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Epsilon!
Mad Scientist
Canada
276 Posts |
 Posted - Oct 04 2003 : 9:29:02 PM
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ok, i dont know if this would be better in the power supply section.. but since it has to do with telephone lines, here it is...
I got this calendar and radio thingy thats also a phone.. It runs on AAA batteries. You put 2 batteries for the radio, and 2 for the phone/calendar. Now, there is a backlight that lights when you pickup the phone thats powered by the phoneline, its a blue LED.
i wanna forget about the batteries and power the whole thing off the phoneline. How much current can i draw safely form the phoneline? And how would I do this, transformer, diodes, regulator?
note: there is DSL filters on the line.
Thank you for your help
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YS
Nobel Prize Winner
USA
1132 Posts |
Posted - Oct 13 2003 : 6:27:20 PM
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Powering things from the phone line is possible but somewhat tricky. There are basically three states of the phone line - all with different power abilities:
1. On-hook. Voltage is about 42VDC but you can not get any significant current from the line - if you draw few milliamps the central office equipment will think you picked up the phone and will switch to state 2.
2. Off-hook. Voltage is about 7VDC and current is 26 mA, more or less. In this state, you can get up to 10 mA, I should guess.
3. Ring. About 100Vpp AC, 20 Hz or so. You can get a decent couple of milliamps there, but it needs to be rectified.
Overall, it is tricky. You may think of using AAA NiMH cells charging them from the line in off-hook state, but it may not be enough for radio, which needs about 50 mA. Say, taking 5 mA from the line even with 100% efficiency you will get radio talk time about 0.1 of your phone talk time.
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Epsilon!
Mad Scientist
Canada
276 Posts |
Posted - Oct 14 2003 : 07:54:12 AM
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thanks, looks like ill just install an ac adapter..or ill just leave it on batteries it they last a reasonable amount of time.
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Powering stuff of the phoneline
