The .gif attached is a circuit from Electronics Today International
(UK) of March 1996, part of a 9 page article called "Driving Triacs
With The PIC Microcontroller" by Bart Trepak. Much of the text
is relevant to the ongoing thread of lighting control. If the halogens
talked about this week can run OK with AC this circuit could be
easily adapted to work with a lower AC voltage. The circuit is for
a 240VAC line voltage. Note that Triac terminal MT1 is connected
to Phase/Live and MT2 to Neutral. Two push buttons are in the
circuit as up/down dimming controls.

Suggestions and points (and there were a lot of them) raised in the
article include ;

Using 0 and -5V as the PIC supply because the triac triggers better
with a negative voltage pulse

The internal static-limiting diodes of the PIC CMOS inputs are
used to clip the input mains voltage. Two 2M2 are better used
than a 4M7 because of the voltage rating of 1/4W resistors.

The diode in series with the gate resistor is to prevent the AC that
comes out of the gate, when the triac is conducting, getting back into
the PIC. Although the author says it's already limited by the gate
resistor (100R) it's better to be cautious and block it altogether.

RTCC input requires an external diode to V+ (in this circuit 0V line)
To use RTCC as a zero-crossing detector, set it to FFh. The next
positive transition on RTCC will cause it to rollover to 00h. Check
it with mov RTCC,1 and test Z flag. (My take on this is that RTCC
can't be used to detect both positive- and negative-going crossings)

Detect both the rising and falling approach to the zero voltage
point. This way, integral numbers of both positive and negative
half cycles can be used. For example, loads with high inertia
such as heating elements would be better suited being supplied
with complete half cycles. With motors and lights this would
cause jerking or flickering and they are best supplied with a part
of every cycle.

Put a choke in series with the triac to minimise EMI and suppressor
cap(s) across the load.

If you assume the threshold for the CMOS inputs is -2.5V (with
this PSU 0V is high and -5V is low) then the positive- and negative-
going zero crossing points are slightly assymetrical, which should
be accounted for if very fine control is needed.

To detect the zero crossings - When reading the input pin a change
from a stream of 1's to 0 is a negative-going zero crossing ie the
mains is heading into its negative half cycle. A change from a stream
of 0's to 1 is a positive-going crossing.

The 470n (250V) cap is there for continuous gate current with a large
triac (50mA gate) in integral cycle applications. If a stream of micro-
pulses is used instead (the author suggests 100 every half cycle) the
current going out of the PIC is reduced and the cap can be cut back to
100n. To reduce PIC power further use sensitive gate triacs. A transistor
can be used to boost the PIC pulse current. From PIC o/p pin an 8k2
to npn base, emitter to -5V, collector to gate via 390R.

Trigger early in the cycle when using whole half cycles to minimise
current rush and electrical noise. However with inductive loads, the
current and voltage through the triac may not be enough to cause it
to latch properly and the trigger pulse should be lengthened or a
series of micropulses sent throughout the cycle.

For dimming, divide a mains half cycle into a finite number of equal
time intervals, say 50 or whatever is convenient for the frequency
you have the PIC running at. The triac will continue to conduct after
being triggered so the power to the light will come from the moment
the triac is triggered until the next zero voltage point. For 100% power
send a gate pulse out as soon as a rising or falling voltage is detected
after a zero point. For other brightnesses, start counting the time
intervals after the zero point and then send a gate pulse. As this
means the power will be applied at a non-zero voltage, steps will
have to be taken to cut noise, such as chokes or suppressor caps.
This may be difficult with high wattage applications.

I hope this will help anyone with mains interfacing, I know I learned
a thing or two. Mains is not my favourite stuff to play around with and
the article has given me a bit more confidence than I had before.

Jinx








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