Specifications

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5.1.2 Impedance Detector
RF transformers T1 and T3 drive the impedance bridge that is
balanced at 50 ohms. T3 samples the line current and thus D7 out-
puts a negative DC level proportional to line current. A tertiary
winding on transformer T1 provides a line voltage sample to D2
that provides a positive voltage proportional to line voltage. R18
and R11 act as a summing network for the current and voltage
signals, with ratios chosen, such that at 50 ohms, the summed
signals result in a balanced or zero voltage condition. If the line
impedance goes too high, the signal from the voltage sensor will be
relatively higher than the current sensor, which will result in a net
positive output voltage from the summing network. Similarly, a
low line impedance will result in more output from the current
sensor, resulting in a net negative output voltage from the
summing network. The summing network output is shifted to a 0 to
5v range, then fed to the processor’s A to D converter port, and
used within the micro-controller.