I have an idea for a gizmo that would be attached to a telephone line in a place where mains power might be difficult to get. The device would only need to operate intermittently - say 3 minutes per hour. Could it be designed to use the telephone line to charge a battery? If so, how much power can be drawn from the line?
We discuss this in detail in AoE pages 933 to 937.* You get very little for an "on hook" phone, just a leakage current really, under 1 uA at -48 volts. If you act like 5 phones, REN = 5, you can draw 5uA. You get more in the "off-hook" condition, of course, up to 150mW of guaranteed power, 26mA at 6 volts, see figure 14.7 page 936. Can you take the phone off hook now and then, to charge your battery?
Hmmm... And they don't charge any fees (other than the monthly baseline) until you actually complete a call. I wonder: how good a modern life one could one live on 150 mW average?
In off-hook state, you can drain the current of 20mA @ 5V across the load. You can also charge a battery in the hooked-up state. However I would not advise to load the telco line to anything lower then 100k in this case. Otherwise the line detector will sense an excessive leak.
Vladimir Vassilevsky DSP and Mixed Signal Design Consultant
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