Abstract: An overcurrent detection and protection device for stepping motors has means for detecting an overcurrent condition in the power supply to a stepping motor. Means responsive to the detecting means rapidly provides a signal to the stepping motor drive system causing the current to terminate and to remain terminated for a predetermined interval of time, at the end of which interval the device resets itself causing the current to resume. The device is sufficiently fast-acting to protect electronic components such as transistors.

Abstract: An overcurrent protection apparatus for a DC motor which can be used with a speed control system of a DC motor to prevent an overcurrent condition arising when the DC motor is run with a heavy load or locked by an overload from burning or overheating windings of the motor. The apparatus is constructed using at least one integrated circuit and includes a current mirror circuit for detecting an overcurrent and for cutting off the supply current of the DC motor. A timing circuit prevents overcurrents lasting only a short time from triggering the apparatus to shut down the motor. Once the apparatus is triggered to shut off the motor, the motor will remain off for an indefinite period of time until the apparatus is reset.

Abstract: Methods and apparatus for controlling an electric motor effect operation of such motor by electric energization thereof, and derive from a motor winding or from the back electromotive force of the motor an electric signal indicative of motor speed. The motor is thereupon mechanically stalled, and such stalling is detected from a comparison of the derived speed signal and the electric energization. Deenergization of the motor is effected in response to the stalling detected from the latter comparison. Limit switching of electric motors and actuators without limit switches may be provided in this manner.

Abstract: A current detector comprising a current-pulse duty conversion element including a core of an amorphous, magnetically soft material having a first and a second coil disposed thereon. A current which is to be determined is passed through the first coil while the current flowing through the second coil is detected. A pulse of a given frequency is applied to one end of the second coil while the other end of the second coil is connected with an element which converts the current flowing through the second coil into a corresponding voltage. A voltage developed across the element is integrated and then converted into an analog voltage. The duty ratio of the voltage pulse appearing across the element which is connected to the other end of the second coil changes in accordance with the value of current passing through the first coil, whereby the analog voltage shifts.

Abstract: A method of short-circuit protection of an electrical three-phase machine, realized in a short-circuit protection device for a three-phase electrical machine, provides that phase currents are formed in phases of the stator winding of a three-phase electrical machine and, concurrently, a magnetic flux is produced by the end loops of coil groups of each phase in the stator winding, and a reverse current is separated from phase currents by a reverse current discriminating unit. Afterwards, current measuring and comparing units simultaneously measure the reverse current and the total electromotive force, produced by the magnetic flux in conditions excluding short-circuits in the stator winding and added up by a means for summing and converting magnetic fluxes into the electromotive force. In case the measured magnitude of the current and the value of the electromotive force exceed the maximum, three-phase electrical machine disconnection signal shapers produced respective signals which cut out the machine.

Abstract: In a motor control system of the present invention, a series circuit comprising a current sensing resistor, a DC motor and a power transistor, is connected between the output terminals of a DC power source; and a diode and a capacitor are connected to the series circuit. The diode is connected in parallel to the current sensing resistor and the DC motor, so that energy stored in the motor, corresponding to a drive current of the motor, is discharged through the current sensing resistor. The capacitor is connected in parallel to the DC motor and power transistor, so as to increase the power supplied to the motor when the load of the motor is momentarily increased. The current limiting circuit of the invention, enables the power supplied to the motor to be continued to be supplied when a momentary overload occurs, and to cause a limitation of the power supplied to the motor when the overload is for a longer period of time.

Abstract: The level and duration of RF current flowing in the neutral grounding circuit of a generator is monitored to determine internal arcing conditions.

Abstract: A short-circuit protection device for a DC control element has at least one transistor which is addressed by timing pulses obtained by comparison of a triangular voltage with a control voltage. Each such transistor is provided with a separate overcurrent protection device which, after a short circuit, blocks the associated transistor until the beginning of a new switching-on pulse. The control voltage assumes values which are above the maximum value of the triangular voltage.In the event of a short circuit, the transistor concerned is switched off very fast, but is switched on again by the next succeeding switching-on pulse and a determination is made as to whether the short circuit is still present.

