At 06:45 AM 11/27/02 -0700, you wrote:
>ok..so this might be more PIC...but in general since the PWM doesn't have to
>be generated by the PIC itself.
>
>If I have a 0-15VAC that I want to ramp....or actually follow the PWM
>output, can this be done with MOSFETS or triacs?  I *think* this falls under
>more like soft start or dimming, but I'm not sure.
>
>I'd just like to be able to ramp the AC with the PWM output, so kinda like a
>voltage follower circuit, but its not DC, its AC

If you just chop up the 15VAC (presumably mains frequency?) with a PWM
signal, that may be what you want.

If the 15VAC is isolated from your power supply, you can use a bridge
rectifier and a single MOSFET driven by the PWM signal. The load goes
in series with the "AC input" of the bridge and 15VAC power supply, the
MOSFET shorts the "output" of the bridge when it is on (- tied to ground).

You could  use phase control too (thyristors), but you'd have to synchronize
with the zero crossings for the trigger, and the uncorrected power output
linearity would be an "S" shaped curve rather than a straight line.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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