RE: Ignition Coil as a TC Transformer?

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

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Gary.Lau-at-compaq-dot-com>

Regarding the need to invert the 555's output to achieve a duty cycle of
less than 50%, I can see that greater than 50% applied to the power
transistor would result in a DC bias on the coil and excessive current
through the transistor, but I would think that one should strive for exactly
a 50% duty cycle at a suitably high frequency (?) such that the core doesn't
saturate.  As noted, the usual 555 circuit will generate an output with
greater than 50% duty cycle, but the 555 circuit that Terry used in his
Tesla Coil tuner uses an ingenious configuration that always guarantees a
50% duty cycle over all frequencies.  I've not tried it but I think that
circuit could be used to directly drive an IGBT.  See 
http://hot-streamer-dot-com/temp/TCT1.doc

Gary Lau
Waltham, MA USA



The schematic is now posted at:

http://hot-streamer-dot-com/temp/Schematic.gif

Cheers,

	Terry

At 06:47 PM 3/3/2001 -0600, you wrote:
>Hi Matt,
>
>I have never used a 2n3055, but have built some flyback drivers. The last
>one I whipped up just for fun last December uses an IGBT. The 2N3055 is a
>popular transistor because it is cheep and universally available. Any good
>high power transistor will work. The trick is to use something like a
2N2222
>to drive the power transistor more or less in a Darlington configuration.
In
>the case of a MOSFET or IGBT which is controlled by voltage rather than
>current and could be driven directly by the 555, a transistor like the
>2N2222 is still needed to act as an inverter because the 555 output will
>tend to be high longer than it is low. You really want a very narrow pulse
>rather than a wide one, and inverting the output of the timer does that. If
>the pulse is to wide, too much current can pop the MOSFET or IGBT.
>
>This circuit is designed for the development of a flyback driver and
>includes adjustment for frequency (from approx. 18KHz to 80KHz) and pulse
>width. I was able to get from approx. 2KV up to more than 50KV using this
>circuit and a flyback out of an old color TV. It features an offline power
>supply of around 170VDC, and a low voltage power supply for the timer and
>switching voltage. There are no over current protection circuits in this
>bread boarded prototype. I intend to add over current protection as soon as
>I get back to the project, but am just recovering from the financial
>disaster which is January. (Taxes, insurance, more taxes, etc.) I will send
>this to you directly as an attachment. If anyone else is interested, email
>me at deano-at-corridor-dot-net and I will be happy to send you a copy as well.
>
>later
>
>deano