Abstract: The current in each phase of a multiple phase chopper is sensed and compared to a reference overcurrent value. Any chopper phase reaching overcurrent and any chopper phase on for a longer period of time are turned off immediately. The conduction periods of chopper phases that have been on for a shorter period of time than the overcurrent phase are similarly shortened to maintain balanced phase conduction and minimize the ripple content of the source current. The phase relationship among the various chopper phases is adjusted in response to an overcurrent turn-off to allow the commutation network associated with such phase to be adequately charged before the generation of a current turn-on for another phase.

Abstract: A d.c. motor, particularly a paper-feed drive motor for printers, is monitored and controlled (e.g., for protection against paper jamming) by controlling a motor switch to turn the motor off temporarily when excess current is monitored, but turning it on to probe persistence of the condition. This turn-on will remain short if, indeed, overload current conditions are still present. The circuit includes a bistable multivibrator with input circuit rendering it monostable for normal operating conditions and astable for the overload case.

Abstract: A Faraday rotator optical sensor in combination with optical fiber transmission means is employed to monitor overload current conditions in a high voltage electrical load and transmit this information to a low voltage signal processing circuit while providing desired isolation between the high voltage load and the low voltage signal processing circuit.

Abstract: A control device for a small press is provided with means preventing overloading of the drive motor, a timer for detecting when an operational cycle of the press has exceeded a normal value, and a position detector which notes the position of a cam of the press. The power switch of the drive motor is tripped to suspend the flow of current to the motor upon an indication of the foregoing, e.g., a detected over-current, an excessive cycle time, or an operation end position of the cam.

Abstract: A circuit control device combining the functions of a current sensor, a time delay circuit, an undervoltage sensor, and a lockout such that, once tripped, the device must be reset intentionally. The current sensor includes a coil (16a) surrounding two reed switches (18, 20), the reed elements (18a, 20a) of each of which being perpendicular to those of the other for vibration resistance. The coil is then surrounded by a magnetic shield (28) in order to shield out some of the leakage flux from the main relay coil (14c). The time delay circuit includes a PUT (Q1) which compares a steady voltage from a voltage divider network with the increasing voltage across a capacitor (C3). The undervoltage sensor includes a second PUT (Q3) which compares the voltage of the DC source (10) with the steady voltage across a zener diode (D5).

Abstract: A circuit, safe against an overload, for incrementally varying the power to an electric blower motor used for heating, ventilating and conditioning vehicles, having a PTC resistor in series with other series resistors in the circuit.

Abstract: A control system for an automatic damper in a heat generation system. The damper is provided with a spring biasing arrangement to open the damper when the plant is generating heat to permit unobstructed discharge of the products of heat generation. A motor maintains the damper closed when no heat is being generated. Provision is made for disabling the control system permitting the damper to be opened by the spring and the heat generator to function normally when a malfunction occurs in a switch which normally performs in a safety capacity. Included in this arrangement is a fuse in a circuit which opens the circuit when the switch fails as the result of a short circuit or welding of the contacts. The circuit includes means for preventing short circuiting during normal operation.

Abstract: A DC Solid State Overload Relay for use with a DC motor controller is provided with functions to provide instantaneous and delayed tripping of an overload relay by monitoring a standard ammeter shunt signal placed in the power line. The relay circuit uses dropping resistors and a sensing unit with the sensing unit containing separate presetable adjustment for the instantaneous and delayed tripping function and further has automatic reset provisions and a trip indicator which indicates the cause of relay tripping.

Abstract: A motor control circuit for disconnecting the motor's power supply when the motor is stalled but maintaining it connected when it is running comprising a power loop and control loop, the former including a signal sensing means which provides an output sensing signal whose magnitude is a function of the motor's state in series with an ON, OFF switching means' input-output terminals which is responsive to the magnitude of a control signal applied to its control terminal to control its state. The control loop comprises a counter which provides a control output signal in response to a predetermined count and whose input is fed by the output of a voltage controlled oscillator whose frequency is a function of the magnitude of the output sensing signal of the signal sensing means to which it is connected. A resetting means is coupled to the reset terminal of the counter for periodically resetting the counter. The output control signal from the counter is fed to the control terminal of the switching means.

Abstract: A switching assembly to be incorporated in a window-regulator for a motor vehicle and the like, which includes paired units of switching members, each of which is arranged to serve for operating an electric motor in either of two rotational directions in a manual mode or in an automatic mode subject to the displacing amount of a single switching knob, and an electric circuit means including a solenoid for selectively self-retaining and self-releasing the automatic operating mode. The switching assembly is arranged to be capable of selectively energizing and de-energizing the electric motor incorporated in the window-regulator together with operating the solenoid simultaneously.

