Jim asked:

>or knows
>how to drive a full wave AC. load with one SCR from a PIC using a
>nonisolated supply , even better.


  Can be done. I'm ASCII-art-impaired or I'd try to draw it out
  for you. Put your load in series with a full-wave bridge. SCR
  connected from (+) side of bridge to (-) side, so that it
  shorts the bridge when ON and permits AC current to flow
  through the load. Now make a power supply for your logic by
  connecting a separate rectifier and filter cap from either
  side of the line to SCR cathode.

  Disadvantages:

  1. As you mentioned, common return is NOT ISOLATED from
     the power line.
  2. Requires extra part (bridge rectifier)
  3. Logic supply is only half-wave, requires heavier filtering.
  4. Extra two diode drops in series with the load.

  Probably more cost-effective with larger loads, where the
  cost of the SCR tends to dominate.

  The classical way to drive two SCRs in inverse parallel is
  with a pulse transformer that has two isolated secondaries,
  but this requires a big slug of current to generate the
  pulse, more than an unassisted PIC can deliver. The pulse
  transformer itself is probably pretty cheap, though.

  Hope this is helpful. Email me privately if you need me to
  fax you a sketch.

  Reg Neale