BACKGROUND OF THE INVENTION

This invention relates generally to solid-state circuit interrupters havingmicroprocessor-based tripping systems and more particularly, it relates to an improved trip energy monitor for microprocessor-based tripping systems used in circuit interrupters for determining if there is sufficient energy to activate a trip solenoid coil.

As is generally well-known in the art, circuit breakers have been widely used in commercial and in¬ dustrial applications for protecting electrical conductors and apparatus connected thereto from damage due to excessive current flow. Circuit breakers typically include trip systems which were designed to interrupt when the current flowing through them exceeded a predetermined level. Specifically, most simple trip systems utilized an electromagnet to trip the circuit in response to current or voltage fluctuations. The elec¬ tromagnet provided a magnetic field in response to current flowing through the circuit breaker. When the current level increased beyond the predetermined level or trip point, the magnetic field "trips" a mechanism which

causes a set of circuit breaker contacts to release, thereby "opening" or "breaking" the circuit path.

Gradually, however, there has arisen a need in the industry for more sophisticated and elaborate tripping systems as the complexity of electrical distribution systems increased. For example, in many commercial and industrial equipment today it is desired to have circuit breakers that perform both an instantaneous and delayed tripping (i.e., time-current interrupting characteris- tics) so as to provide improved accuracy and flexibility on the equipment to be controlled. For this reason, many microprocessor-based solid-state circuit interrupters have been also developed in the prior art in an attempt to provide more accurate and reliable control operations on the electrical distribution system on which the circuit interrupter was being employed. To this end, a microcomputer is provided which is coupled between the current path and a trip solenoid controlling the mechan¬ ism for breaking the current path. The microcomputer stores trip points which activate the trip solenoid when the current within the current path exceeds the trip points.

Nevertheless, these microprocessor-based tripping systems are not without disadvantages since they inter¬ rupt the current path in response to power faults using techniques that are inaccurate or unreliable under certain conditions. For example, it is generally only after a power fault is detected in the current path that the microprocessor-based tripping system attempts to engage the trip solenoid to break the circuit breaker current path. However, after a power fault has occurred, the system power is sometimes insufficient to successfully engage the trip solenoid. While this at¬ tempted energization of the trip coil may not only fail, it will further pull down the power supply voltage used by the microprocessor thereby resulting in erroneous control logic signals.

Therefore, in order to insure reliability of the tripping system, there is generally required some type of monitoring circuit for determining whether or not the power supply is capable of supplying the trip solenoid coil with a sufficient amount of power to effect inter¬ ruption of the current thereby avoiding a failure by attempting prematurely to engage the solenoid coil and further causing a microprocessor malfunction. A prior art monitoring circuit is shown in Figure 1 which in-

cludes an operational amplifier and a number of asso¬ ciated circuit components interconnected thereto so as to perform a comparator function. While this prior art monitoring circuit performed its function satisfactorily, it suffers from the disadvantages of requiring a relatively high number of electrical components for its implementation, being high in cost, and having a low re¬ liability.

Accordingly, the present invention is directed to an improved trip energy monitor which is formed of a minimum number of circuit components and has increased reliability and reduced cost. Accordingly, the present invention represents an improvement over the prior art monitoring circuit of Figure 1. Specifically, the trip energy monitor of the instant invention includes a Zener diode, a current-limiting resistor, and an inverting buffer.

SUMMARY OF THE INVENTION

Accordingly, it is as general object of the present invention to provide an improved trip energy monitor for microprocessor-based trip units which is relatively

simple and economical to manufacture and assemble, but yet overcomes the disadvantages of the prior art monitor¬ ing circuits.

It is an object of the present invention to provide an improved trip energy monitor for microprocessor-based trip units which is formed of a minimum number of components and which has a high reliability in its opera¬ tion.

It is another object of the present invention to provide an improved trip energy monitor for micro¬ processor-based trip units which is formed of components with relatively low cost.

It is still another object of the present invention to provide an improved trip energy monitor for micro- processor-based trip units which includes a Zener diode, a current-limiting resistor, and an inverting buffer.

In accordance with these aims and objectives, the present invention is concerned with the provision of an

improved trip energy monitor for microprocessor-based trip units which includes a Zener diode, a current- limiting resistor, and an inverting buffer. The Zener diode has its cathode connected to receive a power signal. The resistor has its one end connected to the anode of the Zener diode and its other end connected to a ground potential. The inverting buffer has its input connected to the junction of the resistor and the anode of the Zener diode. The inverting buffer has an output for providing a reference logic signal. The reference logic signal is read by a microprocessor to determine whether or not a trip signal is to be generated for energizing a solenoid coil to interrupt a current path in a circuit interrupter.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention will become more fully apparent from the following detailed description when read in con¬ junction with the accompanying drawings with like reference numerals indicating corresponding parts throughout, wherein:

Figure 1 is a schematic circuit diagram of a prior art monitoring circuit; and

Figure 2 is a schematic circuit diagram of a trip energy monitor, constructed in accordance with the prin- ciples of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, there is shown in Figure 2 a schematic circuit diagram of an improved trip energy monitoring circuit 10 for use in association with microprocessor-based electronic trip units employed in solid-state circuit interrupters. As is well-known, the trip units (not shown) include a trip solenoid coil for breaking the current path in the circuit interrupter in response to a trip signal generated by a microprocessor.

