Importance

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Abstract

A voltage comparator compares the DC current in active elements of a trunk amplifier with a variable reference voltage for detecting when to switch to a bypass mode. Its output is normally low for passing a current through it, an LED, and relay coils when the amplifier operates satisfactorily. Circuitry for establishing an allowable amount of decrease in DC current in a current sensing resistor R5, prior to switching the comparator for de-energizing the relay coils, comprises a first transistor Q4 that continuously conducts collector current through a load resistor, a second transistor Q2 that is only responsive to a low output voltage of the comparator for passing a prescribed current through the load resistor for decreasing the first transistor Q4 collector voltage a prescribed amount, and circuitry including voltage dividers for coupling a greater percentage of the change in Q4 collector voltage to the positive input of the comparator than to the negative input thereof which also senses the level of the DC current.

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First Claim