Re: SSTC Singing Arc Design - Help needed

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dhmccauley-at-spacecatlighting-dot-com>


> > Well, I'm off to begin my SSTC Singing Arc project.  I checked out
Richie
> > Burnett's site and Alan Sharpe's site but didn't really find anything
useful
> > as far as an actual implementation of the circuit.  So I think i'm on my
> > own.
> >
> > 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.
> >
> > 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???
>
> Usually one feeds a regulated DC voltage into one of the opamp inputs
> to give a reference standard. You might try to feed an audio signal
> into this input so that it varies about its DC mean (which should be
> arranged to set the duty cycle to 45%).
>
> ?
> Regards,
> Malcolm

Yes.  With the TL494 PWM controller, the feedback comparator range is
approx. 0.5V to 3.5V so the bias state would be around 2V.  This can be
easily done by ac coupling the audio signal (using with a 0.1uF series
capacitor) and then into an op-amp voltage follower circuit where at the
input to the op-amp (positive input) has a resistive divider which is set to
the bias voltage.  At the output of this op-amp, you would get the input
audio signal centered about this bias voltage.  There are otherways, but
this seems simplest I think.

Then you can have additional stages which can scale the voltage level down
to the necessary 3V swing required by the PWM circuit.

Dan