Abstract: A voltage stabilizer with a very low voltage drop includes a series voltage regulator circuit having a first PNP transistor connected to an input terminal of the stabilizer via a second PNP transistor and being connected to ground via a capacitor. The stabilizer has biasing and switching circuits disposed between the input terminal thereof, a base terminal of the second transistor and ground.

Abstract: Two or more buck converter circuits are cascaded in such a manner that the output of one serves as the input to the next, with the input voltage to each succeeding buck converter stage being reduced in magnitude. The total circuit losses are substantially reduced as compared to the losses generated in a single buck converter having the same input voltage range and the same output voltage and output current. Both positive and negative output terminals may be provided for an output stage.

Abstract: The switching power supply has high output power, high efficiency and fixed or variable output voltage. The input voltage may be AC or DC. Preferably, the input current is constant. Smallest possible peak currents of switches result in best possible line and component utilizations at any time.In one embodiment, a first converter has an output for converting DC input voltage into a base voltage. A first capacitor is coupled between the DC input voltage and first converter output for storing the base voltage. A second converter is coupled to the first converter output for determining the output voltage. A second capacitor filters the output voltage. Both converters are referenced to ground. A diode applies the DC input voltage to the output.

Abstract: A low DC voltage supplied to a low DC voltage receiving terminal is converted into a high DC voltage by a DC-DC step-up circuit. The obtained high DC voltage has an optimal value for driving an electro-luminescence device. This high DC volatage is converted into an AC voltage through a pair of series-connected transistors in a switching circuit ON/OFF-controlled in response to an oscillation output from a multivibrator. The converted voltage is supplied to the electro-luminescence device through a series resonance circuit including the electro-luminescence device and a reactor as an AC drive voltage. At this time, a resonance voltage is fed back to the multivibrator and the switching circuit so as to obtain an appropriate luminance correction capability.

Abstract: A power supply (ringer power supply) for a telephone exchange comprising a first switching circuit for receiving and subjecting direct current to high frequency switching by means of a switching element, a converter transformer connected to the switching circuit, a smoothing circuit connected to the secondary side of the converter transformer, a second switching circuit connected to the output stage of the smoothing circuit and used for switching the input current by means of a transistor bridge of a plurality of transistors for being alternately turned on, a direct-current bypassing diode connected between the input and output terminals of each transistor, a driving circuit for driving each of the transistors in accordance with a predetermined driving signal, and a driving signal generator circuit for generating the driving signal for setting timing to prevent the period during which a transistor is turned on from overlapping the one during which another transistor is subsequently turned on, permitting a red

Abstract: A preliminary stage of a voltage regulator with a low voltage loss to be connected with a further stage of the voltage regulator and comprising an input and output voltage terminals, a common voltage terminal for input voltage and output voltage, a capacitor serving as a charge storage element at an output side of the series preliminary stage, a branch including a transistor having an emitter connected with the input voltage terminal, a collector connected with the output voltage terminal, and an auxiliary collector; a power means for feeding the transistor with a base current, and a current reducing element for reducing current flowing in the capacitor to zero before the transistor reaches saturation when the input voltage and the output voltage falls below a predetermined amount, and including an element for influencing a potential of the collector and connected with the auxiliary collector and the input voltage terminal, and a voltage regulator containing such a preliminary stage.

Abstract: An inverter circuit includes a transformer having a primary winding with a first end adapted to be connected to a d.c. source, a switch connecting the other end of the primary winding to a reference potential, a secondary winding coupled to the primary winding, a rectifier for rectifying the output of the secondary winding, and a bridge switch circuit connected to invert the rectified secondary winding voltage to produce an a.c. output voltage. The inverter circuit further includes a first control circuit for producing a first pulse train of pulses having a first given frequency and a pulse width responsive to the rectified output voltage of the secondary winding, and a circuit for applying the pulse train to the switch for controlling the switch to open and close at the given frequency.

Abstract: A power supply with a pulse width modulation switching mode voltage regulator having an output filter with an attenuation notch at the switching frequency of the pulse width modulator. The output filter comprises an input inductor and a series connected tank circuit with a shunt capacitor. The tank circuit is comprised of a shunt connected L-C circuit tuned to the switching frequency.

