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Re: transistors instead of diodes?

Original poster: "Jolyon Cox" <jolyon@xxxxxxxxxxxxxxxxxxxxxxxx>

Steve,
you say you use the 4046 PLL -what other PLLs are suitable for tesla coil
use -I mean would it be possible to use the 567 tone decoder/PLL chip for
instance -or even the 555 timer with additional circuitry?

Jolyon

----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Wednesday, March 23, 2005 7:08 PM
Subject: RE: transistors instead of diodes?


> Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>
>
>  >I am wondering if in place of fast-recovery diode it would be possible
to
>  >use a second transistor in anti-parallel with each transistor of the
>  >H-bridge
>
> Hi Jolyon,
>
> If you use MOSFETs then this happens automatically since MOSFETs conduct
> equally well in either direction when they're on. The trick is used in
> industry (it's called a synchronous rectifier)
>
> However, there is no real advantage to it since the fast recovery diodes
> already make a good job of returning as much spike energy as possible to
the
> DC supply. It's only useful in low voltage high current circuits where the
> MOSFET has lower conduction losses than the best schottky diodes.
>
> IGBTs only conduct in one direction so you would need two in
anti-parallel.
> But it's totally pointless in this case since an IGBT actually has a P-N
> junction, so it always has a higher volt drop than a diode of the same
> current and voltage rating. (so does a 600v MOSFET for that matter)
>
> As for your other questions on suppressing unwanted harmonic modes, I use
a
> 4046 PLL chip in my controller and I can set it to reject any modes
outside
> of a narrow frequency range. Steve Ward uses a somewhat different logic
> circuit but the result is similar- it won't allow the driver to drive
modes
> above a certain frequency.
>
> I don't think there's any point in adding extra coils and capacitors to
the
> primary circuit, it just increases IR losses.
>
> Steve Conner
>
>
>