JPH0462209B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an AC two-wire type electronic switch in which a load is directly controlled by a switching element, and more particularly to an electronic switch featuring short circuit protection.

Conventional technology and its problems A two-wire electronic switch such as a photoelectric switch or a proximity switch that is configured to directly control a load has a detection circuit section and a switching element such as a thyristor. The switching element is driven to control the supply of power to the load. However, if the load becomes internally short-circuited due to an accident, or by mistake, the power source may be directly connected to the electronic switch without connecting the load. If the switching element is closed in such a case, a short-circuited large current will flow through the switching element, which may destroy the element or even cause it to explode, potentially causing damage to humans and surrounding objects. It was hot.

It is also practiced to connect a current limiting resistor in series with the switching element to limit the current flowing through the switching element. However, it has been difficult to miniaturize such a current limiting resistor because it requires a large power capacity. Furthermore, if the resistance value is increased in order to suppress the short circuit current, there is a problem in that the residual voltage of the electronic switch increases during normal operation.

Purpose of the Invention The present invention solves the problems of conventional electronic switches, and provides an alternating current switch that can reliably protect switching elements in the event of a short circuit and can reduce residual voltage during operation. It provides a two-wire electronic switch.

Structure and Effects of the Invention The present invention is an AC two-wire type electronic switch that directly controls a load using a switching element, and includes a detection circuit section and a current limiting resistor connected between two terminals of the electronic switch. A series connection body with the triax that is driven only in the event of a short circuit, an output switching element connected in parallel to the triax and driven by the output of the detection circuit to control the load, and a series connection element connected to the output switching element. a first overcurrent detection resistor that is connected to the circuit and supplies a trigger signal to the gate of the triac in the event of a short circuit; a second overcurrent detection resistor that is connected in series to the output switching element and detects an overcurrent; When the voltage across the second overcurrent detection resistor exceeds a certain level, the thyristor is triggered before the tri-attack and resets the operation of the detection circuit, and is maintained by the power supply supplied to the detection circuit. It is characterized by comprising the following.

According to the present invention having such features, when a predetermined overcurrent flows, the thyristor is immediately driven and the operation of the detection circuit section is reset. Subsequently, the triax is triggered by the trigger signal of the first overcurrent detection resistor, so the short-circuit current flows through the triax and no longer flows to the switching element for switching the output, making it possible to protect the switching element for switching the output. becomes. Further, the short-circuit current flowing through the triax is limited by the current limiting resistor, but the surge current resistance of the triac is greater than that of a thyristor, so the resistance value of the current limiting resistor can be made small. Therefore, it is possible to reduce the residual voltage generated by this resistance during normal operation. Furthermore, since the thyristor that resets the detection circuit section is maintained by the power supply supplied to the detection circuit section, short circuit current will not flow after the zero cross of the alternating current, thereby protecting the electronic switch.

DESCRIPTION OF EMBODIMENTS FIG. 1 is a circuit diagram showing an embodiment of an electronic switch according to the present invention. In this figure, a current limiting resistor R3 and a triac 4 are connected in series between terminals 1 and 2, and a diode bridge 5 and an overcurrent detection resistor R are connected in parallel to the triac 4.
6 is connected. The connection point between the overcurrent detection resistor R6 and the diode bridge 5 is connected to the gate of the triac 4. The current limiting resistor R3 is a low resistance selected so that the current flowing through the triax 4 in the event of a short circuit is below its surge current value, and the overcurrent detection resistor R6 detects the short circuit current and sends a trigger signal to the triax 4. The resistor has an extremely low resistance value of, for example, 0.1Ω or less. Since the resistance value of this resistor R6 is extremely small, the C/R circuit normally provided at the gate of the triac as a surge absorber becomes unnecessary. The diode bridge 5 rectifies the AC voltage applied between the terminals 1 and 2, and has an avalanche diode 7 and a thyristor 8 for absorbing surge voltage, and a resistor R9 for overcurrent detection between its positive and negative output terminals. are connected in parallel, and a constant voltage circuit 10 is further connected in parallel to them. The thyristor 8 is an output switching element that switches the load current depending on the load connected between the terminals 1 and 2. This thyristor 8 has a necessary and sufficient rated current (e.g.
2A), and avoid using an unnecessarily large thyristor. This is the rated current
For thyristors below 2A, the gate trigger current is several
The gate trigger current may be about mA, but in the case of a thyristor with a rated current of 3A or more, the gate trigger current requires several tens of mA or more. The constant voltage circuit 10 supplies a constant voltage via a trigger circuit 12 to a sensor circuit 11 which is a detection section formed as an IC. The trigger circuit 12 is provided with a transistor 13 for current amplifying the output of the sensor circuit 11 and providing a gate signal to the thyristor 8, and a transistor 14 for charging a power supply capacitor. The base of the transistor 14 is connected to the collector of the transistor 13 via a Zener diode 15, and the collector is connected to the power input terminal Vcc of the sensor circuit 11 via a diode 16. A smoothing and power supply capacitor C17 is connected between the power supply terminals of the sensor circuit 11. Further, a bypass resistor R18 is connected between the collector and emitter of the transistor 14 for charging the capacitor C17 and providing a power supply voltage to the sensor circuit 11 when the transistor 14 is off. In addition, a resistor R19 and a thyristor 2 are connected to the power output terminal of the trigger circuit 12.
0 is connected in series. A reset terminal of the sensor circuit 11 is connected to the common connection end of the resistor R19 and the thyristor 20 via a diode 21. Further, one end of the overcurrent detection resistor R9 is connected to the gate of the thyristor 20 via a resistor R22. Since the overcurrent detection resistor R9 provides a gate signal to the thyristor 20 in the event of a short circuit, a low resistance is selected, but the thyristor 20 shall be triggered at a current value lower than the short circuit current value at which the gate signal is provided to the triac 4. . The sensor circuit 11 also includes a detection coil 23 and a capacitor C24.
A resonant circuit is connected to the

