On Thu, 27 Mar 1997 13:15:44 -0800 "Aaron J. Miller"
<millera@LELAND.STANFORD.EDU> writes:
>Hello all,
>
>This question is only marginally related to pic's since I am going to
>use
>a 16C84 to control the chopper, but there seem to be quite a few
>bright
>people out there who may be able to help...
>
>I am trying to make an incandescent lamp dimmer from a 16C84 and
>planned
>on doing a simple waveform chop using a triac.  I realized, however,
>that
>since the pic is going to be doing a bunch of other time sensitive
>stuff
>that I don't have the processor time to manually sense the zero and
>chop
>the wave using the pic.
>
>So, is there any easy method to passively (from the pic's point of
>view)
>chop the AC wave?  i.e. use 8 I/O pins to a D-A to give a reference to
>the
>chopper circuit and have it do the rest?  That way I can just output
>the
>light level to the pins and it would stay put leaving the rest of the
>processor available?
>


        We are using the 16c74 to phase control four triacs.  We detect
zero cross by doing a negative edge interrupt on the external interrupt
pin.  Positive zero cross is determined based on a timer.  We determine
when to turn on a triac by using the compare mode of a capture/compare
timer register.  At zerocross we determine when the first turn-on will
be, then put it in the compare register.  On receiving the interrupt from
that compare, we turn on that triac and load the next.  The external
interrupt port pin is driven directly from the power supply transformer
secondary with a series current limit resistor.  The clamp diodes in the
16c74 square it nicely and the hysteresis on the input prevents noise
problems.  Note that we actually get the interrupt a little BEFORE the
zero-cross, but that's a lot better than a lot after (which would be the
case on a positive edge trigger).
        A 16c74 running at 16 MHz handles this quite nicely.  While doing
the phase control it is also receiving a 250 Kbps datastream and scanning
4 A/D inputs.

        If you want to generate triac firing externally, you can do it
with an op-amp ramp generator (that is synced with the AC line) driving a
comparator for each triac.  You can get analog output of many of the PICs
by using pwm into a simple low pass filter.  We're doing something
similar in a product where we take the 250 Kbps datastream and pull out
48 bytes, converting them to analog signals.

        Now... speaking of AC choppers...  Anyone aware of a bunch of PWM
generators (like those in PIC chips) but all in one chip (maybe a dozen
of them or so that could be driven by SPI or I2C)?

Harold