while it isn't a lot of power in the big picture, there is a passive version, just get the right resistor and capacitor and put them in series with the coil.
discussion here:
http://www.diyelectriccar.com/forums/showthread.php/evdl-coil-economizer-13711.html

discussion here:
http://www.diyelectriccar.com/forums/showthread.php/evdl-coil-economizer-13711.html