From: "David VanHorn" <
microbrix@...>
> You're allowed a few microamps or so. You can also draw power when
> you're off-hook, or when the line is ringing, to charge up a supercap,
> if you can do it without upsetting the impedances. Constant current
> sources are your friend.
To make clear what David said:
When the line is on-hook you can draw _very_little current (otherwise the
Telco thinks you have gone off hook). This is a matter of microamps. The
good news is the on-hook (open loop) voltage is pretty high, somewhere
around 48V.
When the line is off-hook you can draw quite a bit more current (but of
course the voltage will be quite a bit lower).
You can also extract power from the ringing voltage (about 20Hz AC, maybe
100V or so). The source impedance is pretty high and the Telco only allows a
certain load on the line (measured in "Ringer Equivalences").
---- Bob Ammerman
RAm Systems
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
