Abstract: A switching module having a plurality of switching channels responsive to a microcontroller for coupling electrical power to a load, and decoupling electrical power in the event an undesired operating condition is sensed. Each switching channel includes a switching transistor and transistor driver. Monitoring circuitry senses the actual voltage drop across the load to provide the controller with an indication of undesired operation. A limiting resistor limits the transistor drive such that the transistor is forced out of satuaration during a high power condition, thereby avoiding undesired power dissipation which would otherwise occur if operation in the saturated condition continued.

Abstract: Apparatus for detecting malfunctions of an electrical device comprises a current transformer, connected to the electrical device a current-sensitive switch connected to the current transformer, a power supply connected to the current sensitive switch, and an indicator connected between the switch and a power supply. The switch is adapted to be switched on and prevent the indicator from being actuated by the power supply while the energy in the electrical device is greater than or equal to a predetermined level, and is adapted to be switched off and permit the indicator to be actuated by the power supply while the energy in the electrical device is less than the predetermined level.

Abstract: A switching system for powering first and second electrical loads from a voltage supply, the electrical loads being operable from controls and adapted to be connected to the voltage supply by the switching system. The switching system includes switching circuitry having a control element which is responsive to a control voltage difference to cause the switching circuitry to become conductive, the switching circuitry having a switch terminal connected to a first supply line and another switch terminal connected to the first electrical load. The switching system also includes additional circuitry responsive to the controls for generating the control voltage difference when the voltage supply is connected to the first supply line thereby powering the first electrical load from the first supply line. The switching system further includes further circuitry responsive to the controls for generating first and second control signals.

Abstract: A reverse battery and loss of positive supply voltage detector circuit includes a power field effect transistor for switching a stored inductive energy to the ground supply voltage terminal if there is a disruption in the power supply. A detector circuit senses a reversal of the supply voltage connections as would happen if the battery were connected backwards and enables the power field effect transistor for discharging a negative voltage that may have developed across an inductive load. The detector circuit further senses a loss of the positive supply voltage and enables the power field effect transistor for discharging a negative voltage that may have developed across an inductive load due to the disruption of applied power.

Abstract: A power interrupt switching device for independently controlling normal and remote loads from a single control switch. The device includes an edge detector, an energy storage element, and a latch. When power is applied to the switching device, the edge detector detects the power transition and generates an output pulse for a predetermined time. The storage element charges and generates an output after a second predetermined time greater than the first. Consequently, both outputs are not present simultaneously upon the first application of power. If power is interrupted momentarily and then reapplied, the edge detector again generates an output while the storage element maintains its output during the interruption. With both outputs present, the latch is triggered to deliver power through the switching device to a remote load. The latch remains closed until power is removed.

Abstract: An electrostatic discharge protection circuit includes a P.sup.- doped channel and an N.sup.- doped channel that form a serial path between a signal pad and a transistor. Holes are depleted from the P.sup.- doped channel in response to a negative electrostatic discharge on the input signal pad; and electrons are depleted from the N.sup.- doped channel in response to a positive electrostatic discharge on the input signal pad. When either depletion occurs, the path from the signal pad to its transistor is open circuited; and so the transistor is protected. Conversely, when no electrostatic charge exists on the signal pad, the path through the P.sup.- doped channel and the N.sup.- doped channel is highly conductive; and so signals pass between the pad and the transistor very quickly.

Abstract: An integrated protection unit is a circuit breaker which includes basic overcurrent protection facility along with selective electrical accessories. A molded plastic accessory access cover secured to the integrated protection unti cover protects the accessory components contained within the integrated protection unit cover from the environment. A combined overcurrent trip actuator and multiple accessory module is either field-installed or factory-installed within the integrated protection unit. A separate actuator-accessory module is selected for different combinations of accessory functions.

Abstract: A drive circuit for terminators in a SCSI (Small Computer Standard Interface) system comprises a bipolar transistor operating in saturated condition to keep the voltage drop to less than 200 millivolts. A second transistor serves to detect when, due to overcurrent conditions, the first transistor goes out off saturation and turns it off. Also provided is means for ensuring that the first transistor turns on before the second transistor on power-up.

Abstract: An operation halting circuit comprises a circuit for controlling a system which executes a specified operation and a circuit for shutting off every input to the control circuit so as to prevent the supply voltage from being applied to the control circuit when the supply voltage drops below a specified threshold value.

