Martin Kirk writes:

           I am using a 16C57 in a project driving nitinol "muscle wires"
   which contract when heated.  I believe that I will use alternating
   current to heat the wires so that I don't have to regulate the all that
   power just for heating wires.

           1)  Would someone suggest an efficient way to switch A/C power using
   PIC outputs.

           2)  I have been looking into using a triac but I have little
   experience with them and I am not sure just how they work.  Does a triac
   switch on and off with it's control or does it stay triggered as long as
   the A/C power is applied?


A triac will switch on when a gate signal is applied and if there's enough
forward current through the device, it will continue to conduct until voltage
across the main terminals drops to zero (ie. there won't be enough holding
current to maintain the conduction state.)

Martin, I'll assume you need to have a continuously variable capability when
driving the wires, so this implies using AC phase control rather than the
zero-crossing circuits which have binary capability only, if you're not going
to get too complicated.

Motorola has a pair of devices, TDA 1085/1185 which provide all the functions
to drive triacs for phase control. The input is a continuously variable
voltage, so you'll have to generate that somehow by the PIC, which, if I'm
correct, doesn't have a DAC. Maybe you could get PIC to generate a PWM signal
which could be averaged with appropriate signal conditioning, shouldn't be
hard, and apply this to the triac controller. Any fuzzy stuff in my message
should be cleared up with a quick look at the data and application sheets.


Anthony Lithgow
Dept. of Electrical & Computer Engg.
Ryerson Polytechnic University
Toronto, Ontario, Canada
email: alithgow@eccles.ee.ryerson.ca