Re: Low Voltage Tesla Coils

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

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, July 13, 2002 8:06 PM
Subject: Re: Low Voltage Tesla Coils


> Original poster: "David Sharpe by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<sccr4us-at-erols-dot-com>
>
> Terry
> This is excellent and makes good sense!  Only issues I see
> with this concept are:
>
> 1. Current limiting, but if you use a IGBT SS-Variac on
>     front end in dynamic current limiting pulse by pulse, you've
>     pretty much made the system blow-out proof.  "I" would
>     still opt for a isolation transformer (read impedance
>     "reactor" to limit current when the "OH SH..Ts" occur.

That's the "max surge current" issue (and where that "amps interrupting
current" (AIC) rating of the circuit breaker becomes important)...

A big series choke might be sufficient... just something to limit the
current to 100Amps or so...



> 2.  Opto-couplers are limited to about 4kV hold off ("I"
>      wouldn't feeld comfortable with only 0.3" spacing between
>      1-2kV peak x 150-200A impulse.  Toshiba's IC opto
>       is listed at Digikey (TLP250) and has ~60ns switching
>      transistions and << 1microsecond delay.  It is good \
>      enough to run a FET SSTC at 100kHz (although separate
>      isolated power supplies are now needed).

Transformers.... especially if you are doing a series thing with a bunch of
devices stacked up.  It's easy to build/buy suitable isolation transformers
for the gate pulses that have 20 kV standoff.

> 3.  What is maximum surge current a 50A 1200V totem pole
>       IGBT for motor control going to take before it goes POOF?
>       Most are rated at 10 microsecond short circuit current
>       ratings, and since you are decrement wave firing, peak
>       currents up to perhaps 70% of this rating "may" be ok.
>       I really get concerned about extreme silicon die thermal
>       excursions at these obsince peak power levels.

Depending on how much these babies cost, and how brave you are, one might
want to derate them a bit.  Say long term RMS of half, likewise for peak.
70% would be pretty brave.

> 4.  One to three turn primary, Vin =<1-1.5kV, Zo <=10 ohms.

A simple offline boost converter might not be a bad idea... 240VAC  (half
wave doubled..) in, then boosted to a few kV.  It would be a tradeoff
between the boost converter hassle and the multi device "sparkgap" hassle.

>
> Lets Roll!
> Regards
> Dave Sharpe, TCBOR
> Chesterfield, VA. USA
>