Electric circuit for operating trip circuit of a circuit breaker

摘要

A removable rating plug for a circuit breaker provides art active variable impedance in series with a reference resistor between the sensing terminals of the circuit breaker trip circuit to adjust the sensing voltage produced in response to the input current received front current monitors in the protected circuit. The active variable impedance, which is energized by the input current, includes a high gain transistor amplifier, preferably a Darlington transistor, controlled by an operational amplifier. A control circuit includes a pair of reference diodes which match the forward drop across the Darlington transistor when full on, in series with a reference voltage selection circuit which applies a reference voltage to the non? inverting input of the operational amplifier. The voltage at the connection between the Darlington transistor and the reference resistor is fed back to the inverting input of the operational amplifier so that the gain of the amplifier, and therefore the variable impedance in series with the reference resistor, varies with the reference voltage.