Abstract: A low winding resistance protective system for electrical motors comprising sensing apparatus for coupling to at least one motor winding and for comparing the winding resistance of a motor when at rest to a predetermined threshold and for providing an output indication in response to exceedance of that threshold and decoupling apparatus for decoupling the at least one motor winding from the sensing apparatus such that high voltages applied to the at least one motor winding during operation of the motor are prevented from reaching the sensing apparatus. An embodiment particularly suitable for use with high voltage motors is also described.

Abstract: A Faraday rotator optical sensor in combination with optical fiber transmission means is employed to monitor overload current conditions in a high voltage electrical load and transmit this information to a low voltage signal processing circuit while providing desired isolation between the high voltage load and the low voltage signal processing circuit.

Abstract: A method is provided for detecting the existence of arcing faults in the phase windings of a polyphase electric machine and for simultaneously identifying the specific phase winding in which the arcing fault exists. Arcing faults in phase windings of electric machines cause high frequency currents to flow through all phase windings of the machine with a drastically higher magnitude in the phase winding in which the arcing fault exists. These currents can thus be measured to determine the existence of the arcing fault and selectively determine which phase is affected.

Abstract: An A.C. overload current sensing, remote power controller of minimum size and weight and integral construction. Current rate sensing wound toroids are used that are small enough to fit around the line terminal studs without having to enlarge the prior insulator space available therein. The electronic overload current detecting and tripping control circuit is replaced by a pair of modules, detector and control modules, having improved performance and a shape and size such that they fit within the relay housing alongside its operating magnet. The detector module has integrator means for converting current rate of change signals into current magnitude indicative signals thereby enabling use therewith of rate sensing wound toroids of about one-tenth the size of equivalent current transformer wound toroids.

Abstract: A solid state overload relay protective apparatus for electric motors which includes not only the overload feature, but the additional features of underload and single phase protection. Small current transformers for sensing the current flow in each phase of the motor are utilized, like those found in the ground fault sensing art because a current-to-voltage converter circuit reflects a short circuit back to the secondary of the transformers. The signal from the current transformers which is proportional to the sensed current in the motor is fed through the converter, a scaling and summing amplifier adjustable for a wide range of motor full load currents, an ideal diode peak detector, a time integrator amplifier, to one input of a comparator circuit having a trip reference voltage signal at another input. The comparator provides an output signal to a transistorized trip level circuit that deenergizes a coil of an overload relay opening its contacts and interrupting power to the motor.

Abstract: An electronic safety device circuit is disclosed. This circuit includes two integrated circuit operational amplifiers, the first connected as an operational amplifier and the second connected as a follower. The output pin of the first amplifier is connected to the positive input pin of the second amplifier through a diode. The positive input pin of the second amplifier is connected to a memory capacitor. The output pin of the second amplifier is connected to the negative input pin of the first amplifier. A switching network is provided for grounding the positive input pin of the first amplifier and discharging the memory capacitor.

Abstract: A solid-state, multistage temperature controller for automatically controlling the operation of fans to maintain the temperature of an associated engine within preselected limits.

Abstract: A protection circuit protects a motor against excessive energizing current and exposure time conditions in accordance with the inverse relationship existing between these quantities. The circuit includes a pulse generator which generates a pulse train signal having a pulse rate proportional to the magnitude of the excess current applied to the motor. The pulses of the pulse train signal are counted up in a counter to a predetermined number. When that number is reached, the circuit operates to disconnect the motor. The greater the excess motor current, the higher the pulse rate of the pulse train signal, and the shorter the counting time required to obtain the predetermined number. The inverse relationship between excess current magnitude and exposure time is thus obtained. The circuit indicates when excess current is being applied to the motor and retains that indication after disconnection.

Abstract: A d.c. motor whose armature is supplied from an a.c. power source through an a.c./d.c. thyristor converter and is subject to damaging commutator flash-over, as from loss of a.c. voltage during regeneration is provided with a protective circuit which includes a normally charged capacitor and a diverter resistor which are connectable across the converter and motor armature, respectively, by SCR means in response to a fault condition such as either predetermined high output current from the converter (i.e., indicative of high armature current) or excess a.c. input current to the converter. Connection of the capacitor across the converter and its discharge instantaneously and forcibly but temporarily blocks current flow from the converter thyristors to the motor armature.

