powering CalerID from a phone line.

Dgswaner

This is way off topic but it is the sand box! I pay a small fortune in batteries each year for 4 phones with caller ID. why can't the tiny LCD screens be powered from the phone line and not batteries?

the only think I can think of is power consumption. but would it really be that big of a deal?

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DGSwaner

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"When in doubt, use C4" - Jamie Hyneman, Myth Buster

Mike Green

The telephone company thinks it's a big deal. They have to provide enough backup power for the phones (and the central office equipment) to function for hours in the case of a power failure. If every telephone consumed a few milliAmps when idle, it would add up to a lot of power. Theoretically, you could rectify and store the ringer power, then run the caller ID unit off the stored energy for a few minutes. The data could be stored in an EEPROM for when there's no power. All this extra circuitry would add to the cost of the unit when batteries and/or wall warts are relatively cheap.

Update: I found an EDN article talking about a telephone line monitor and indicating that, for FCC approval, there has to be at least a 5M Ohm leakage resistance across the on-hook phone line. At the normal 48V on-hook voltage, that's less than 10uA.

Post Edited (Mike Green) : 11/17/2008 9:37:57 PM GMT

Paul Baker

Also most phone companies will fine you for drawing more than a very small amount of current when the circuit is not active, they take thier emergency communications role _very_ seriously.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Dgswaner

I thought that might be the case, it just seems that I'm always replacing the batteries in my corded phones, which only power the Caller ID. I just might do this tho.. I use Vonage so I'm the one who is actually powering the phones, although I'm not sure my router can handle the extra mA. My phone use 4 AA batteries and at 4-5 a year the extra circuity would pay for it's self eventually. or the easiest solution. just get cordless phones.

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DGSwaner

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"When in doubt, use C4" - Jamie Hyneman, Myth Buster

Paul Baker

You can always hack the phone to accept a wall wart instead of powering it from batteries, especially since it's a corded unit to begin with. If you are especially industrious, you can make a pack that will fit in the battery compartment. I did this once for a 9V battery, I disassembled one, made a little regulator circuit on perf board with a DC jack on one side, soldered the regulator's output to the battery's terminals, reassembled the psuedo battery using thick paper to line the can with·insulator and plugged a wall wart in to the underside of the battery where the cardboard insert once was. Then finally I filed a corner of the compartment cover for the wire to come out of the device.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Post Edited (Paul Baker (Parallax)) : 11/17/2008 9:55:40 PM GMT

kwinn

If your phone wiring in your house is 2 or 3 pair and they are not all in use you could use a spare pair to provide power instead of the batteries. You would of course need to run
four wires to the phone and modify it a bit. This is much neater than wall warts everywhere. If you do this make sure to use a current limiting resistor in case of a short.

john_s

Allow me to disagree with wall wart or batteries supporters a bit as the whole stand alone caller ID design to start with should be designed in the 1st place to be able to "harvest" energy from just about any source these days - RF, sound, solar, RING voltage - you name it and operate under 10uA easily. But that is really off-topic

Some time ago MAXIM had an app-note on that subject.