Re: SSTC Singing Arc Design - Help needed

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

Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi,

> The first thing I need to do is get a DC input to my power supply circuit.
> For this, I'll just use a full-bridge rectified mains (115VAC) with some big
> caps for smoothing filters.

OTOH less filtering tends to make sparks less bushy and look more
like "real" streamers... With TL494 or LM3524 the audio sounds distorted,
so IMHO an extra 60 Hz humming doesn't bother the experience that much. ;-)

> The next thing is I need to figure out how to implement am modulation (via
> audio signal) to my FET drive circuit.  Right now
> I'm using a TL494 PWM Driver IC.  Someone how I need to am modulate the
> output of this guy.  Anyone have any ideas???

There's a very quick and simple, but crude way which isn't too hi-fi:
 http://www.hut.fi/~jwagner/tesla/tv-driver/schematics.htm
(TL494 is the ancient predecessor of UC3524, and IIRC they have the same
pin-out)


The error comparator setup described by Malcolm and Dan is a lot better.
It is the same as used in hi-fi class D (class S) power audio amplifiers.

The picture on the site below might help you to understand why you're
doing what you're doing:  ;-)))
 http://members.hometown.aol-dot-com/_ht_a/lmdmkm/Class-d/class-d1.htm

Or
http://power.ece.uiuc.edu/Balog/images/PAP2001/PWM%20AMP%20PAP%202001_files/
frame.htm#slide0003.htm
http://power.ece.uiuc.edu/Balog/images/PAP2001/PWM%20AMP%20PAP%202001_files/
frame.htm


Watch out for the freewheeling diodes. They may get quite hot as you are
running each mosfet at an average duty cycle of only 25%.

This is unless you tie the TL494 outputs together and thus get an average
50% duty cycle per mosfet.


In addition, you shouldn't excpect any stunning audio quality from TL494
or UC3524, the ramp signal isn't really so linear, and this causes a lot
of audio distortion. In my first class D amp (driving a speaker, not
a TC) I used a LM3524 - while it did work ok, it didn't sound very
convincing.

A discrete triangle wave generator (NE556) and comparator (LM319) works a
lot better - in fact these two IC's would be almost all that is required
even for a SSTC PWM & audio controller. :-) Dump the TL494?

Or then again, as this setup drives a TC instead of a speaker, audio
quality probably isn't a big issue... :-)))


cheers,

 - Jan

--
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 high voltage at http://www.hut.fi/~jwagner/tesla
 Jan OH2GHR