-=> JASEN BETTS wrote to MIKE ROSS <=-

 MR> Alternatively instead of a transformer with extra taps, a second smaller
 MR> transformer with a higher voltage could be used to drive the bases.
 MR> Comments welcomed! 

I snagged an article from Electronic Design News that used a
voltage bucking circuit to switch an SCR on at AC zero crossing.

Basically a small full wave rectifier with current limiting
resistor charges a capacitor bypassed by a low voltage zener.
An optoisolator remains off until such time as the voltage falls
below 3.1 volts at which point the cap dumps to the base of the
transistor turning it on lighting the LED in the optoisolator
which in turn switches on the SCR at mains voltage.

The reason for the 3.1 zener is to make up for the diode
and transistor drops between the charging source and the 30 uF
cap - zener combo.

Much simpler than it sounds and could be easily adapted to your use.
The optoisolator would not be needed in your case and could
be replaces by a bridge rectifier - SCR arangement
where the bridge passes current when the SCR
turns on.

With the voltages and currents your talking here
you could use a small transitor, an SCR, two small bridge rectifiers,
current limiting resistor, diode, zener and cap.
Perhaps less if one more clever or if I thought about it some more.
 
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