Molded case circuit breakers or the like include control circuitry which may be of the discrete nature or of the integrated circuit nature. Regardless, the reliability of the control circuit becomes questionable in high temperature environments such as may be found in a desert during the daytime or such as may be caused by local electromechanical problems such as poorly connected load conductors. Because of this, circuit breakers and the like are provided with overtemperature protection which causes the circuit breakers to automatically trip when the temperature of the circuit breaker exceeds a predetermined value which may vary according to local standards, designs, specifications, etc. A typical example of a temperature above which circuit breaker tripping is required for failsafe operation is 90.degree. C. In the past, thermistors have been utilized to cause over temperature tripping in the firing circuits for silicon-controlled rectifiers which are connected in series with circuit breaker trip coils or trip devices. Typically, the silicon-controlled rectifiers are gate-controlled to cause the trip coils or trip devices to be energized under certain Well-known circuit breaker conditions such as over-load, short circuit, undervoltage, etc. The thermistor circuit operates independently of the latter mentioned to cause the silicon controlled rectifier to be fired to cause the circuit breaker to trip because of the presence of ambient over-temperature conditions.
Circuit breaker control systems often include large scale integrated circuits and precision voltage references. A band gap reference is a typical precision voltage reference. The band gap reference circuit utilizes the base to emitter voltage of a transistor--which has a negative temperature coefficient connected in series with the voltage developed across a resistor which has a positive temperature coefficient--as a reference voltage. The voltage developed across the resistor is a function of a predetermined fixed current which is supplied to the resistor from circuitry internal to the band gap regulated reference. The difference in temperature coefficients between the base-to-emitter voltage of the transistor and the series-connected resistor provides an output voltage reference signal which is relatively independent of the temperature to which the control circuit is exposed. As the base-to-emitter voltage of the transistor decreases with an increase in temperature, the voltage across the current fed series connected resistor increases generally proportionally to provide an output reference voltage which remains relatively constant. It is to be noted that the series-connected resistor though heretofore utilized as a temperature compensating device for a voltage regulator has properties which make it useful as an over-temperature detector replacement for the thyristor circuit previously described. It would be advantageous if a way could be found to utilize the latter concepts to provide a reliable, efficient, inexpensive over-temperature protection circuit for a circuit breaker.
It is also well-known in the prior art to trip a circuit breaker as a function of the current squared in a conductor protected by the circuit breaker times time (I.sup.2 t). This is represented by the well-known I.sup.2 t control circuit characteristic for a circuit breaker. The I.sup.2 t characteristic is utilized because that characteristic is most representative of the amount of heat which is built up in the conductors which are protected by the circuit breaker. Said in another way, the heat in a conductor protected by a circuit breaker is proportional to the square of the current flowing through that conductor. The larger the build-up of heat, the higher the temperature of the conductor. The higher the temperature of the conductor, the more likely it is to cause insulation failure, personnel burns, ignition of closely-located flammable materials and the like. To provide the I.sup.2 t protection in a circuit breaker, typically a circuit is provided within the circuit breaker control system which charges a capacitor with a relatively small charging current which is proportional to the square of the current flowing in the conductor to be protected. The resultant voltage developed across the capacitor is thus a proportional indication of the heat which is being built up in the conductor which is being protected by the circuit breaker. Voltage sensitive tripping circuitry then utilizes the voltage across the capacitor to cause the aforementioned silicon-controlled rectifier, for example, to be fired to actuate a tripping mechanism. It is undesirous, however, to cause a circuit breaker to trip when the need ceases to exist. Such a ceases. For example, when overload current in a conductor which is to be protected drops to a lower value, the conductor begins to dissipate heat so that the danger associated with previous heat build-up lessens. The electronic circuitry which models the build-up of heat through the charging of a capacitor must also represent the heat dissipation, otherwise the circuit breaker may be unnecessarily tripped at a later time when the overload current increases again. In order to accomplish this, typically a memory resistor is utilized in the prior art which slowly discharges the previously-described capacitor with a time constant which is deliberately chosen to approximate the time constant associated with the dissipation of heat in the protected electrical conductor. Typically, this memory resistor in the prior art is connected in parallel with the capacitor in question. When the capacitor charging current drops to a low value, the capacitor may begin to discharge through the resistor with a time constant which approximates the heat dissipation characteristics of the conductor to be protected. This arrangement, however, has a disadvantage during the charging cycle or portion of the operation when the capacitor is being charged by the previously-described electrical current which is related to the square of the line current. The reason for this lies in the fact that the memory resistor provides a parallel path for the aforementioned charging current and consequently, some of the current which should be provided to the capacitor for producing a voltage which is representative of the heat in the conductor to be protected in fact flows through the resistor thus introducing an error into the control system as a result thereof. This is especially critical at low values of overload currents where the error becomes significant. It would be advantageous, therefore, to provide circuitry which would allow the use of the memory function as previously discussed without presenting the problem associated with the parallel path for the capacitor charging current.
Circuit breaker apparatus in which it is desirous to use the teachings of the present invention are found described in the following patents assigned to the assignee of the present application: U.S. Pat. No. 4,554,421 issued Nov. 19, 1985 to K. A. Grunert et al. and entitled "Molded Case Circuit Breaker With Handle Lock", U.S. Pat. No. 4,639,701 issued Jan. 27, 1987 to A. B. Shimp entitled "Circuit Breaker With Interface Flux Shunt Trip" and U.S. Pat. No. 4,540,961 issued Sept. 10, 1985 to Maier entitled "Molded Case Circuit Breaker With An Apertured Molded Cross Bar for Supporting a Movable Electrical Contact Arm".
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