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PICList Thread
'Building a better Tesla coil'
1997\11\04@084942 by stephens

picon face
For my purpose a Tesla coil is for generating an aestheticaly
interesting and exciting non-lethal display. I realy want to see some
10 foot sparks (Would be non-lethal?)

The idea is to build a smaller, cheaper more powerfull, Tesla coil by
oscillating smarter and not harder, with modern semiconductors and
ferrites. Maybe I've been zapped a few times too many. I would
appreciate some feedback, and hopefully can find some others who have
always wanted a mega-volt multi-kilowatt Tesla coil but just can't see
providing the money, space and power for one. I hope to shrink it and
improve its performance by, hopefully, (now don't laugh please) 2
orders of magnitude.

The info I've found and will refer to include:

"A high potential Tesla coil impulse generator for lecture
demonstrations and science exhibitions by Kenneth Skelden, Alastair
Grant and Slan Scott, in that great little American Journal of
Physics, 65 (8) August 1997, pages 744 to 754." recommended and
graciously offered by Winfield Hill

And many (but not all!) of the text files at
ftp.funet.fi/sci/electrical/tesla/misc/

And sept 1991 RE "Solid State Tesla Coil" Duane Bylund

********

Begin by sacrificing a pole pig. Maybe the most expensive, rare,
bulky, massive and nearly impossible to fabricate component. The pig
(or neon, oil burner, copier transformer) used to turn 120VAC to 6 -
20 KV. The big ones are rated at 10KVA.

In Bylund's RE article, he does this using a 300 watt transistor
switching supply to boost 120 VAC to (is it?) 5000 volts. This is
continous (CW) power at 100KHz. The secondary of this compact coil is
around 10" dia tall, wrapped with #30 wire (IMHO he should have use 24
and made it 30" high, for higher Q )

It's SG3524 switcher can be tuned (YEA!) but no power control (Boo).
It ONLY spits out 100KV, 7" streamers. It needs a fist-sized $40
ferrite transformer to excite the secondary, to which the ferrite
transformer is directly connected. No spark gap, to waste power, or
loose primary to loose more energy. Direct connection, like Tesla's
"Magnifyng coil" and more familiar to my mind as a PI filter, or 1/4
wave matching transformer. Driven end is low V, hi I.

Wouldn't it be nice to just switch 120 VAC directly into the
secondary, and not pay for $50 bucks worth of ferrite, in ADDITION to
making the secondary too?

I was planning on using FETs & a 1KV microwave xfrmr. But after
reading some theory, came to the conclusion that I might get a
multi-kilowatt (FET's power-dependant) output, but no mega-volt
>1 meter arc's, which is what I'm after.

The problem here is the secondary's Q. If Q were high enough, CW power
could build up high voltages. The Q is the killer here. Unless a
physicaly LARGE secondary with nasty Litz and/or large gauge wires
used, you loose - air core's, copper loss and small size don't all
fit.

What about impulses? They will give you impressive, cooler, arc's.
Then you go back to needing Semiconductor un-freindly high voltage to
charge a big cap to several KV. And expensive or hard to implement
solid state switching. I've seen circuits to series connect, isolate
BJT's & FETs. But I don't like thier complexity. A spark gap is EZ'r,
but were back to another transformer power supply, and waste. And a
big secondary.

I propose the following design:

* A simple exciter, consisting of oscillator as simple as a 555 or
complex as a PIC oscillator/controller

* BJT's to drive a half-bridge MOSFET's 500V 7A, which could be
paralleled for upgrading

*A low ESR .1uF cap to couple the bridge directly to the secondary

Now the hard part...
And I'll admit I'm too lazy to get out my books & programs to
calculate and model this system, because I'm hoping to get
a reality check from someone with experience and/or more knowledge.

The Plan:
A compact secondary around 4" dia by 15" long, encapsulated in two
hacked-together 3-liter bottles. Maybe pressurized & dessicated, or
oil filled.

So now I can:
Use a ferrite rod core in the secondary! This will multiply the Q by
ferrites mu by increasing inductance, lowering frequency, et.
Use multiple windings maybe (quit laughing, they'll be teflon/mylar
insulated)
Use <32 AWG copper because frequency's lower
Around .5H inductance, around 200 pf capacitance, around 20KHz
resonance, Q around 1000?