Abstract: Apparatus and method for controlling one or more electrically powered devices. A control for a temperature conditioning system is shown which is operative to monitor the electrical current supplied to component devices, i.e., compressors, auxiliary heating elements, indoor fan, and outdoor fan; and to cause an indicator means to indicate that one or more of the devices has failed to operate in a manner consistent with the controlled status of the system. If a failure is detected, control means are operative to lock out one or more of the devices from operation.

Abstract: A power interruption protection system for AC electric motors. An electroresponsive relay (OL) in the energizing circuit of an electroresponsive main power contactor (MC) for an AC motor is responsive to deliberately deenergize the contactor if a power supply interruption persists for more than a few cycles of the supply frequency. A motor current level detector (CLD) monitors the motor current and if it falls below a predetermined level activates a pulse counter (CTR) to register pulses from a clock pulse generator (CPG) which produces pulses each half cycle of the supply frequency. If the pulse counter registers a certain number or more of such pulses an amplifier-driver (AD) energizes the relay to effect disconnection of the motor from the AC supply. The system is disclosed both as a stand-alone system, and as combined in a system affording current overload protection modes as well for AC motors.

Abstract: In order to prevent dangerous operation of machinery, for example excessive speeds in internal combustion engines upon electrical failure in the fuel management system or associated sensors, the current passing through the electromagnetic final control element which engages the fuel control rod is monitored. In normal operation of the final control element, when the armature is displaced, a back EMF opposes the applied voltage, thereby reducing the effective current. Accordingly, a jammed armature and similar malfunctions will produce increased current through the final control element. The threshold of a threshold response switch is set to lie between these two values of the current, whereby, when the armature is jammed or short circuits occur, the current will rise above the threshold and permit fuel shut-off or a switchover to mechanical fuel control.

Abstract: Disclosed is an overload protective circuit for an electric motor which is adaped to monitor and track motor temperature as a function of an operating parameter, namely armature current. The motor's armature current is sensed and converted to a multi-bit digital signal which is used to address a programmable read only memory (PROM) which has been pre-programmed with discrete digital values of temperature for a plurality of armature currents for the motor being protected. The digital address signal for the armature motor current is periodically updated at a relatively rapid rate and depending upon the armature current sensed, multi-bit temperature values are outputted from the PROM to a digital latch which operates a relatively slower rate to output another address back to the PROM so as to predict what the temperature will be at the next time increment based on the present current value and the present temperature.

Abstract: A solid state protection system for AC electric motors driving high inertia loads requiring long acceleration times. An electroresponsive relay (OL) is separately responsive to permissible acceleration time, circuitry (DT, DC), acceleration overload current circuitry (ACA), and to normal running overload current circuitry (ACR). Up-to-speed detector and latch circuits (USD, USL) deactivate the acceleration time circuitry and overload current circuitry, and activate the running overload current circuitry upon attainment of normal running speed by the motor. The response levels of the overload and time response circuits are individually settable.

Abstract: This disclosure relates to apparatus and method for controlling and protecting an electric motor. A transmitter is coupled to the motor and to the power lines for generating pulses on the power lines in certain circumstances such as during an overheated motor condition. A control unit is coupled to the power lines and senses the pulses and monitors the power current magnitude in the lines. The unit compares the current level with previously established upper and lower limits. If the current level is outside the limits for a preset length of time, a fault condition exists and the motor is turned off for a length of time that depends upon the severity of the fault. The unit also turns off the motor upon the receipt of pulses from the transmitter when a certain number of pulses regularly recur. The unit includes indicators for displaying the nature of the fault.

Abstract: A motor is energized by a source through first and second transfer switches. Each transfer switch is operated by a switching transistor. The switching transistor for the first transfer switch is driven by a drive signal while the switching transistor for the second transfer switch is driven by the output of a delay circuit responsive to an excessively large current flowing through the motor. A self-holding switch is provided to connect the motor to the source through the second transfer switch.

