Okay, since I'm sure none of you care
here's the details of power and phones:
On hook -- -48 VDC. Yes, negative. It makes sure the lines don't get electroplated (honestly). Approx. 50 mA of current can be drawn at this voltage before the phone senses an off-hook condition (may be less in your area, however, phones are supposed to display a 600 Ohm load to the line when off hook).
Off hook -- 9 VDC (max). Approx. 40 mA of current can be drawn at this voltage. Much more than that will likely show as a short. You can (sort of) measure your distance to your exchange by measuring this.
Ringing -- 90 VAC. I wouldn't bother trying to tap this...
The ringing voltage (and, to some degree, the on hook voltage) are why you should NEVER work on a live phone line unless a phone is off the hook.
There's more here, if you care to read.
BTW: Tapping this power could get your phone disconnected. It might even be illegal, since phones are _supposed_ to be powered by battery banks in the CO (helps ensure they work during power outs) and if you are draining them for no reason during an emergency, you're going to look like an idiot. And if you're in the US, like that store (I think) it's definately illegal as per the FCC.
BTW: The current available is more than enough to make a decent high-intensity LED flashlight with. Could come in handy during power outages (but read the above before you do something stupid like use it). Parts required: LED ($0.25) + 2.2k resistor ($0.05). Wire in series. Reverse if it doesn't light. Enjoy. Don't zap yourself.
Oh, and many of those devices (especially the car charger) will draw so much power they'll be sending a telco van to your house to see what the short is in NO time. Your crappy phones (not quality ones) will start to make chirping noises at night when the CO automatically runs tests on them, and other fun things if you start to do this stuff.
Just to let you know...
posted by shepd at 5:30 PM on December 5, 2002
yhbc, as much as i'd like to take credit for that stylish purchase, the phone belongs to my parents. i have no idea how i ended up with it all these years later.
and shepd - in particular, i would advise against working at the external interface box standing in damp grass with bare feet and the stripped leads gripped between your teeth. an incoming call WILL result in an embarrassing display for the neighbors. i found out the hard way.
posted by quonsar at 5:40 AM on December 6, 2002
http://www.metafilter.com/22064/Free-Electricity-from-the-phone-company