Abstract: In a method of and apparatus for connecting a storage battery to an elevator inverter, the storage battery being connected to a d.c. circuit of the elevator inverter power stage, a voltage in the d.c. circuit is higher than the battery voltage. Power flows bidirectionally from the storage battery to the d.c. circuit and vice versa. When power is flowing from the storage battery to the d.c. circuit, the battery voltage is matched to the voltage in the d.c. circuit by the aid of a semiconductor switch, by means of which an energy store is charged, and a diode, by which the energy in the energy store is discharged into the d.c. circuit. When power is flowing from the d.c. circuit to the storage battery, the voltage of the d.c. circuit is matched to the battery voltage with the aid of a second semiconductor switch, by means of which the energy store is charged, and a second diode, by which the energy in the energy store is discharged into the storage battery.

Abstract: A high-efficiency programmable power supply including a linear regulator and a switching pre-regulator. The switching pre-regulator monitors the voltage drop across the series pass regulator pass transistor and maintains a predetermined voltage drop thereacross. The programmable power supply output voltage is varied according to a digital signal applied to the linear regulator. The switching pre-regulator, in providing a generally constant voltage drop across the linear regulator pass transistor, increases efficiency of the power supply by reducing excess voltage which appears at the input of the linear series pass regulator to a minimum level below which the linear regulator can operate. Therefore, the power supply according to the present invention maintains high efficiency over a wide range of selectively programmed output voltages.

Abstract: According to the invention, a buffer capacitor (4) is connected, on the one hand, via a series controller (5) limiting the charging voltage of the buffer capacitor (4), to supply voltage terminals (2, 3) and, on the other hand, via a voltage stabilizing circuit (6), to lead terminals (1a, 1b) of the electronic device (1). Thus the buffer capacitor (4) can be charged to a charging voltage which is clearly above the operating voltage of the electronic device (1), so that the period until the minimum operating voltage of the electronic device (1) is reduced, is lengthened. In addition, a constant operating voltage is available for the electronic device (1).

Abstract: A linear regulator employing a field effect transistor (FET) pass element to supply a pulsed load is programmed by a cooperating switching regulator such that an increased voltage differential is maintained across the FET to reduce response time during the leading edge of the load pulse and to thereafter reduce the voltage differential to achieve reduced power consumption and increased efficiency during the remainder of the load pulse.

Abstract: A power supply for an arc lamp. It includes a DC converter which has an input, an output, and a circuit for increasing the voltage received at the input and for supplying the increased voltage to its output. A current sensing circuit is provided for controlling the amount of current delivered to the output. An output "H" bridge is coupled to the output of the DC converter for generating a squarewave in response thereto. The power supply is capable of quickly igniting and re-igniting arc lamps, is relatively inexpensive to manufacture and it relatively light in weight. The power supply can accept either an AC or DC source over a wide voltage range and increases or decreases the input voltage compared to the output voltage.

Abstract: A power supply for an arc lamp. It includes a DC to DC converter which has an input, an output, and a circuit for increasing the voltage received at the input and for supplying the increased voltage to its output. A current sensing circuit is provided for controlling the amount of current delivered to the output. The power supply is capable of quickly igniting and re-igniting arc lamps, is relatively inexpensive to manufacture and is relatively light in weight. The power supply can accept either an AC or DC source over a wide voltage range and increases or decreases the input voltage compared to the output voltage.

Abstract: A power supply for an arc lamp. It includes a DC converter which has an input, an output, and a circuit for increasing the voltage received at the input and for supplying the increased voltage to its output. A current sensing circuit is provided for controlling the amount of current delivered to the output. An output "H" bridge is coupled to the output of the DC converter for generating a squarewave in response thereto. The power supply is capable of quickly igniting and re-igniting arc lamps, is relatively inexpensive to manufacture and is relatively light in weight. The power supply can accept either an AC or DC source over a wide voltage range and increases or decreases the input voltage compared to the output voltage.

Abstract: An improved power supply capable of driving an external device at relatively high power levels for intermittent periods of time with the power supply driven by relatively low current input power which can vary widely in voltage. The power supply incorporates a step-up switching voltage converter module for increasing the input voltage to a higher value so as to charge an energy storage capacitor to an energy level sufficient to power the external device for said intermittent periods of time at relatively high current. The energy storage device voltage output is regulated by a step-down switching voltage converter module. Control signals sensing the voltage of the energy storage capacitor can be used to control energization and de-energization of the external device.