Abstract: A three phase electrical load protection device is disclosed including an input transducer for detecting each phase voltage and the direction of rotation of the phases of a three phase power system. A control monitor is disclosed for monitoring signals from the input transducer representative of the highest and lowest phase voltages and the direction of rotation of the phases for controlling an output device for disconnecting the electrical load when improper supply conditions are detected and monitored.

Abstract: A scheme for detecting reduced source of voltage of a controlled switching power supply including a transformer employs the inherent insolation characteristics of the transformer to yield an isolated alarm signal indicative of a failed or inadequate source voltage.

Abstract: A method and an apparatus which delay initiation of a shut down procedure in a computer to allow the riding through of variations in a power supply. An interrupter receives and interrupts an asserted shut down signal and outputs an interrupt signal when the shut down signal has been asserted. A gate trigger is coupled to the interrupter and triggers an energy booster to raise the power supplied to the computer when the gate trigger receives the interrupt signal from the interrupter. The amount of energy remaining in the energy booster is monitored by a monitor and a depletion signal is outputted when the amount of energy remaining in the energy booster is below a reference value. An output stage has inputs coupled to the interrupter and the monitor, and receives at these inputs the interrupt signal and the depletion signal and outputs a re-asserted shut down signal to the computer when both the interrupt signal and the depletion signal are present at the inputs of the output stage.

Abstract: A voltage fault detector (10) for producing a trip signal causing switching from a first AC voltage source (12) to a second AC voltage source (14) is disclosed. A plurality of comparators (18,20,22) monitor a signal which is proportional to the magnitude of the voltage of a first AC voltage source (12) to determine if sample values fall within predetermined ranges. A counter (30) counts the number of samples falling outside the predetermined range to generate the trip signal when the count reaches a predetermined limit. The counter (30) is augmented for each sample which is outside the predetermined range and decremented for each sample which is within the predetermined range.

Abstract: An overcurrent protection system (10) for a power supply is disclosed. The system monitors the instantaneous flow of current between a power supply (12) and an electrical load (16) to detect when the flow of current exceeds a maximum rated amount. A counter (30) is used for counting the number of times that samples of the flow of current exceed the maximum rated amount synchronous with zero current crossing to generate a trip signal when a predetermined count is reached which disconnects the power supply (12) from the load. The counter (30) is decremented each time a predetermined time interval elapses in which the flow of current above the maximum rate is not detected as measured from a point of zero current flow between the power supply (12) and the electrical load (14). The invention may be used for both AC and DC power supplies.

Abstract: A system for sensing the phase of a three-phase AC system and detecting phase reversal and undervoltage and phase unbalance and for providing a switching function when the system is not functioning properly.

Abstract: A system for effecting parallel operation of a pair of AC power sources includes a monitoring circuit for monitoring the difference voltage occurring between output voltages on corresponding phases of the power sources. A capacitor is connected to a charging circuit and to an output of the monitoring circuit. When the difference voltage occurring between the output voltages on corresponding phases of the power sources exceeds a predetermined threshold level, the capacitor is shorted. Paralleling of the power sources is inhibited when the capacitor voltage is below a second predetermined threshold level.

Abstract: A method and apparatus for detecting voltage losses in a power distribution line wherein a plurality of voltage sensing devices are used to generate an alarm signal which identifies the phases and locations corresponding to points at which voltage losses are detected.

Abstract: In an "AND" logic operation-oscillation circuit in which an oscillating output is produced when a signal of a voltage higher than the voltage of a circuit power source is applied a diffused resistor is used for a resistor to which a voltage lower than the circuit power source is applied and a non-diffused resistor is used for a resistor to which a voltage higher than the voltage of the circuit power source is applied, whereby there is constructed a fail-safe circuit in which a short circuit trouble is not caused even if an input signal of a level higher than the voltage of the circuit power source is applied and a phenomenon that an oscillating output is produced even in the absence of an input signal is prevented.

