Tim:

          D1..D4 = 50V 3A

L o-------*->|-->|-*----o Lamp phase
           |        |
           |        |
           *-|<--|<-*
           |        |
           *---R1---*  R1 =  50 ohms 0.25W
           |        |
           R2       |  R2 =  50 ohms 0.25W
           |        |
           +--->|---+
              OC LED   Optocoupler LED

The diodes can be a part of an inexpensive rectifier bridge. It will 
dissipate at most 5W (at 3A load).

The higher the load, the more interesting it is to use a thermistor type 
scheme on the switching element, and sense its temperature.

Peter
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