Abstract: A stabilized power supply unit designed to supply the power from the commercial power source as far as the commercial power source is normal but supply the power from a battery that can be made available by an automatic switching device when the commercial power source has failed, wherein the AC voltage from the commercial power source is rectified by a rectification circuit to obtain almost equal DC currents on both the positive and negative sides of the common line while the commercial power source is normal. Said DC voltage is chopped by the boosting chopper circuit to boost the voltage on both the positive and negative sides. The boosted voltage is converted into the AC voltage through the DC-AC inverter, and the high-frequency component of the AC voltage is compressed through the filter in order to be outputted as an AC voltage of specified frequency.

Abstract: There is disclosed a diode snubber for the voltage transients generated in the parasitic inductance of a switched resistor when current therethrough is interrupted. Also disclosed is the use of such a diode snubber in: a series switched resistor regulator; a shunt switched resistor regulator; a hybrid switched resistor regulator and a passbank switched resistor regulator. Also disclosed is the use of a linear dissipative regulator in all these forms of switched resistor regulators to maximize the effiency of power delivery to the load.

Abstract: A power converter for use in converting a variable DC input voltage into a variable DC output voltage is disclosed which uses control circuitry on both the input and the output to optimize converter performance. By charging the inductor up nearly to the saturation point, but completely avoiding saturation of the inductor, power capability at the input end is maximized. By discharging the inductor completely without either losing power capability by continuing to attempt to discharge the inductor after the charge is depleted or by failing to nearly discharge the inductor thereby presenting the possibility of saturation to the boosting of current through the inductor during the next charge cycle.

Abstract: There is disclosed a diode snubber for the voltage transients generated in the parasitic inductance of a switched resistor when current therethrough is interrupted. Also disclosed is the use of such a diode snubber in: a series switched resistor regulator; a shunt switched resistor regulator; a hybrid switched resistor regulator and a passbank switched resistor regulator. Also disclosed is the use of a linear dissipative regulator in all these forms of switched resistor regulators to maximize the efficiency of power delivery to the load.

Abstract: A circuit for operating high-pressure gas discharge lamps comprises a full-wave rectifier (2) which is connected to an alternating voltage supply source and supplies an output direct voltage to a combinatorial circuit part comprising a switching transistor (3), a choke coil (4), a fly-wheel diode (5) and a storage capacitor (6), from which the lamp is energized. A further combinatorial circuit part (7, 8, 10, 11) comprises at least one electronic circuit element (7) coupled between the storage capacitor (6) and the lamp (9) and is controlled by a control device (12). The control device compares an instantaneous lamp current of higher frequency with a nominal-value signal composed of a sinusoidal voltage component having double the supply frequency and a d.c. voltage component having a value of at least the maximum amplitude of the sinusoidal voltage.

Abstract: Disclosed is an electrical power system adapted to receiving power from a source of alternating current, typically a small alternator, and supplying a direct current potential to a load. This power system includes two stages, the first stage comprising switchable rectifying devices such as silicon controlled rectifiers which interact with the impedance of the alternator or other source of alternating current power as a switching regulator to create an intermediate direct current potential. The second stage operates from this intermediate direct current potential to produce a slightly lower output voltage, and creates a more accurate regulation than is possible with the first stage alone, the second stage greatly reduces ripple and high frequency noise components contained on the output of the first regulator stage. The second stage serves as an active low pass filter tuned to a desired DC voltage.

Abstract: A resonant forward converter at a switching frequency of 10 MHz provides a small volume point-of-load converter for distributed power systems. The leakage inductance of a transformer is made negligibly small relative to the magnetizing inductance of the transformer. This enables a resonant controllable switch on the primary side of a transformer and a secondary side rectified diode to be switched either on or off at the same time. The capacitance across the primary side switch and the secondary side diode rings with a resonant inductor connected in parallel with the transformer. Both the switch and diode, therefore, have zero voltage switching transitions. Further, the resonant inductor being connected in parallel with the transformer provides a resonant ring that is independent of load.

Abstract: An X-ray generator system has first and second switching devices such as transistors which are complementarily turned on and off to intermittently apply a DC voltage to the primary winding of a transformer for thereby inducing a high voltage across secondary windings of the transformer for enabling an X-ray tube to emit X-rays. The X-ray generator has a control mode in which the first and second switching devices are simultaneously turned on immediately before they start being complementarily turned on and off, for allowing the high voltage to have a sharply rising positive-going edge.