Abstract: Switched body circuitry is provided to prevent a system I/O from being effected by the loss of power supply or ground to an MOS integrated circuit within the system. A semiconductor substrate of a first conductivity type has formed therein a well region of a second conductivity type opposite to that of the first conductivity type. First, second, third and fourth spaced-apart shallow diffusion regions of the first conductivity type are formed at the surface of the well region. A first gate electrode and the second and third diffusion regions combine to form an MOS transistor which is either an input pull up or pull down device or an output pull up or pull down driver of the MOS circuit. A second gate electrode and the third and fourth diffusion regions combine to define a first MOS switched body transistor. A third gate electrode and the first and second diffusion regions combine to define a second MOS switched body transistor.

Abstract: An integrated protection unit is a circuit breaker which includes basic overcurrent protection facility along with selective electrical accessories. A molded plastic accessory access cover secured to the integrated protection unit cover protects the accessory components contained within the circuit breaker cover from the environment. A combined overcurrent trip actuator and multiple accessory unit can be field-installed within the integrated protection unit.

Abstract: A power supply which automatically configures itself to correctly respond to either a low or a high level of an A.C. supply voltage to supply a single level D.C. output voltage. The supply switches between a voltage doubler and a full wave rectifier operation dependent on the level of the A.C. supply voltage. Once the supply switches to the full wave rectifier configuration, a latch prevents the power supply from switching to the voltage doubler configuration until the A.C. supply voltage is removed from the power supply. The latch prevents damage to components by preventing the power supply from switching to a low input voltage configuration when, in fact, a high input voltage is present.

Abstract: A voltage shock protection circuit having two anti series connected field effect transistors adapted for being connected between a conductor and a protected circuit for connecting the protected circuit to the conductor and also protecting the protected circuit against voltage transients possibly occurring in the conductor, the voltage transients having excessively high or low potentials that could damage the protected circuit. The voltage shock protection circuit has a voltage sensing circuit connected to the protected circuit and to the conductor so that it is sensitive to voltage drops across each of the anti series connected field effect transistors and adapted such that when either of the voltage drops becomes sufficiently great one of the field effect transistors is pinched off. The voltage sensing circuit may include a field effect transistor.

Abstract: A permanent magnet generator and rectifier is connected to the exciter of an alternator and a separate rectifier provides a D.C. voltage supply to a voltage regulator. A thyristor is connected in series with a second fuse across the exciter rectifier to short circuit the supply in series with a main external fuse. If the main fuse fails to open, the second fuse opens. The second fuse is potted in the regulators to prevent tampering. An overexcitation and a low voltage demand signal are sensed. The two signals are coupled to a NAND logic gate to produce an output shut-down signal. If either even is established and the first produces a signal, a shut-down signal is generated to a second comparator amplifier of latch circuit. A delay circuit connects the shut-down signal to a triggered oscillator for firing the thyristor.

Abstract: A low voltage reset circuit includes a first transistor having a base connected to a voltage divider and an emitter connected to the junction of a resistor-diode arrangement such that when the supply voltage falls to a trigger voltage level, the transistor is driven conductive. A second transistor is connected to the collector of the first transistor and is driven conductive therewith for applying a reset voltage to the reset terminal of a microprocessor. The collector of the second transistor is returned through a resistor to the base of said first transistor to provide positive feedback such that the first transistor is maintained in conduction until the supply voltage rises above the trigger voltage level.

Abstract: A method and apparatus for providing enhanced security to the non-volatile memory of an electronic postage meter during power up and power down of the meter is disclosed. The apparatus comprises means for detecting a regulated voltage level of the meter, means for detecting an unregulated voltage level of the meter, means for detecting a predetermined frequency from a system clock associated with the meter and a reset delay circuit responsive to the detected unregulated voltage, detected regulated voltage and the predetermined clock frequency for allowing access to the non-volatile memories if the unregulated voltage level, the regulated level and the clock frequency have reached their appropriate levels and preventing access to the memories otherwise. Through the use of this circuitry the critical accounting information within the memories is protected during the powering up and powering down of the postage meter.

Abstract: A power status monitor for an electronic system includes a charge/discharge model of the system power supply. The charge/discharge model contains an estimate of the amount of time during which energy-storage devices in a system DC power supply are capable of maintaining system operation following loss of AC power. The power status monitor monitors the frequency and an amplitude of the voltage of the AC power source for signs indicating imminent failure of the AC power source. In the event that such signs are detected, the estimate in the charge/discharge model is decremented at a rate corresponding to the rate at which charge is consumed. A trip value, applied to the charge/discharge model, determines the time at which a signal is generated enabling an orderly shutdown of the system load. If acceptable AC power is resumed before the trip value is reached, the charge/discharge model increases its estimate of the time available at a rate corresponding to the charge accumulation in the DC power supply.