The windings are spaced in oil. The core's insulated by wrapping with
aluminized mylar sheets. Individual mylar sheets are cut to 'C' shapes
to prevent magnetic coupling. Aluminization would make the dielectric
self healing. Otherwise, regular mylar. Or teflon. More is at top,
less at bottom to match potential gradient along coil core. (I'm
guessing even "high resistance" ferrite isn't at megavoltages.)

Well, even if I can shrink the secondary AND boost Q with ferrite, I
could still have the CW-small, hot spark problem. Now I'm not too well
magneticaly coupled to the ferrite, having an inch of teflon. So
hopefully the bar core doesn't saturate. But even if I am, maybe I can
use an old radar trick to turn FM CW into a high voltage impulse!

In certain all-pass or delay-equalizing filters, and dispersive delay
and transmission lines have the usefull property of delaying some
frequency's more than others, without effecting amplitude. So imagine
the driver FM chirping into such a filter. With the proper design, the
chirp is compressed into an impulse, as the initial high frequencies
take longer than the subsequent low frequency's to propagate through
the filter/line.

OK, so now I have to build not a tesla coil, but a Tesla-Chebyshev (or
something) filter. Insanely difficult to make a lumped element HV
filter right? I have this silly hunch, that if I make some type of
"SPECIAL" core, kind of like microwave plumbing filters or wave guide,
maybe a cone shaped dielectric or ferrite core or winding, maybe
relying on the core saturating at higher frequencies, dissapearing
like guannella transmission-line transformer cores?

Or, yea, got the idea? I need a high voltage, ferrite-shrunk Tesla
chirp-to-impulse matched filter. Who's done this and wrote about it at
these VLF frequency's and high voltages?

Then maybe I can get 6 foot sparks out of a tiny little bottle plugged
(almost) right into an outlet, using a couple chips, couple FET's and
ONE coil, for only a few bucks worth of surplus ferrites & MOSFETs, a
PIC and an old motor or wall wart. And a few rolls of teflon tape.
wouldn't you love to play with one of these?

No, I don't want to wake up now :(

*************************************************

Ok, so what the hell does this have to do with PIC's, the PIC'rs are
wondering? Here are some requirements I began spewing out. Obviously,
I need to include a kitchen sink and the DOD. But if thier is enough
interest, maybe we can collaborate. Otherwise, I'll probably loose
interest after my current 35W, 5"x20" spark-excited tesla kills my few
remaining neurons.

TESLA PIC functions-
PIC has dual purpose software,

1) to generate a swept square wave for network analysis of a complex
coil. Controlled by PC software, and generates smith chart.Controls
4046 VCO to generate square wave, and reads its phase detectors.
Also need analog switch for VCO range.

1)Testing and Coil Evaluation:

Sweep Oscillator PB1,PWM output controls 4046 VCO 1KHz to 1MHz
               PB2,analog switch for 4046 VCO range cap
               PB3, analog switch for 4046 VCO range cap

Amplitude meter - diode detector, 3 port line diode/cap A/D (ya, its
cheezy but I just want to see a peak


Tesla Coil PIC
Cheep Transformer-less cap coupled CPU power
FET driver oscillator, Chirp, CW, mono-pulse, pulse width, impulse and
repitition rate
Power consumption (120VAC) measure
E-field meter - analog switch chopping electrometer, to PIC A/D
Monitor FET temp (OK, I need a muxed A/D too)
Emergency shutdown solid state relay. 15 Amp :)
Read safety interlock switch
Read power input control - plastic knobed HOT pot
Read frequency input control - plastic knob Hot pot
Operation LED
Standby LED
Danger LED
Lethal Power LED
Piezo buzzer for command acknwlg & warning/fault
Safe remote IR or TV/VCR format remote interface
Serial IRLED or hardwire (cheap) to PC for hot op data dump

****************************************

Epilogue

The Burgler-Quter

The 5 mega-volt pop bottle resides inside a 1meter dia plexiglass
sphere 1/16" thick, filled with Krypton and neon to glow orange, and
decorated like a vicious. sadistic pumkin.