The trip energy monitoring circuit 10 serves to monitor the power supply voltage connected to the solenoid coil and generates a reference logic signal to the microprocessor to indicate the availability of sufficient energy to fire the trip solenoid. In other words, prior to attempting to energize the trip solenoid

coil, the microprocessor will check the logic level of the reference signal to determine whether or not the power supply voltage is at that time capable of supplying the solenoid coil with a sufficient amount of power to effect interruption of the current path thereby elimi¬ nating a failed attempt to energize the solenoid coil and/or causing a microprocessor malfunction. If the power level is sufficient to energize the trip solenoid coil, the microprocessor will generate the trip signal so as to interrupt or break the current path.

The trip energy monitoring circuit 10 is comprised of a Zener diode CRl, a current-limiting resistor Rl, and a Schottky inverting buffer INVl. The Zener diode CRl has its cathode connected to an input terminal 12 for receiving a power signal V _τ . The power signal V _τ is generated from a power supply (not shown) and is used to operate the trip solenoid coil. The anode of the diode CRl is connected to one end of the resistor Rl and to the input of the inverting buffer INVl at an internal node 14. The other end of the resistor Rl is connected to a ground potential GND. The output of the inverting buffer INVl is connected to an output terminal 16 for generating a reference logic signal V _ref . This reference signal is

monitored by the microprocessor to determine when the trip signal is to be generated.

The Zener diode CRl has preferably a breakdown voltage of +9.1 volts, such as Part No. 1N5239B. The resistor has preferably a resistance value of 10k ohms. The inverting buffer is preferably comprised of a hex Schmitt-trigger inverter of the type similar to MC54/74HC14A, which is manufactured and sold by Motorola, Inc., Schaumburg, Illinois.

Before the microprocessor generates the trip signal in an attempt to energize the trip solenoid coil, the reference signal V _ref is checked to determine if it is read as a high logic or a low logic. If the reference signal V„ _f is read as a low logic, the microprocessor determines that there is sufficient power to activate the solenoid coil and attempts to do so. If the reference voltage V _ref is read as a high logic, the microprocessor determines that there is insufficient power to activate the solenoid coil and waits, but checking periodically the logic level of the reference voltage V _ref . Once the reference signal V,^ is switched to the low logic, the microprocessor will attempt to activate the solenoid coil once again.

Thus, the Zener diode CRl is non-conductive until the power signal V _τ reaches a sufficient voltage, which maintains the output of the inverting buffer INVl at a high logic level. As the power signal V _τ reaches the sufficient voltage (i.e., approximately +11.4 volts), the diode will be rendered conductive and the input of the inverting buffer will be at a high logic. Accordingly, the output thereof will be switched to the low logic. This indicates to the microprocessor that sufficient energy is available to trip the solenoid coil.

As a result, the trip signal will be generated by the microprocessor which is a cyclical waveform providing increased trip energy due to better synchronization with the maximum available energy. The Schmitt-trigger in- verter has a hysteresis voltage so as to produce a high immunity against noise. Further, this hysteresis voltage insures that the trip signal from the microprocessor will be of a repeatable length to the trip solenoid coil.

By comparing the trip energy monitoring circuit of Figure 2 with the prior art monitoring circuit of

Figure 1, it can be seen that the instant invention is

formed with a substantially reduced number of electronic components. Accordingly, there is achieved increased reliability and reduction in manufacturing and assembly costs.

From the foregoing detailed description, it can thus be seen that the present invention provides an improved trip energy monitor for microprocessor-based trip units which includes a Zener diode, a current-limiting resistor, and an inverting buffer. The trip energy monitor provides a reference logic signal which is read by a miccroprocessor to determine whether or not a trip signal is to be generated for energizing a solenoid coil to interrupt a current path in a circuit interrupter. The trip energy monitor of the present invention provides for more reliable operation and performance at reduced cost than those traditionally available.

While there has been illustrated and described what is at present considered to be a preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of

the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the central scope thereof. Therefore, it is intended that this invention not be limited to the particular embodi¬ ment disclosed as the best mode contemplated for carrying out the invention, but that the invention will include all embodiments falling within the scope of the appended claims.