Abstract: Electric control apparatus (20) for equipment (1) comprising a driving circuit (5) for generating a signal (S.sub.1) to drive the equipment as installed in a vehicle, the driving circuit including circuitry having a first specified working voltage range, and a breaking circuit (6) operatively interposed between the driving circuit and the equipment, the breaking circuit including a first switching circuit (100) for operatively interrupting the supply of the drive signal to the equipment under a condition that the supply voltage (Vdd) is lowered below a first predetermined level (V.sub.

Abstract: A control circuit for shutting down a constant-current supply suitable for a double-end feeding power supply system for a long-haul wire telecommunication system. The shutdown control circuit includes a voltage detecting circuit, a current detecting circuit and an AND gate. The voltage and current detecting circuits provide detection signals when the constant-current supply provides an output voltage greater than a predetermined voltage value and the current at the output terminal is less than a predetermined current value. The predetermined voltage and current values are determined to correspond to a load circuit input resistance at which the supply of a current by the constant-current supply is to be shut down.

Abstract: The system includes a battery state-of-charge monitoring circuit which is operable to produce a signal change when there is a change in the battery charge from an acceptable charge to a substantially reduced charge. The signal change from the monitor is connected to control a lock-out relay to disable a control circuit for a load controller for battery powered equipment which is to be locked out. A current detecting device is connected to detect the presence of current in the control circuit of the controller and is connected to hold off the lock-out operation of the lock-out relay while current is flowing in the control circuit to thereby assure that the lock-out of the load occurs after a normal interruption of the load control circuit.

Abstract: A voltage regulator includes an output power element and protection circuit connected to a control terminal of the power element in such a way as to cause the element to conduct in a direction opposite to that of normal operation, when the transient overvoltages of polarity opposite to that of the generator reach a predetermined value. The energy associated with the transient is thus discharged via the output power device and it is not therefore necessary to use additional power components to carry out this function, leading to reductions in size and production costs.

Abstract: This detective circuit detects an abnormal drop of an input voltage of a switching regulator of the flyback system. From the secondary winding of the switching regulator, a constant voltage controlled dc voltage is delivered through a diode. This circuit is provided with a peak voltage charging circuit which detects the peak value of a voltage produced in a direction opposite to the forward direction of the diode, and a voltage comparator which compares the peak value with a setting value. Since this peak value is in proportion to the input voltage, the output of the voltage comparator indicates an abnormal drop of the input voltage.

Abstract: A device is provided for digitally determining and safeguarding a thermal image including a processor connected to at least one sensor able to form a digital image of the thermal state of an apparatus, a random access memory containing the thermal image formed by the processor and a safeguard for storing, should there be a voltage cut, the instantaneous thermal image. The safeguard includes a programmable digital counter fed by a capacity curing power-cuts and equipped with a clock which modifies its counting state for the duration of the absence of voltage. The processor reads the state of the counter after restoration of the voltage.

Abstract: A system for rapidly detecting power disturbances with ferroresonant transformers having a decay profile comprising the steps of rectifying a power signal; comparing the rectified signal against a reference signal; comparing the rectified signal against a dynamic reference signal; and generating a power-fail signal in response to either comparison falling below its respective reference.

Abstract: A low voltage shutdown circuit for a PWM power supply includes a voltage divider supplying bias for a transistor having a load resistor that is part of an integrating network. The capacitor in the integrating network is coupled to a voltage sensitive switch (Diac) which feeds an LED detector in a photo optic coupler. Under normal conditions, the transistor is in saturation and the capacitor is discharged. The transistor comes out of saturation when the voltage level across the divider drops to a predetermined level and the capacitor charges to the trigger voltage of the Diac, which fires the LED in the photo optic coupler for developing a shut-off signal.

Abstract: A continuous power source operating as an uninterruptible power supply functions as an on-line power management device for output loads such as computers. During normal power flow operation, alternating current power passes from a conventional AC line power input through a power line filter and a power monitor and phase shift circuit to output terminals coupled to the load. The power monitor circuitry provides control signals for synchronizing the frequency and phase of a sine wave generator, the output of which is coupled to a normally inoperative static inverter as the control signal for controlling the frequency and phase of operation of such static inverter. The output of the static inverter is coupled to the load terminals through a transformer. If the voltage of the line alternating current signal falls outside a predetermined window, the power monitor switches off power from the line to the load terminals and enables the static inverter for operation.

