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Subject: Re: Solid State Tesla Coil Success!
From: Mike Poulton
Organization: MTP Technologies
Date: Wed, 25 Dec 2002 21:04:36 GMT
NNTP-Posting-Date: Wed, 25 Dec 2002 16:04:36 EST
On 25 Dec 2002, "Frank Babis" said:
> Could you post your coils details along with a parts list. I'd be
> really interested in the information. I'm fascinated with Tesla coils
> and have spec's for about 30 differant units. Eventually I'll beuild
> my own, until then I'm sorta archiving and gathering ideas on other
> How long were the sparks?
I got 6-8" discharges for a long time with no excessive parts stress.
The maximum discharge length would be about 24" to ground, down through
the center of the secondary. This is with no top-load on the coil.
Once I wind a new secondary and put the proper top-load on it, I expect
much better results. I have been running it VERY conservatively, not
even approaching the power levels for which it was designed. I'll get
to that later, when I feel like blowing some more mosfets.
The driver circuit is not terribly complicated, but is far too large for
an ASCII schematic. I'll describe it instead:
Make an H-bridge with four IRFP350LC mosfets. These are rated 400V 16A,
with low gate charge for fast switching. Use a 16A 30V ultrafast diode
in series with the drain of each mosfet, and a 400V 16A ultrafast diode
in reverse parallel with the mosfet and other diode. This effectively
bypasses the slow internal body diode in the mosfet and replaces it with
the external ultrafast diode. This is absolutely necessary. Put 10uF
of 600V polypropylene capacitors and 1000uF of electrolytic capacitors
across the + and - rails of the bridge. The output of the bridge goes
to the coil primary, through 10uF of polypropylene capacitors (for DC
blocking in case of mosfet failure). The coil primary is CAT-5 cable
with all conductors in parallel (or RG-174U coax, both work well). The
number of turns is adjusted to change the input impedance.
Now for the mosfet drive circuit. Wind two identical drive
transformers. Each transformer is wound on a ferrite pot core, and all
windings are 30AWG kynar wire-wrap wire. The primaries are 4 strands in
parallel, 20 turns. Each secondary is 30 turns, one strand. There are
two secondaries on each transformer. The primaries of the two
transformers are in parallel and connect to the driver circuit through a
10uF bipolar electrolytic and a 1uF polypropylene capacitor (in parallel
with one another) for DC blocking. The secondaries are attached from
gate to source on each mosfet, with a 10 ohm 1W resistor in series with
each. Make sure they are phased appropriately before applying power to
the H-bridge, or you will be sorry.
The transformer drivers are a pair of TC4420 and TC4429 IC's operating
as a full bridge. They are in parallel pin-for-pin except for the
outputs. Plenty of low-esr bypassing is provided right on the power
pins of these IC's, plus a 2200uF electrolytic to smooth the input from
a 12V 1A wall-wart. The drive signal is provided by an Exar XR-2209 VCO
IC. This is a great little variable frequency squarewave generator IC.
It is set up for 100kHz to 300kHz operation, with coarse and fine
frequency adjustments. The entire circuit is built on an extruded
aluminum heatsink chassis from a big car audio amplifier. The power
semis are all bolted down along the edges on the right, with the
polypropylene bypass caps in the middle and the drive circuit on the
left. The power connections in the H-bridge are made with RG-174U coax
(shield and center in parallel), and are completely straight and as
short as possible. The low-voltage driver is enclosed in a little
faraday cage made from brass sheeting. Input power is provided via a
10A variac and a bridge rectifier. The variac and rectifier are
three-phase, but only one phase is presently connected. As soon as I
wind a new secondary, I'll play with it some more and try a higher input
voltage. I expect at least 24" discharges with no problems once I get
it all optimized.
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