wrote in message ...
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

It seems counter-productive to build booby-traps into smoke detectors
and the like.

Unlike a wall-wart, it will require special precautions not to accidentally
touch anything when you go to change the battery or maybe even set the time
on a clock. You will never know whether a capacitor has shorted and whether
you might be killed the next time you provide a path to earth ground.

There's nothing to limit the current when capacitors fail, so that failure
will cause a fire. There's no transient voltage protection, so the odds
of capacitor over-voltage failure is pretty high.

Don