I have built several 200W AC dimmers using Microchip 8-bit CPU and the illustrated back-to-back MOSFET circuit.
I have seen on Internet schematics with the D1 diode in parallel with the R1 resistor, or without.
Assuming it should be a regular diode (tried 1N4148), I connected a diode and compared it on oscilloscope, but did not see any difference.
Can anyone tell me, why to use a diode at that location?
Update: with 200W load of 4 pieces of 50W bulbs, without the diode we often destroy the MOSFET, pretty much immediately after first power-on (transistor becomes short). With the diode, it seems to work even after many hundred power-ons.