Abstract: A molded case circuit breaker provides both overcurrent protection as well as accessory protection function by means of a self-contained electronic trip actuator module. Undervoltage release function is provided by a separate undervoltage release assembly that includes the undervoltage coil and an electronic control circuit. Shunt trip facility is provided by the shunt trip coil and a shunt trip control circuitry. The overcurrent, shunt trip and undervoltage release coils are arranged on a magnetic circuit within the electronic actuator module.

Abstract: To protect electrical equipment against excessive applied input signals which, for example, may be analog signals, excessive with respect to positive or negative excess voltage in relation to an upper reference level (U.sub.Ref) and/or a lower reference level (GND), collector current control of the transistor (12) at the input (3) of the protective circuit by three transistors (11, 13, 14) holds the voltage level at the input (3) at the reference voltage level when a driving or signal supply circuit provides an excess voltage. The input transistors, and the further transistors, are connected in the form of a current mirror. The circuit can be constructed in dual form (11-14; 21-24) to provide for protection against negative over-voltage as well. Due to the symmetrical circuit structure, excellent temperature compensation is obtained. The circuit can easily be constructed in the form of an integrated circuit.

Abstract: A reserve power source, switchably connected to a network between a primary power supply and a load, provides an auxiliary power signal to the load when a power failure condition is detected in the signal from the primary power supply. The reserve power source includes a circuit for instantaneously detecting and switching upon the occurrence of a power outage. An overvoltage/noise detector circuit is included to detect when either a sustained overvoltage condition or a high, short duration overvoltage condition occurs. A low voltage detector circuit is included for detecting a low voltage, or brown-out, condition. When an overvoltage or low voltage condition is detected, a preferred embodiment of the invention switches the load to the auxiliary power signal within one-half cycle of, but in zero-crossing synchronization with, the primary power signal.

Abstract: An underload protection system for an electric motor connected to first and second power supply lines, the lines being connectable to an AC power supply. The system comprises a line voltage sensing circuit connected to the first line and having a line voltage signal impressed therein, and an amplitude adjustment circuit connected to the voltage sensing circuit for adjusting the line voltage signal to produce an amplitude adjusted voltage signal. A line current sensing circuit is connected to the second line and has a line current signal impressed thereon, and a phase adjustment circuit is connected to the current sensing circuit for adjusting the phase of the current signal to produce a phase adjusted current signal.

Abstract: Motor control circuits having solid-state switching elements are commonly used on industrial vehicles such as lift trucks. A short circuit condition in the motor circuit can cause catastrophic failure of the semiconductor switching element. The instant invention detects such a short circuit condition and modifies the motor control signals supplied by a chopper circuit to a switching element. The apparatus includes a trigger circuit for receiving the motor control signals and sensing a voltage across the motor produced in response to electrical power delivered from a power supply.

Abstract: A power latch circuit uses first and second bipolar transistors responsive to an actuate voltage to latch into a conductive state with power supplied to a computer and a third bipolar transistor powered by the latch circuit and responsive to a deactivate voltage from the computer to place the first and second transistors in a deactivated state to remove power from the computer and itself. It further uses an FET in parallel with the third bipolar transistor and activated directly from the DC power supply when the second transistor is deactivated to latch the first and second transistors in the deactivated state in spite of voltage fluctuations on the deactivate line which might otherwise cause the latch circuit to reactivate. As an FET, it uses little current through its gate compared to alternative bipolar devices while the latch circuit is deactivated.

Abstract: A control circuit combined with a magnetically bistable solenoid armature provides a bistable undervoltage release function to a circuit breaker. The solenoid is fabricated from a material having high magnetic remnance and low coercivity. A first signal to the solenoid coil develops a high remnance flux to hold the armature against a spring force. A second signal, of opposite polarity and predetermined voltage, produces an opposing magnetic flux to cancel that retained by the solenoid. The armature then becomes extended under the influence of a charged spring to trip the breaker.

