An idle telephone line tempts designers to use its 48 V potential as a power source. However, Part 68 of the US Federal Communications Commission's telecommunications regulations states that any device that connects to the phone line and is not actively communicating must present a resistance of at least 5 MΩ (Reference 1). To meet this requirement, a device's continuous-current drain must not exceed 10 µA. Fortunately, many devices that connect to the phone line do not require continuous power and can remain off for long intervals, awakening only for a short time before relapsing into power-off mode. Providing power for these applications from the phone line presents obvious advantages by eliminating the need for a battery or another power source and the cost of battery maintenance.
The circuit in Figure 1 charges a 1.5 F supercapacitor, C1, from the phone line through a diode bridge and a 5.6-MΩ resistor. A Maxim MAX917 nanopower comparator, IC1, consumes only 0.75 µA from its power supply. Resistors R2 and R3 halve the voltage across C1 and apply it to IC1's positive input voltage at Pin 3 for comparison with its built-in 1.245 V reference. For voltages across C1 that do not exceed 2.49 V, IC1's output at Pin 6 remains low. When C1's voltage reaches 2.5 V, Pin 3's voltage exceeds the reference voltage, and IC1's output goes high, turning on Q1 and Q2.
|Figure 1.||This power-conversion circuit delivers intermittent bursts of regulated
voltage from a supercapacitor charged by a trickle of current
from a telephone line.
Several days must elapse before C1 becomes fully charged, given its huge capacitance and a charging current of less than 10 µA. The voltage on C1 can never exceed 2.5 V because, once it reaches 2.49 V, Q1 and Q2 turn on, connecting C1 to a switched-mode-power-supply circuit. Because the power-supply current exceeds the charging current, the voltage across C1 starts to decrease when Q2 turns on. Transistor Q3 holds Q2 on when C1's decreasing voltage causes Q1 to turn off.
The switched-mode-power-supply circuit comprises a Linear Technology LTC3459 micropower boost converter, IC2, and its associated components, which deliver 5 V at 10 mA. A fully charged C1 can supply power to a 10-mA load for approximately 40 sec. With no load, the circuit can sustain its 5 V output for more than 10 hours. For greater output current and shorter operating time, select another boost converter that can operate at a low input voltage.
Mechanical switches, open-drain MOSFETs, open-collector transistors, or a microcontroller's open-drain output pins can drive two external control inputs to force the circuit on and off. Pulling the On input low forces Q2 to turn on and deliver power from C1 to the power converter, and pulling the Off input low turns off Q2 and removes power from the converter. Note that the power converter's output-return line connects to the telephone line and thus should not connect to an earth ground or to grounded equipment.
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