Thank you for your reply. PWM is more like FM. There are existing
"dimmer" circuits to do this with a TL494.
The TL494 will work for PWM, but I don't think you want the feedback,
boost inductor, etc they show in the application notes - it is
designed or optimized to provide a constant vantage in a boost or buck
regulated power supply. Fixed output voltage - not what you are doing
- but it will work,or can be adapted for PWM "modulation."
Sorry if I was unclear, but what I am looking for is amplitude
modulation. For example, most commercial 12V fluorescent inverters run
at around 20KHz. How can I best adapt such a circuit to modulate, or
gate, the 20KHz with a lower audio frequency squarewave?
Barry
Amplitude modulation is probably not the best idea. That would be,
for instance, connecting the speaker output of an amp into a full wave
bridge rectifier, then feeding that to your lamp. I don't think you'd
like it.
But you did mention PWM and that is the way to modulate a fluorescent
light. So I think we have a communication problem.
With pure amplitude modulation you don't have the "strike" voltage for
the fluorescent tube. The tube really wants to see several hundred
volts to start and warm filaments. Once it is ionized it may only
require 50 volts with a current limiter.
Straight amplitude modulation will be well below the strike voltage
half of the time or more.
Enter PWM - the audio signal is converted to PWM and gives the tube
the full 12 volts (several hundreds after the inverter) and turns it
on and off fast enough that you won't see it with your eyes. The
important thing is that even when the "on" pulse is very short in
duration, it still supplies the full 12 volts so may work for
modulating a fluorescent tube inverter.
Now, you say the tube is working at 20 KHZ - your modulation can't
also go up to 20 KHZ (the oscillator in the pwm) it has to be slower
to look good. The audio modulation source should probably be below
5KHZ - voice only.
You have three things that can "alias" with each other. The tube
should be ~200 KHZ, the oscillator in the PWM at 20 KHZ, and the audio
no higher than 3KHZ for things to be happy.
High frequency is more effective at ionizing lamps than low frequency
at the same voltage (or at some given voltage). So, Ideally you'd
want the lamp to be running at a much higher frequency than 20 KHZ for
audio - and not waiting for a heater to help the gas ionize.
What you want to do, can probably be done. Lamps are moving to higher
and higher frequencies as the switching parts get better and cheaper -
they don't have to spend as much on the inductors and efficiency goes
up.
BUT you may have problems just taking some off the shelf 12v lamp and
just adding pwm to it and expecting it to track an audio signal.