Abstract: A switch-mode power supply includes the series combination of an inductor and a controllable switch adapted to be coupled to a voltage source for producing a succession of increasing and decreasing ramp currents through the inductor. The average value of the ramp currents is controlled by a feedback loop. A second controllable switch arrangement is coupled for causing the current leaving the inductor to recirculate through the inductor during those intervals in which the second switch is conductive. An integrator is coupled to the inductor for integrating ramp current from the inductor during those intervals in which the second switch is nonconductive, for generating load voltage. A second feedback arrangement senses the load voltage and controls the average conduction of the second switch to control the load voltage. In one embodiment, an inverter receives ramp current from the inductor and generates alternating current, which are transformed and rectified before application to the integrator.

Abstract: A high efficiency integrated power converter adapted for direct connection to line voltage and having on chip protection from overcurrent may be implemented utilizing a GTO-SCR as the switch element in a switching power converter such that the converter requires no external transformer or inductor based voltage reducing circuitry. The input voltage may vary over a wide dynamic range without deterioration in circuit performance due to the extremely high anode to cathode breakdown voltage of the switching element and further due to the on chip protection from excess current flow through the switching element.

Abstract: An arrangement for enhancing the available energy storage capability of a hold-up energy storage element for use with the step-up or boost regulator stage of a power supply enables use of smaller hold-up storage elements, such as capacitors. The circuit stores energy at the boost regulator output and returns the energy either to the regulator input in the event of a transient line failure, or to the regulator output during a load transient. The invention provides improved transient fault tolerance by increasing the hold-up time of the storage element.

Abstract: A power supplier for providing a regulated high-frequency AC output with a bucking and boosting section and a push-pull output section in which a common switching element is used in both the bucking and boosting section and in the push-pull output section.

Abstract: A direct-current power supply circuit includes a series-connected switch (TR1) and an inducator (L) connected together with a flywheel diode (D) across an input voltage supply (V.sub.s). A transformer (TX) has a primary winding connected to the inductor (L) and the diode (D) by way of a bridge-connected network of switching devices (TR2-TR5). A reservoir capacitor (c) is connected to a secondary winding of the transformer (TX) by way of a full-wave rectifier (DB), and a load (LD) may be connected to the reservoir capacitor (c). Control means (CM) are provided to control the operation of the series-connected switch (TR1) and the network of switching devices (TR2-TR5) so that each cycle of operation transfers an equal amount of charge to the reservoir capacitor (c) regardless of variations in the input voltage (V.sub.s) or load conditions.

Abstract: A programmable power supply is disclosed which combines a linear power supply with a switching power supply. The switching power supply includes a pulse width modulator, a main power transformer, and a DC to AC inverter which is responsive to the pulse width modulator for varying the input to the main power transformer. The linear power supply includes a FET that is coupled to the output of the switching power supply. The FET is controlled in response to a feedback signal representing the output of the FET and also a voltage programming signal representing the voltage to be programmed. The pulse width modulator is controlled in response to the feedback signal representing the output of the FET and also a feedback signal responsive to the voltage drop across the FET. The output from the switching power supply, which is fed to the FET, is controlled in response to the output from the pulse width modulator.

Abstract: An alternating current sensing circuit for sensing the current supplied by a power supply of an electronic instrument includes an isolation transformer and at least one transistor connected across the secondary winding of the transformer. The isolation transformer provides galvanic isolation between the power supply and internal feedback circuitry that controls the power supply located within the instrument. The transistor clamps the secondary voltage on the transformer to a constant magnitude when the base-emitter junction is forward biased. In response to the sensed current, the secondary current of the transformer flows through the transistor when the base-emitter junction is forward biased to produce the constant secondary voltage. The secondary current provides a current sensing signal which is transmitted to the feedback circuitry.

Abstract: An improved switching arrangement for a direct coupled switching power supply is provided by utilizing a gate turn off SCR as the primary switching element and a second SCR as the turn-off control such that upon detection of any of a fully charged storage element, an excessive current through the gate turn off SCR, or an insufficient input voltage, the switch is turned off.

Abstract: In a circuit supplying a digital system, the frequency at which the switching device therein is controlled is synchronized to be the same as one or several times half the sampling frequency. In this way, the original frequency spectrum can be kept free from interfering signals.