Two 1cm dia plastic jacketed conductors diverge from the cheaks, out 1
meter, and terminate in aluminum spheres, like catfish stingers. The
sphere is suspended by a power cord on a rotating motor mount,
allowing it to rotate and face objects scaned by a PIR sensor. It
scans, oscillating right and left, untill an IR anomaly is detected,
then homes in, powers up, zapping and emitting hilarious laughs &
cackling at 120 dB. (I'll deal with the pyro-acoustic siren design
another day.)

1997\11\05@135401 by lilel

flavicon
face
I've also fantasized about building tesla coils.

Doesn't the FCC get really upset about these things when they go off?
Esp. if you implement your plan to use FM to drive it?  We are
talking, potentially, about a multi-kilowatt transmitter here.
Shielding?



AND the idea about the arc spewing jackolantern that tracks burglars
- THAT's way cool.


Best Regards,

Lawrence Lile

1997\11\05@140820 by Pierce Nichols

flavicon
face
On Tue, 4 Nov 1997, Lawrence Lile wrote:

> Doesn't the FCC get really upset about these things when they go off?
> Esp. if you implement your plan to use FM to drive it?  We are
> talking, potentially, about a multi-kilowatt transmitter here.
> Shielding?

       A good layer of copper foil around the room you run it in should
just about do the trick.

       Pierce

"I have a work order for the immediate demolition of your reality tunnel."

       -Bob, RAW Construction Corp.

===========================================================================
Geek Code v2.1: d?H+sg+a-w++v+c++UHS+P+L+E+N+K!WM--!V-po+Y+t+5+j+R+G!tvb+++
D+B---e+u*hf+r+*n-y+

1997\11\07@020906 by Scott Stephens

picon face
At 08:44 AM 11/4/97 +0000, you wrote:
>I've also fantasized about building tesla coils.
>
>Doesn't the FCC get really upset about these things when they go off?
>Esp. if you implement your plan to use FM to drive it?  We are
>talking, potentially, about a multi-kilowatt transmitter here.
>Shielding?

Well, its tuned to 500KHz, so without a tower, and being in my basement,
with its energy heating, ionizing, and ozone-izing the air, hopefully it
won't be heard. By FM chirp, I mean around 10 to 100KHz, not the 88-108MHZ band!

>AND the idea about the arc spewing jackolantern that tracks burglars
>- THAT's way cool.

I thought so. Although its more to amuse my family, than assault. A recent
post on the TESLA mailing list commented that two car coils insulated in oil
with plastic tubing, and phased, produced 30" sparks when pulsed with a 600V
100uF (18 watt) impulse. Ouch!

1997\11\08@074342 by paulb
flavicon
face
Scott Stephens wrote:

> Well, its tuned to 500kHz, so without a tower, and being in my
> basement, with its energy heating, ionizing, and ozone-izing the air,
> hopefully it won't be heard. By FM chirp, I mean around 10 to 100kHz,
> not the 88-108MHZ band!

 I suspect just about ALL of the above things would drive the FCC, as
well as aviation authorities, neighbours etc., WILD!

 500kHz is a distress frequency, or at least always used to be.  Get
the hint?  The ISM (Industrial, Scientific, Medical) allocations are a
bit lower, and constrained.  455kHz is your standard IF frequency for
most radio receivers, so sweep a signal across it and you'll annoy a LOT
of people.

 I do have some experience of diathermy units, the oldest ones of which
(now we are talking at least 20 years ago) did use spark gaps but were
pretty well behaved (read: tuned to their proper frequency).  Later ones
use a quench frequency of a few hundred hertz (by my ear) and are
totally solid state, possibly even crystal-locked.  Bandwidth should
therefore be limited to a couple of kHz.

 In any case, there appears to be considerable confusion here between
FM "chirp" and simple impulse (Capacitive discharge) sytems.  Barking up
the wrong tree I would say.  The major limitation of Tesla coils is
tuning the distributed capacitance of a very large winding.

> A recent post on the TESLA mailing list commented that two car coils
> insulated in oil with plastic tubing, and phased, produced 30" sparks
> when pulsed with a 600V 100uF (18 watt) impulse. Ouch!

 I find that a little hard to believe.  Especially the plural of
"spark".  600V 100µF computes as 18 Joule (Watt-second).  Fairly
standard professional photoflash.   ^^^^^

 Cheers,
       Paul B.

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