Now, connect the AC power supply 30 between terminals 1 and 2 as shown in the figure.
and load L are connected in series. Then the load L
A minute current flows through the resistor R3, and the AC voltage of the AC power source 30 is rectified by the diode bridge 5, converted to a DC voltage of a predetermined voltage by the constant voltage circuit 10, and then passed through the resistor R18 and the diode 16. Power is supplied to the sensor circuit 11 and the capacitor C17 is charged. Then, the oscillator of the sensor circuit 11 oscillates due to the resonance circuit formed by the detection coil 23 and the capacitor C24 connected to the sensor circuit 11, and the sensor circuit 11 enters an operating state for detecting an object. Then, when the oscillation state changes due to the proximity of an object, the sensor circuit 11 outputs an H level output that is approximately equal to the power supply Vcc, the transistor 13 is turned on, and the thyristor 8 is triggered. Then, the thyristor 8 is turned on, current is supplied to the load L via the diode bridge 5, and the load L is driven. At this time, a voltage drop occurs across the resistors R3, R6, and R9, but as each resistor has a low resistance value as described above, the triac 4 and thyristor 20 are not triggered, and the residual voltage of the electronic switch is reduced. kept small. In each subsequent cycle, when the AC power supply 30 passes the zero cross point, the thyristor 8 is temporarily turned off.
The transistors 13 and 14 are turned on and the capacitor C17 is rapidly charged. When the Zener voltage of the Zener diode 15 is reached, the transistor 14 is turned off and the thyristor 8 is turned on. In this way, power is supplied to the sensor circuit 11 and the phase of the thyristor 8 is controlled.

Now, when an overcurrent flows through the load L or when the load L is short-circuited, the short-circuit current flows through the resistor R3, the diode bridge 5, the thyristor 8, and the overcurrent detection resistors R9 and R6. First, the voltage of the overcurrent detection resistor R9 rises instantaneously, so a gate signal is transmitted to the thyristor 20 and the thyristor 20 is turned on. Then, the voltage at the anode end of the thyristor 20 decreases, so the voltage at the reset terminal of the sensor circuit 11 decreases, and the sensor circuit 11 is reset. The short-circuit current flowing through the thyristor 8 also flows through the short-circuit current detection resistor R6, and when the voltage across the resistor R6 exceeds a certain level, the triac 4 is turned on. Here, the resistance values of the resistors R9 and R6 are selected so that the triax 4 is turned on after the thyristor 20 is turned on first, as described above. When the triax 4 is triggered and turns on, the short circuit current flows through the resistor R3 and the triax 4, the voltage across the thyristor 8 drops to a value close to zero volts, no gate signal is applied, and the thyristor 8 turns off. . Therefore, from now on, most of the short circuit current will flow through the triac 4. Furthermore, since the thyristor 20 remains on due to the discharge current of the capacitor C17, the sensor circuit 11 also remains in the reset state. Therefore, the sensor circuit 11 does not output an H level signal. In this way, the trigger signal is no longer applied to the thyristor 8, and the thyristor 8 continues to be turned off. In the next half cycle, the thyristor 20 remains on due to the discharge current of the capacitor C17, so the sensor circuit 11 maintains the reset state and the thyristor 8 does not turn on. Therefore, since no current flows through the resistor R6, the triac 4
The triac 4 never turns on, and only a current limited by the resistor R3 flows through the triax 4 from the point of short circuit to the point of zero cross. In this way, in the event of a short circuit, it is possible to turn off the output switching thyristor by passing the short circuit current through the triac, which has a higher surge current resistance than the thyristor, and it is also possible to turn off the triac in less than half a cycle. Therefore, the capacitance of the diode bridge 5 does not need to be very large, and the resistance value of the current limiting resistor R3 can also be made small. Therefore, it is possible to reduce the residual voltage when the electronic switch is normally turned on. Further, since current is supplied to the sensor circuit 11 and the capacitor C17 through the resistor R18, the thyristor 2 continues until the power supply 30 is cut off.
0 continues to be in the on state, and no short circuit current will flow again.

Although the present embodiment has been described as a proximity switch, it goes without saying that the present invention can be applied to photoelectric switches and other types of AC two-wire electronic switches.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of an AC two-wire electronic switch according to the present invention. 1, 2...terminal, R3...current limiting resistor, R
6, R9... Resistor for overcurrent detection, 4... Triack, 8, 20... Thyristor, 10... Constant voltage circuit, 11... Sensor circuit, 12... Trigger circuit.

Claims (1)

[Scope of Claims] 1. A detection circuit unit, a series connection body of a current limiting resistor connected between two terminals of an electronic switch, and a triax that is driven only in the event of a short circuit, and a triax connected in parallel to the triax, an output switching element that is driven by the output of the detection circuit section to control the load; and a first fault circuit that is connected in series with the output switching element and that provides a trigger signal to the gate of the triax in the event of a short circuit. a current detection resistor; a second overcurrent detection resistor connected in series with the output switching element to detect overcurrent; and a voltage across the second overcurrent detection resistor that is equal to or higher than a certain level. is triggered before the triax and resets the operation of the detection circuit section when
An AC two-wire electronic switch comprising: a thyristor held by a power supply supplied to the detection circuit section. 2. The AC two-wire electronic switch according to claim 1, wherein the current limiting resistor is selected so that the current flowing through the triac in the event of a short circuit is equal to or less than its surge resistance.