Ring-trip detection using ring voltage and current.

摘要

Apparatus and method for detecting a telephone (4) going off-hook during ringing measures the ringing current when the ringing voltage and current have a negative polarity. When the telephone is off-hook, the magnitude of the current will exceed a predetermined amount when the ringing current is negative. This technique is faster and more reliable than prior art ringing current duty cycle measuring techniques. An exemplary embodiment of the detection apparatus involves a resistor (17) in series with the ringing supply (3) to measure the ringing current, and a transistor (19) gated when the ringing voltage is negative.