Re: Low Voltage Tesla Coils

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

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

Hi Dave,

Don't get too far ahead of the "pie in the sky" idea here ;-))

I have certainly discounted a few "tiny details" about what such a system
may do to the AC line input ;-))

Antonio's point about how direct off line voltages are going to not work
due to impossible primary inductances is a "BIG" limiter.  MOTs may be
"in", so we have some natural current limiting right there.

I have always assumed such a system would be controlled through long fiber
optic cables especially when "I" was holding the other end ;-)  Considering
the "noise" of TC's, fiber optics are perfect for such a system.

I can't see a reason why an IGBT would be on longer than 500uS or have a
sustained high current in it unless the darn thing was on fire.  Of course,
before such a thing is "perfected", a few WILL be on fire ;-))

Much to ponder....

Cheers,

	Terry


At 10:57 PM 7/13/2002 -0400, you wrote:
>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.
>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).
>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.
>4.  One to three turn primary, Vin =<1-1.5kV, Zo <=10 ohms.
>
>Lets Roll!
>Regards
>Dave Sharpe, TCBOR
>Chesterfield, VA. USA
>
>Tesla list wrote:
>
>> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>>
>> Hi Richard,
>>
>> Variacs have lots of head room so this does work, to a point (you'll "know"
>> when you reach it ;-)).  The high voltage will saturate the core pretty
>> fast.  However, who said we wanted a heavy expensive o'l variac either.
>> With a bit of thought in the control, we can fire at any voltage we want
>> just by timing when we trigger the IGBT during the primary cap charging
>> cycle.  No need for a variac, but just another few IC's (we trust silicon,
>> don't we? ;-))  Neat isn't it!! :-))  I suppose we could consider matrix
>> converters too, but lets not get too "far out", yet ;-))
>>
>> But before you blow up a nice variac, consider this little circuit for
>> getting close to 700 VDC at !terribly! high power!
>>
>> http://hot-streamer-dot-com/temp/Vx2.jpg
>>
>> BTW - These direct AC line circuits involve giant power levels!!!  Your
>> grabbing the "power devil" by the tail...  Even pig system coilers will be
>> surprised by the explosions these circuits can make!!!  Don't forget fuses,
>> breakers, safety precaution, and getting far away if you dabble in such
>> things!!!  The circuit above does not show breakers, soft start, drain,
>> filtering, and a bunch of other very important "details"...
>>
>> A fine bit of advice for dealing with direct line currents:
>>
>> Imagine the side of your house where the breaker box is...
>> Imagine a six foot hole getting blown through your house there...
>> Imagine how far away you want to be when that happens..............
>>
>> Another note:
>>
>> I once asked a power engineer why the 12 Ga. line cord to his experiment
>> seemed to be rather small?  He replied...  It's the fuse......  3 hours
>> later, it proved to work...
>>
>> I suppose LVTC's will require a number of "new" safety notes...
>>
>> Cheers,
>>
>>         Terry
>>
>> At 08:38 PM 7/13/2002 -0400, you wrote:
>> >> One may even be able to go down to ~300 or ~150 VDC
>> >> (line voltage just rectified) and eliminate that transformer!!  An Off
>> >Line
>> >> Tesla Coil (OFTC - sorry again :-)))
>> >
>> >Terry, list,
>> >
>> >I've considered wiring a common variac as an auto transformer.  Probably
>> >dangerous as hell, but that's what we do the best, eh?
>> >
>> >Put the full 220 vac across one HV leg and the wiper of say a 1256D and
>> >take Vout from the other leg.  Would have to figure the lower limit of
>> >wiper travel and put in a mechanical stop.  Wouldn't want to put 220 vac
>> >and 150 amps across two windings.  Watts per variac wind could easily be
>> >calculated and wiper travel limited to a safe range.  After all, we all
>> >over drive variacs to 260-280 vac now.  What's a little more?
>> >
>> >Conceivably 500 to 800 vac could be available for your IGBT switched coil.
>> >And, no heavy iron transformer.
>> >
>> >Comments?
>> >
>> >RWW
>> >
>> >
>> >
>