> >> Does anyone have any idea how much power you could tap from
> >> the line without causing it to go off hook?
> >
> >The phone companies have some definite ideas about this :-)
>
> 3ma is allowed.
> in the real world, your on-hook voltage can be as low as 12v, though this
would be a rare case.


I'm not saying that that isn't true.
I've been out of the formal telcom field for a decade or so now.

 BUT that is truly astounding if it is true.
3 mA is effectively 16 k. That is such a low value that line testing
equipment would have no prospect whatsoever of testing the line for
incipient fault conditions.

A 12v on hook voltage with a 1k line (nominal limiting loop std line) and a
300 ohm feed bridge (say) and a 1k loop gives about 38/1300 = 29 mA. R
effective for the looping device with 12v across it is 12/.039 =~ 310 ohms.
This is far lower than what I recall (a decade) on as a device that would
"pull dial tone" in worst case conditions. A shorter loop gives more current
still.

So I'm surprised at these figures and the generosity of the Telcos concerned
:-)
FWIW 29mA at 12v is worth about $US0.20 in mains electricity costs per year
at NZ rates ;-) (or about $US0.80 at the exchange supply end at 50V ).


        RM

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body