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PICList Thread
'[PICLIST] (PIC):Driving a Triac'
2001\01\25@064344 by Anand Dhuru

I am workingh on a 16F84 based project, the output of which is to control a mains lamp thru' a triac. Now, because of the way I have written the software, I cannot afford to use the classic (PWM) timing based strategy for contyrolling the intensity of the lamp. However, I do have 4 pins from port B at my disposal. Is there any way I could use a R-2R-4R kind of a strategy to control the triac, which in turn would control the intensity of the lamp?

Ant suggestions would be highly appreciated.


Anand Dhuru

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2001\01\25@075737 by Jinx

face picon face
I'm wondering if there's a way to use a magnitude comparator like
the 7485 to compare the value of the 4 PIC pins to the number
of mains cycles for a given brightness. In other words, say your
dimming down sequence goes from (15 cycles on/0 off) then (14 on/
1 off) and so on down to (14 off/1 on) then (15 off/0 on), perhaps
you could design a digital circuit that could set the on/off ratio as
set by the 4 PIC pins

The analogue way I guess would be to compare a reference
voltage generated by the 4 PIC pins and the voltage of the AC
wave. When the AC exceeds the reference the mains to the light
is turned off. This could get tricky resetting the circuit after each
cycle, coping with equal voltages either side of the peak, and
perhaps also handling the negative portion of the AC

If you can swing it then direct control of the triac by the PIC would
give the best results, ie the usual digital way. Is there no way you
can squeeze any "standard" dimming s/w into your code ?

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2001\01\25@083346 by Jinx

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Are you able to put another PIC in there ? A 508 or 509 would
do, could be more economic and give better results. The F84
could send control pulses to it with one pin - Start/ Stop, Up/Down.

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2001\01\26@134718 by Peter L. Peres

picon face
>R2R to control triac

The oddly true anser is that yes, you can. If you steer a triac with DC
then there is a range where by varying gate current you can vary the
firing angle in the fourth quadrant between 270 and 360 degrees (in theory
between 180 and 360 degrees). I would not recommend this for a series
product however. The why this is so has to do with how the triac is fired
by DC when the polarity is 'wrong' (by gate hot carrier current injection
-- the firing point depends on the relationship between -Vak, Cg'g and Ig
which leads to phase control in ac).


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