Re: True solid state Tesla coil

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <cwillis-at-guilford.edu>


 Hi Matt,

I think you definitely have a good idea in wanting to start small and simple. 
You'll probably get a better understanding of why certain improvements are made
in the more complicated circuits, and you'll waste less money in blowing
transistors!  I'll briefly describe a very crude lashup that can give you
something to play with- but you'll want to improve on it later, trust me.

You can use a 555 timer, one of the most abundant oscillator chips available,
to directly drive a IRF540 MOSFET, which switches current through the primary
winding on a tesla coil.  Make a normal secondary (with fine wire so it
resonates at ~300 kHz or less) and use a primary of about eight or so turns (I
am remembering my own experiments with coils wound on 3" PVC pipe.) Wind this
at the bottom of the secondary, directly on top of it.  Mount the FET on a
heatsink.  Use a < 25VDC supply that can provide several amps.  A current
limiting feature helps preserve the FET and a variable supply is a real help
here.  This is just about as simple as it gets.  I was able to get a 1.5" spark
from such a coil, and it turned out to be good for plasma displays.  The
downside of something this simple- the FET runs hot and fails occasionally. 
Better yet is to use the 556 dual timer, with one timer making low-frequency
pulses and triggering the second timer to produce bursts of the resonant
frequency.  This gives some neat visual and audible effects and also keeps FET
duty cycle low.

As Richie Burnett's website (www.richieburnett.co.uk) explains quite well, you
can protect an FET with a series schottky rectifier and external fast diode.  

I was very nervous about my first attempts at solid-state coiling, the approach
described above.  After all, those transistors are expensive (from the point of
view of a "starving student" budget at least) and fail quickly, without
warning.  Definitely not as robust as tubes.  But there are ways to get into
this hobby cheaply.  Some semiconductor companies are real nice in that they'll
give you "free samples" of their loot.  Of course, the idea is that you develop
commercial goods around these components.  Maybe someday we can sell  SSTC's
for some sort of commercial purpose!  But in the mean time, you can amass
enough free parts to figure out how to not blow them up; troubleshooting an
SSTC demands some blown parts.  (Anyone: please let me know if you consider
this free part procurement to be an unethical practice.  I suppose after
achieving reliability, you should buy them to be fair.)  Try ON semiconductor
(ONSemi-dot-com), Texas Instruments (ti-dot-com) and Fairchild Semiconductor
(Fairchildsemi-dot-com).  They all have extensive online sample programs.  ON makes
a lot of those nice power FETs, for example...

Good luck.  I'm just procuring the parts for a medium SSTC using a half-bridge
FET approach.  I plan to use a ti UC3825A PWM chip as the oscillator (it is
much faster than the 555), some driver chips, and IRF740's switching rectified
wall current.  I am finally doing a real SSTC because I love tube coils, but
not the added bulk and expense of the plate supply!  

-Carl


"Tesla list" <tesla-at-pupman-dot-com>
10/23/2001 06:58 PM CST

To: tesla-at-pupman-dot-com
cc: 
bcc: 
Subject: Re: True solid state Tesla coil


Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Fucian-at-aol-dot-com>

In a message dated 10/23/01 7:44:54 PM Eastern Daylight Time,
tesla-at-pupman-dot-com writes:

<< You can also follow a link from TCBOUK to Alan Sharp... He ran a nice SSTC
at Corby this year.
Cheers, Mark H. >>
I have looked at these.These are complex circuits.Are there any that use very
minimal parts?Im not looking for expensive high output device.Just the
equivialent of a 1in disruptive coil.

Matt