Saturday, March 23, 2013
Telephone Line Indicator
For a 100µA instrument, this resistor value will have to be increased to about 680 kΩ. The starting point, when selecting a resistor value is that when the telephone is not in use, the meter should deflect about 2/3rd of full scale. The amount of meter deflection indicates the three different states of the telephone line: 1. The deflection is very small: the line is in use (voltage 5 to 12 V). 2. The deflection is 2/3rd of full scale: the line is not in use (voltage typically 48 V). 3. Full-scale deflection: ring signal (60 to 90 V AC). Because the idle voltage and certainly the ring voltage are high enough to be dangerous, it is recommended that the circuit is constructed in such a way that it presents no hazard when touched.
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