Abstract: A current sensing circuit that controls a contactor that connects a load to power lines. The circuit provides an initial momentary delay period after energization of the circuit and before operation of the contactor to permit transients and other short duration anomalies to die out. An immediately succeeding second momentary delay period during which the contactor operation cannot be changed, assures that an initial inrush of current to the load will not cause a fault signal to erroneously operate the contactor. When a fault signal does cause the contactor to be operated, the circuit cannot be set back to its intended operation condition until energization of the sensing circuit is manually interrupted and reestablished. During normal operation of the circuit when current on the power lines is within acceptable limits, the circuit is latched in its proper operating mode.

Abstract: A static type overload relay is used to protect a motor against overload. The overload relay comprises an integrator for integrating the difference between the motor current and a continuous current carrying capacity of the motor. When the output of the integrator exceeds a predetermined value, a circuit breaker connected in series with the motor is tripped.

Abstract: A load indicator for an air cleaner associated with an air movement system which conducts air to a designated area in response to the supply of electrical energy from an energy source. An energy sensor includes a closed loop flux conducting circuit which is removably connected to surround at least one lead of the energy supplying circuit for generating a magnetic flux signal substantially proportional to the energy flow in the lead. The indicator operatively responds to a flux signal of a predetermined magnitude for providing a predetermined load indicating signal for the filter.

Abstract: A passive circuit for protecting an AC driven load controlled by a solid state device such as a triac or the like. The circuit is coupled to the load and the solid state controller element and effects shutdown of the AC power to the load and the load control element whenever an asymmetrical AC power input measured over a predetermined number of cycles is received by the load from the controller element. The device includes an inductor having a thermal link coupled thereto, the inductor being connected in parallel to the AC load which is being controlled by a triac or other similar device. Should asymmetrical input power be received from the controller, which could be occasioned by failure of the controller element itself, the inductor across the load will become heated by asymmetrical input power, causing heating of the thermal link. At a predetermined temperature, the thermal link will open, which removes AC input power to both the load and the controller element.

Abstract: Motor overload current is sensed and coupled to the input of an operational amplifier, causing the amplifier output to invert and charge a plurality of parallel connected capacitors. When the overload condition is present for a predetermined time, the capacitors are sufficiently charged to change the output state of a second operational amplifier. A diode is coupled between the output and the input of the second amplifier and is poled to latch the second amplifier into its inverted state when the capacitors have reached the predetermined charge. A luminous source, such as a light emitting diode, is coupled to the output of the second amplifier and becomes non-luminous in the inverted state of the second amplifier. The diode is photo-coupled to a photo conductive circuit for disabling the power coupling to the motor circuit when the diode is non-luminous.

Abstract: A compact resistor device comprises a body of ceramic material of positive temperature coefficient of resistivity having a large number of passages extending through the body between opposite ends of the body, thereby forming very thin webs of the resistor material between adjacent passages. Coatings are formed on the resistor material along the inner walls of the passages to serve as ohmic contacts. The coatings in alternate passages are connected together at one end of the resistor device and the coatings in the other passages are connected together at the opposite end of the device to serve as device terminals. When the device terminals are connected to a power source, current flows through very thin webs of resistor material between ohmic contacts in adjacent body passages. The resistor device is particularly useful in current limiting applications requiring low, room-temperature resistance and in heat-exchanger applications where a fluid to be heated is directed through the passages.

Abstract: A circuit arrangement is disclosed for protecting against overload electrical equipment such as an electric motor, and any mechanical equipment driven thereby, such as a power actuator. Two light emitting diodes are connected in parallel with each other (with opposite polarity) and in parallel with a series impedance in the power supply circuit of the motor so that each produces a light output in response to an overload current of appropriate polarity in the motor. Associated with each diode is a respective phototransistor which responds to the light output from the associated diode and, via a respective control circuit, operates a switch to interrupt the load current.

Abstract: A prime mover adapted for energization to a preselected speed therefor across a power source. The prime mover has a pair of winding means for excitation so as to effect the energization of the prime mover generally to the preselected speed when it is energized across the power source. Means is provided in circuit relation with one of the winding means and the power source for inhibiting the excitation of the one winding means generally as the prime mover attains its preselected speed, and means is operable generally in response to the occurrence of a preselected condition with respect to the excitation of the other of the winding means for placing the inhibiting means in open circuit relation with the power source generally as the prime mover attains its preselected speed.A method of operating a prime mover, a method of interrupting the operation of means for inhibiting current flow therethrough, and a circuit are also disclosed.

