DC Resonant charging tesla coil with the spark gap
replaced by an IGBT. Rather than triggering the 'gap' as soon as the
charging current hits zero, I used a de-Qing diode like in normal DC coils,
so I can run at any break rate I want. Does anyone have any thoughts on the
validity of this approach?

The main issue is that the primary ! current is going to hit 400A peak but
the biggest IGBT I can find just now (Infineon part# BUP314) has a pulse
rating of 160A. So I have to either parallel three IGBTs (using a separate
primary coil/tank cap/gate drive for each one to ensure sharing) or use a
tank cap one-third the size and then go up to 1200bps to get the same power
as before. Can anyone comment on whether increasing the BPS would impact
the spark length even if the power level stays the same? I.e. do a few
large bangs give a bigger spark than many little ones?

Your thoughts much appreciated

Steve C.





Jimmy



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