Hi Folks , Sorry to  drag this thread on but I have a question with
respect to Peters circuit.
http://www.vertonet.com/members/pvh/scr.pdf
In my application I am doing as Glen suggested. I have another diode on
the anode side of D6 going to the -ve rail.  I have found ,and it seems
to make sense, that my circuit will work fine when the SCR is on however
when it is off there is a problem. If one removes the SCR from Peter's
circuit there is no longer a current path for the DC portion of the
circuit if the input going to the load is -ve. I put a large (47K)
resistor in parallel with the SCR which seems to work but I don't know
if this is the best solution. I am  using a 5 Mohm resistor going to the
PIC to look for zero crossings so it is important that the PIC be
referenced to one side of the AC line. If anyone has any better
solutions than the 47K resistor across the SCR I'd be interested to hear
it.
Thanks
Jim