Abstract: A trolley wire for conveying direct current power to an electrically powered vehicle in an underground mine is connected in series with a shunt to a power source. The current drawn by the trolley wire passes through a circuit breaker and the shunt. The shunt generates a voltage input signal proportional to the current drawn by the trolley wire. A solid state comparator device receives the input signal and compares it to a reference signal having a magnitude representing the maximum level of current permitted to flow through the conductor. If the reference signal exceeds the input signal, the comparator device supplies an output signal that oscillates between a positive and a negative voltage to a trip circuit. The output signal actuates the operating coil of the circuit breaker so that the circuit breaker contacts remain closed to permit current to flow through the shunt to the trolley wire.

Abstract: A method is provided for detecting the presence of arcing faults in the stator windings of large dynamoelectric machines, such as turbine generators. Arcing faults, such as broken strands in a winding conductor with continued arcing, give rise to high frequency currents which flow in the neutral lead of the generator winding. The presence of the arcing fault can thus be detected by observing the high frequency neutral currents and monitoring them for the presence of the characteristic frequency distribution resulting from the resonance characteristics of the current path.

Abstract: An overload-protection circuit for an electric motor has a current transformer whose primary winding is connected in series with the windings of the motor to be protected while two secondary windings are provided. The first secondary winding is connected to an actual-value current-monitoring circuit while the second secondary winding forms part of a voltage-supply network for the circuit. The primary winding is subdivided into a plurality of winding sections having respective taps for different nominal values of the overload current. The product of the number of turns of each winding section and the nominal-current value of the respective tap (ampere-turns) is the same for each winding and the respective tap.

Abstract: An inrush current restraint circuit, for use with a typical overcurrent minimum pickup circuit of a power system switching device, such as a recloser or sectionalizer, for raising the initial overcurrent minimum pickup value upon closure of the switching device to a selected value, then linearly returning this pickup value to normal within a selected period of time. The minimum pickup circuit is actuated by the voltage drop across a current sensing resistor, through which a current signal proportional to current in the power system is passed. The initial pickup overcurrent value is increased by providing another current path in parallel with the current sensing resistor when the switching device is closed. A timing capacitor is discharged each time the switching device is open, then charged from a constant current source each time the switching device is closed.

Abstract: Disclosed herein is an electromechanical actuator which acts as an analog of a fluid operated cylinder. A tubular piston moves along the axis of a fixed outer cylinder. One end of the piston is fitted with a threaded stud to which a clevis or other load attachment device can be attached. The other end of the piston, inside the outer cylinder, is attached to a coaxial drive nut. The drive nut is threaded onto a coaxial lead screw which passes through the drive nut and clears the inside of the piston. The drive nut is notched at a plurality of locations, the notches corresponding to a plurality of axially extending protrusions on the inside of the outer cylinder. Thus, when the lead screw is rotated, translational motion is imparted to the drive nut due to its inability to rotate relative to the outer cylinder. A small bar magnet is affixed to the drive nut and cooperates with an external reed switch to provide intermediate shutoff points.

Abstract: A circuit for a polyphase motor which operates in response to undesirable operating conditions so as to prevent damage to the electric motor. The protection circuit converts current to voltage over a range of operating currents. The current sensitive voltage in response to an undesired current condition triggers the operation of a gating circuit to apply potential to an output switch which in turn controls the current supply and stops the motor. The gating circuit is also triggered by a detector circuit sensing a deficiency in motor current to thereby actuate the output switch and interrupt the motor operation and thus protect the motor. A timing circuit provides an inverse time function in conjunction with the voltage applied to the gating circuit. The timing circuit is adjustable to provide adjustability in the elapsed time for providing a triggering voltage to the gating circuit.

Abstract: A device is described for converting solar energy into electric power for a load, which device is provided with photocells and electric accumulators. Switching means are provided which at least during starting of the load connect this load to the output terminals of the array of photocells so that during starting the current through the load is determined by the current supplied by the photocells. The device makes efficient use of the available solar radiation, is of simple design and is reliable.

Abstract: A combination starting and protecting device particularly useful with split phase motors such as those used with refrigeration compressors is disclosed in which an element of material having a positive temperature coefficient (PTC) of resistivity characteristic and a thermally responsive switch are disposed in the same housing. A thermal coupling of a preselected value between the PTC element and the thermally responsive switch is provided so that a desired reset or "off" time is obtained for the thermally responsive switch without affecting the "on" time.