> The voltage I'm looking at is the voltage gated by the triac going into a
> transformer.  The voltage data is used for PID control of that same
measured
> voltage (something else I have to figure out yet).
> ...
> The load on the transformer (off of the secondary), is extremely
resistive,
> almost a dead short (A short nichrome heating element, attached to a
"pen",
> which is attached to a 4 foot long 16 or 18 gauge cord, that is connected
to the
> transformer).  Nothing is rectified or filtered after the transformer
(voltage
> goes from about 0.4vac to 2.0vac max)

I think you need to start from scratch with the original problem statement.
You are chasing design problems do to an inappropriate architecture.

The problem as I understand it is this:  Provide user control over the power
to a heating element.  The power must be regulated to compensate for power
source fluctuations, changes in the heating element, variations between
interchangeable elements, and some amount of unpredictable additional
influences (oxidized connectors, different guage wires, etc).  The power
source is the AC line.  The load must be isolated from the hot side of the
AC line for safety.  (See how you can state your problem in one little
paragraph?)

A triac on the input to a transformer is only one way of achieving this.
Note in this case that if the AC line voltage is reasonably sinusoidal, you
can measure its voltage easily.  If you know the triac firing times
accurately, you can compute a reasonably good approximation of the RMS
voltage delivered at the output of the triac.

However, I think you ultimately need to measure the power into your load.  I
wouldn't be too interested in the RMS voltage going into the transformer.

A totally different approach would be to run the AC line directly into a
transformer to get an isolated low voltage high current out.  You could then
use switching elements to chop this into your load.  Your entire circuit
would now be on the same side of the transformer, making direct measurements
of voltages and currents easier.  There are a number of choices on how to
chop the voltage into the load.  You could rectify the secondary voltage
first and use a single switching element driven from the PIC PWM output, or
you could use a triac on the AC.  In either case, I think you will need to
measure or compute both the load voltage and current to know the power.
Your load looks like a resistor over the short term, but that resistance
will change quite a bit with temperature.

I don't know if you have any control over the plug in tips.  It would be
useful if these had 4 wires instead of 2.  2 of the wires would carry the
power to the load as usual.  The other two wires could be much thinner and
would connect accross the load close to the load.  These last 2 wires would
be used for sensing the true voltage accross the load, thereby eliminating
effects from bad socket connections or losses in the power leads.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body