Abstract: A protection circuit for monitoring and recognizing an undervoltage condition in the electrical system of a motor vehicle having an anti-skid brake control system in order to minimize and reduce any adverse effects on the braking action of the vehicle wheels. The protection circuit includes at least one comparator circuit to recognize if the voltage falls below a predetermined reference threshold voltage. When the supply voltage falls below the threshold voltage, the comparator circuit deactivates at least one of two diagonal switching circuits to remove automatic brakes on at least two wheels of a vehicle by the anti-skid brake control system.

Abstract: An anticipatory power failure detection apparatus and method, for use in connection with power supplies providing a regulated voltage signal to an electronic load such as a computer, employing circuitry for determining whether a control signal, within the regulated power supply circuit, is in a saturation state. Upon detecting a saturation state for the control signal, circuitry signals that an input power failure is occurring. The regulated power supply system can be a switching-type voltage regulation control system, wherein a pulse-width modulated switching system controls the periodic input of energy to an energy-storage network which generates the regulated voltage output signal. A derivative of the error signal is periodically sampled for determining whether it is in the saturation state. The regulated power supply control signal will be in the saturation state when the unregulated input voltage supply is insufficient to maintain the regulated voltage signal at the specified optimum operating point.

Abstract: A time delay undervoltage release for an electric circuit breaker causes the circuit breaker to trip when the control voltage drops below an acceptable level and prevents the breaker from being closed until the control voltage rises to a predetermined level. When the control voltage drops below an acceptable level, the undervoltage release energizes a trip solenoid to trip the breaker. A variable delay is interposed between the control voltage dropping below the acceptable level and the trip solenoid being energized. At the same time the trip solenoid is energized, an inhibit solenoid is deenergized to block the operation of the push-to-close button. When the control voltage increases to a predetermined level, the inhibit solenoid is energized, allowing the push-to-close button to be manually operated and the breaker returned to service.

Abstract: When a d-c bus voltage is produced by rectifying applied a-c power line voltage in a rectifier bridge (such as a phase-controlled SCR rectifier bridge), controlled by a feedback loop which compares a feedback signal representing the d-c bus voltage with a command signal representing a desired set point level and from the comparison automatically maintains the bus voltage at the set point level, unwanted a-c line voltage interruptions may have a deleterious effect on the rectifier bridge itself and on a load driven by the bus voltage. This occurs because during a power interruption (when the bus voltage drops) the command signal causes the feedback loop to impose a control on the rectifier bridge which attempts to increase the d-c bus voltage back to the magnitude represented by the command signal. When power is subsequently restored, the d-c bus voltage will suddenly increase very sharply and may destroy electrical and/or mechanical components in the system.

Abstract: An electronic apparatus including a circuit generating a control signal activating at least one heavy load includes a voltage comparision circuit for comparing a power source proportional voltage with a reference voltage and outputting a power-off signal when the level of the power source proportional voltage is smaller than that of the reference voltage, and a power-off detection circuit responsive to the power-off signal of the comparision circuit for detecting the level of the control signal associated with the at least one heavy load and detecting a power-off condition of the electronic apparatus.

Abstract: A printer comprising a printing mechanism, a control circuit which controls said printing mechanism and a power supply including battery, wherein said power supply provides the first stabilized power source circuit which converts an output voltage of said battery to the predetermined first supply voltage and then supplies it to said printing mechanism and the second stabilized power source circuit which converts an output voltage of said battery to the second supply voltage which is lower than said first supply voltage and then supplies it to said control circuit, and moreover a detecting circuit, which is composed of a transistor and resistors and detects that difference between the supply voltages of said first stabilized power source circuit and second stabilized power source circuit has reached a value lower than the predetermined threshold value, is also provided, and moreover the printing operation is controlled on the basis of an output signal of the detecting circuit.

Abstract: Current in the drain-source path of a microwave high-power GaAs MESFET is controlled by a regulating circuit including an integrating differential amplifier whose output controls the gate of the MESFET. One input of the differential amplifier is supplied with a fraction of the supply voltage, and the other is supplied with a voltage dependent upon the current to be controlled. During switch-on and switch-off of the supply voltage, the risk of Gunn effect oscillation of the MESFET is avoided by a switching circuit which controls the differential amplifier to maintain substantially zero drain-source current when the supply voltage is below a predetermined level less than its nominal level.

Abstract: A bistable multivibrator circuit responds to the voltage across a device to be protected. The circuit changes its state when this voltage drops below a predetermined value.