>Hi all,
>
>        I have an interesting challenge.  I am using a 16C57 to drive a
>nitinol wire actuator with PWM.  The drive goes to multiple actuator and
>as it turns out I need a whopping current to drive them.  I am lifting
>something heavy.  Each takes 6 amps average.  Everything was going along
>happy (in denial) until I started to purchase the power supply to provide
>the current.
>
>        Now I am wondering if any of you have experience with sourcing
>PWM current from an un-regulated supply.  I am thinking that maybe I can
>simply step down the ac line, full wave rectify it, and synch my PIC PWM
>timing to the zero crossing.  I use IRF520 MOSFETs to switch the current.
>This would insure that my driving waveform would be uniform for each cycle.
>I do not believe that having a non-DC source current is any problem.  After
>all, I am only heating wire.
>
>        Also how about some clever (meaning cheap/simple) ideas for PIC
>sense zero crossing from the transformer secondary.
>
Use a full-wave rectifier to develop a bus having positive half-cycles of AC
(i.e. 120 Hz pulses).  The transformer and diodes have to be rated for the
sum of the current through all the wires that will be heated at the same
time.  Connect one end of all the wires to this bus, and the other end to
the anode of a SCR that controls that wire.  The cathodes of the SCRs go to
ground, and the gates are driven by the PIC through resistors (as if they
were NPN transistor bases).  About any SCR will work, for example the S2006L
(about $1.20 each in Digi-Key catalog) will switch 6A RMS and turn on with
15 mA of gate current.  If you're driving multiple wires from each channel,
70 Amp SCRs are available (about $7.00), but you'll need to add a driver
transistor (emitter follower from PIC pin, with collector on 5V bus) to
supply the 50 mA gate current required.

The SCRs will turn on when positive gate voltage is applied, and turn off
during the zero-crossings of the bus voltage.  If you want on/off control
with 1/120 second resolution, synchronizing is unnecessary.  Just leave the
gate drive on while the wire is to heat up, and turn it off when not.  If
proportional control is required, synchronize the PIC with a zero-crossing
voltage derived from the bus (by comparing when the bus voltage rises above
1V or so), and generate a delayed pulse for the gate.  The more delay of
course, the less power will be applied.  But the relation between phase
delay to power is not linear (think of integrating the sine squared
function), easily corrected with a table if required.

As far as conditioning the bus voltage for the PIC, it could be simply
applied to RTCC through a suitably large resistor to limit the current to
<500 uA when the protection diode clamps the pin voltage to 5V.  Loading the
bus with a resistor to ensure that it does return to 0V at each half-cycle
would be a good idea.  The turn-on point would be about 3V.  If earlier
detection is required, a more sophisticated voltage comparator, for example
driving the base of an NPN transistor and using the collector with a pull-up
to 5V, would switch on while the bus voltage is more than 1V or so.

-Mike