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Re: Science Fair Project


From: 	ms291[SMTP:ms291-at-mindspring-dot-com]
Reply To: 	ms291-at-mindspring-dot-com
Sent: 	Wednesday, November 19, 1997 11:05 AM
To: 	Tesla List
Subject: 	RE: Science Fair Project

> From:         Robert Michaels[SMTP:robert.michaels-at-online.sme-dot-org]
> Subject:      RE- Science Fair Project

> TL>chips have a max voltage of 16-18 VDC so I can run the whole thing at 12
> TL>volts. The 7555 provides the square wave for the base frequency. The
> TL>one-shot triggers off the 7555 and allows me to adjust the pulse width
>
>         If the chips are rated only 16-18 volts, you'll want to be
>         absolutely dead-dog certain the power source for the
>         chips is as isolated as it can be from the power source
>         which drives the coil primary.  That is, if you want to operate
>         longer than, oh, say 37 mSec.

Nah... the first *few* 7555's lasted way longer.. almost several minutes
:)
that is until I got the separate source for the coil...

>
>         The rapidly collapsing primary field (which is essential to
>         obtaining a high voltage from coil) generates a vicious
>         spike across the primary.  Like, 50 - 100 v.

I was getting about 70V on the transient peak.. interesting to see how
the 
different spark lengths and quench times affected the waveform on the
primary.
Also very interesting when the frequency started to approach the rate
at which the spark didn't have time to quench. Got to screaming 
pretty loudly and drawing *lots* of current...

>
>         The additional good/bad news is that aforesaid vicious spike
>         is of  =reverse=  polarity  (Lenz's Law).   That it is of
>         reverse polarity means that some protection from it may be
>         obtained by the use of reverse-biased snubber diodes across
>         sensitive parts of your circuitry.
>

Right on again.. the HEXFET has a built-in reverse diode..I also
put strategically placed Transorbs across the CMOS chips.. even at that
it eats a 7555 every so often.. think the common ground might still be 
causing a problem.. was going to optoisolate the FET and coil circuit
and give it a completely separate supply.

>                 Most of the electronic Tesla coils in the world
>                 have been made with vacuum tubes,  =not=  solid
>                 state devices.   I do believe you are destined
>                 to find out why.

Didn't really plan on doing much with the solid state circuit
in the coiling world... 
but it was interesting to see it work. We are gathering the pieces for 
a spark gap coil. After that we may put some of the itinerant vacuum 
toobs to use.

Regards,
Jim Horan