Clark wrote:
Looks like a shock hazard to me.



wrote in message
oups.com...

wrote:


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

Caps do have a failure mode, so a series safety R is normally included
with these type circuits.

And at that low a current, why not ferget the caps and just use a couple
of 150K resistors? The additional power loss will hardly spin your
meter off the wall and you'll avoid the possible cap failure problem.

Jeff (Ducking and slinking off...)

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"Truth exists; only falsehood has to be invented."


If your ac supply is polarised, you'd be a bit safer putting one cap in
the live than one in each pole, as the output will then be at apx earth
potential, though not isolated. As it stands its semi-live.


NT