Abstract: A low power high efficiency switching power supply comprises a switching transducer having primary and secondary windings. The primary winding is supplied with a primary voltage over a series connected constant current source and Zener diode. The primary voltage is chopped by an FET which has a gate connected to an oscillator operating at relatively low frequency. The secondary windings of the transformer are connected to full-wave rectifiers which convert the A.C. signal produced by the chopping of the primary voltage, int a D.C. signal. Efficiency is maximized by using fast switching FET's and low flux densities in the transformer. The power supply is particularly useful in 4-20 mA current loops.

Abstract: A d.c. to d.c. converter power supply having two substantially independent regulated outputs. A pulse width modulator (PWM) converts input d.c. to a.c. having a variable frequency and a variable duty cycle. The a.c. is applied to a transformer having two secondary windings for separate d.c. outputs. One of the secondary windings feeds a rectifier having a capacitor input type filter and means for regulating the output voltage by controlling the duty cycle of the PWM. The other secondary winding feeds a rectifier having a choke input type filter and means for regulating the output voltage by controlling the frequency of the PWM.

Abstract: A high voltage power supply system includes a DC-to-DC converter for producing a DC low voltage, a DC-to-AC inverter connected to a step-up transformer for producing an AC high voltage by transforming an AC low voltage obtained from the DC low voltage, and an inverter controller for controlling an operation time period of the DC-to-AC inverter longer than that of the DC-to-DC converter. Since a residual charge stored in a capacitor of the DC-to-DC converter is rapidly discharged by lengthening the operation time period of the DC-to-AC inverter, an unwanted AC high voltage is induced at a secondary winding of the step-up transformer.

Abstract: A switched power supply has a rectifier arrangement for generating a DC voltage from an input AC voltage. A converter arrangement thereof has a switching transistor and a transformer. The converter arrangement is designed as a free-running flow converter wherein the switching transistor is switched on in a current-free condition. A secondary side of the converter arrangement comprises a secondary switch controller which is synchronized by the transformer and which regulates an output DC voltage.

Abstract: A power supply circuit for a television receiver used in an automobile having either a 12 volt or 24 volt storage battery. The power supply circuit outputs a constant voltage whether connected from a storage battery of 12 volts or 24 volts thereby effecting stable operation of a TV set. A first constant voltage source is activated by an input of 12 volts and the output therefrom is boosted by a booster voltage circuit. A switch means is activated by an input of 24 volts and functions to disable the first voltage source means. A second voltage source means is activated by an input of 24 volts and outputs a constant voltage directly to a horizontal output circuit.

Abstract: A capacitor-type power supply for electrical machining has a capacitor connected across a machining gap while a high-frequency power generator is disposed at a location remote from the machining gap and provides a high-frequency electric power. A feeder connects the power generator with the capacitor to permit the capacitor to be charged with pulses of the high-frequency electrical power. The system permits the high-frequency source to be located distally from the machining gap.

Abstract: An output circuit for diode-or connected positive three terminal voltage regulators. A voltage divider develops a voltage signal representative of the output voltage from the positive three terminal regulator. An adjustable precision shunt regulator connected to the voltage divider provides a control signal representative of the variance of the voltage signal from a predetermined threshold signal. The positive three terminal voltage regulator provides a variable output voltage in response to and representative of the control signal.

Abstract: A regulated power supply that provides two desired direct current voltages at respective outputs thereof, the supply maintaining the difference between voltages appearing at the outputs the same as the difference between the desired values, series regulators respectively coupled to each output that are in a saturation mode when a voltage that is a fraction of the voltage between the outputs lies within a given range, where one of the series regulators is placed in a linear mode when the voltage is at or beyond one edge of the range, and the other series regulators is placed in a linear mode when the voltage is at or beyond the other end of the range.

Abstract: A high-voltage stabilizing circuit for stabilizing high voltages such as those generated by the fly-back transformer of a television. The stabilizer includes a common-emitter type PNP transistor and an inverse current-absorbing capacitor with a small capacitance in order to provide a high-speed response. The collector of the transistor is connected with the primary coil of the fly-back transformer and the capacitor. The emitter of the transistor is connected with a power source. With this arrangement, even though parabolic wave voltages are generated at the capacitor due to the small capacitance, the PNP transistor hardly cuts off at all and can therefore supply adequate current to the fly-back transformer.

