I finally got tired of replacing the 2 C-cells in a clock with a moving pendulum, so I measured the current (3 volts at 340 microamps) and built a charge pump like this, viewed in a fixed font:
0.047 uF @400 V || |-----------||--------------------->|------------>
|| | | | | | 3V
120 VAC --- --- to clock ^ _ 0.047 uF @400 V | | || | |-----------||----------------------------------->
||
I used two caps in case one shorts and for some ground isolation. The diodes are 5.1 V zeners ($1.29 for 2 at Radio Shack) to limit the clock voltage if someone unplugs the batteries. This circuit supplies 390 microamps. I hope to avoid replacing the batteries until their shelf life runs out in 2012 :-)
This could be useful for lots of low-power stuff, eg smoke detectors.
Nick