Abstract: A switching circuit for a power supply in a signal processing apparatus for processing a signal in synchronism with a clock signal having a predetermined frequency, comprising a switching element, driven by a signal having frequency which is in an integer ratio relationship with said predetermined frequency, for receiving a non-stabilized DC voltage, thereby to output said non-stabilized DC voltage intermittently. An electric power supply means controls the DC output voltage to be supplied to an electric power supply line by means of the switching element in order to set it equal to a predetermined value.

Abstract: In a circuit arrangement for feeding electrical users by DC series feed, remotely fed users are respectively preceded by a DC-DC converter which contains a switch controller having a switch element parallel to an input thereof and having a capacitor connected thereto via a diode. In such a circuit arrangement, it is desired that the voltage appearing at the input during the start-up operation should remain below a prescribed amount. For this purpose, the control circuit is designed such that it emits a pulse sequence having a fixed or limited pulse-duty factor during the start-up operation and/or such that at least one part of the load is disconnectable with the assistance of a switch means. The circuit arrangement can be advantageously employed in switch controllers which precede remotely fed users in communications transmission devices with DC series feed.

Abstract: A power supply for providing multiple operating potentials to a load switching controller is capable of providing constant voltage d.c. outputs from any of a plurality of a.c. voltage sources including the full range of power supply voltages and frequencies commonly used in various geographic locations throughout the world. The power switching element on-time is controlled by sensing the output voltage levels and providing feedback control of the switching element. A status circuit provides an indication of either an overvoltage of an undervoltage condition.

Abstract: A DC power circuit for converting an input DC voltage into a predetermined output DC voltage includes a first switching device for switching a DC input voltage, an inductance device connected to an output terminal of the first switching device, a second switching device connected to an output terminal of the inductance device for switching an output from the inductance device, a driver circuit for comparing the output DC voltage with a first reference voltage to drive the first and second switching devices in accordance with the comparison, and a control circuit for comparing the input DC voltage with a second reference voltage to enable the driver circuit to selectively drive either the first or second switching device in accordance with the comparison.

Abstract: A series regulator responds to an input voltage to produce a constant direct voltage. A resistor is connected at one end to the output of the regulator. A current carrying bus is connected between the other end of the resistor and the input of the inverter whereby current flows through the resistor producing a voltage drop thereacross and thereafter flows into the inverter to supply power thereto. An electronic switch has a closed position at which current on the bus supplied via the resistor is augmented by the current on the bus supplied via the switch. The switch has an open position at which the augmented current is not present on the bus, the switch being placed in the open position whenever a pulse appears in the control voltage and thereafter is placed in the closed position when the pulse disappears and said voltage drop is at least equal to a preselected value.

Abstract: Adapter for field signals of computers, microprocessors systems or the like digital electronic circuits. The adapter receives either AC or DC signals, from external or field sources or from computers, microprocessor systems of the like digital electronic circuits, and provides AC or DC control voltages to external or field controls or to computers, microprocessor systems or the like digital electronic circuits. The adapter includes a first switching circuit which responds to the level of an input signal, and which actuates an oscillator which includes the primary of a transformer as its load. Energy is transferred by the transformer secondary to appropriate control circuits for use elsewhere. The adapter also includes a second switching circuit which also responds to a different level of the input signal, the second switching circuit diverting the input signal current into itself across an artificial load when the first switching circuit is inoperative.

Abstract: A power supply circuit for driving a motor includes a rectifier circuit and a converter circuit for stepping down the DC voltage from the rectifier circuit to drive a motor. The power supply circuit further includes a regulator circuit which is energized by the DC output voltage from the converter circuit to supply a constant voltage or current to the motor.

Abstract: A reference frequency and low voltage detector circuit for use with a constant frequency A.C. voltage source is disclosed in which a first supply circuit supplies a first voltage wave form having a repetition rate and magnitude proportional to the A.C. source voltage. A second supply circuit supplies a regulated D.C. voltage of a fixed magnitude as long as the amplitude of the A.C. source voltage is normal. The voltages produced by the first and second supply circuits are supplied to noninverting and inverting input terminals of a differential amplifier which produces a rectangular wave output signal only when the A.C. source voltage is normal. The differential amplifier output signal provides a frequency reference under normal conditions and signals a user device to switch to a secondary mode of operation when the repetition rate of the rectangular wave form changes.