JPH053765B2 - - Google Patents

Description

[Detailed Description of the Invention] Field of the Invention The present invention has a sensor circuit such as a proximity sensor or a photoelectric sensor, and is configured to be connected in series between a load and a power source and to directly control the load based on a detection output. This relates to two-wire electronic switches.

Prior art and its problems Two-wire electronic switches such as photoelectric switches and proximity switches that are configured to directly control a load have a detection section and a switching element such as a thyristor, and switch based on the detection output. The switching element is driven to control the supply of power to the load. Such a two-wire electronic switch turns the load on and off simply by connecting it in series with the power source and the load, so it can be used in the same way as a normal mechanical contact switch, making it extremely convenient. However, if the load becomes internally shorted due to an accident, or by mistake, the power source may be connected directly 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.

To solve this problem, a low resistance for overcurrent detection is installed in series with the switching element, and when the voltage across the resistance exceeds a certain value, the short-circuit protection circuit is activated and the switching element is opened. Various protection circuits have been proposed to control this. However, all of these protection circuits require a terminal to be connected to the current detection terminal or the output of the detection section, and must be integrated with each circuit section of the switch. There was a problem in that the configuration of the protection circuit for this purpose became complicated. Also, two-wire electronic switches often use AC power as the power source, but AC two-wire electronic switches turn off the switching element once at zero cross and quickly charge the capacitor that powers the detection section. When charging is completed, the switching element is driven again to supply power to the load. Even when such a load is driven, the switching element is turned on and off every half cycle, resulting in a constant voltage remaining across the electronic switch, which causes a problem in that the voltage applied to the load decreases.

Purpose of the Invention The first invention of the present application has been made in view of the problems of the conventional electronic switch, and is aimed at increasing the charging period of the power supply capacitor of the detection section and reducing the residual voltage during operation of the electronic switch. The purpose is to make it smaller. In addition to the object of the first invention, the second invention of the present application provides an electronic switch that can detect a short circuit by providing an overcurrent detection resistor in the switching element itself.

Structure and Effects of the Invention The first invention of the present application is an electronic switch that is connected to a load and a power source and directly controls the load, which includes a detection circuit section, a power storage element that supplies power to the detection circuit section, and a power storage element that is connected in series to the load. A field-effect transistor for output switching is connected to the output terminal and the gate potential is limited to a certain value or less, and a field-effect transistor is connected between the output terminal of the detection circuit section and the gate of the field-effect transistor, and the output terminal of the detection circuit section is connected to conduction. The thyristor is connected between the field-effect transistor and the power input terminal of the detection circuit, and is connected between the field-effect transistor and the power input terminal of the detection circuit to drive the field-effect transistor. a charging circuit that charges the power storage element when it is off; a switching circuit that is connected in parallel with the charging circuit and that charges the power storage element each time the voltage of the power storage element drops and the thyristor becomes non-conductive when the detection circuit section is turned on; It is characterized by comprising the following.

A second invention of the present application is an electronic switch that is connected to a load and a power source to directly control the load, and includes a detection circuit section having a reset terminal that is reset by a voltage signal, and a storage battery that supplies power to the detection circuit section. a field-effect transistor for output switching, which is connected in series with the load and whose gate potential is limited to a certain value or less; and a field-effect transistor connected between the output end of the detection circuit section and the gate of the field-effect transistor, and the detection circuit. A thyristor that becomes conductive by the output of the section to drive the field effect transistor, and becomes nonconductive due to a drop in the voltage applied to the detection circuit section, is connected between the field effect transistor and the power input terminal of the detection circuit section. A charging circuit that charges the storage element when the detection circuit section is off is connected in parallel with the charging circuit, and is connected in parallel with the charging circuit to charge the storage element when the detection circuit section is turned on. A switching circuit for charging, an overcurrent detection resistor connected in series with the field effect transistor, and a reset circuit that detects the terminal voltage of the overcurrent detection resistor and prohibits the output of the detection circuit section for a predetermined period of time. It is characterized by:

According to the first aspect of the present invention having such features, when the electronic switch is connected to a power source, the charging circuit charges the power storage element and the detection circuit starts operating. When the detection circuit section is turned on, the thyristor becomes conductive, the field effect transistor is driven, and current is supplied to the load. At this time, power is being supplied to the detection circuit section from the power storage element, and the voltage thereof gradually decreases. When the supply voltage falls below a predetermined value, the thyristor becomes non-conductive, and the storage element is rapidly charged by the switching circuit. Therefore, the detection circuit section starts operating again, the field effect transistor becomes closed, and the original operation is restored. Therefore, the power storage element is rapidly charged intermittently.

Therefore, according to the present invention, even when an AC power source is used, the power storage element can be charged regardless of the cycle of the AC power source. Therefore, the charging interval becomes longer, and it is possible to lower the residual voltage of the electronic switch, which is the average value of the terminal voltage during charging. Furthermore, by limiting the gate potential of the field effect transistor for output switching to a predetermined value or less, it is possible to limit the overcurrent flowing through the field effect transistor at the time of a short circuit. Furthermore, since the overcurrent limiting resistor connected in series with the switching element can be omitted, this contributes to lowering the residual voltage, and the structure becomes simpler and can be made smaller.

According to the second invention of the present application, when a short circuit occurs and an overcurrent flows, the voltage across the overcurrent detection resistor increases, and if the reset circuit detects an increase in the terminal voltage, a predetermined value is reached. The output of the time detection circuit section is prohibited. Therefore, a special protection circuit is not required, and the configuration can be simplified.

DESCRIPTION OF THE EMBODIMENTS FIG. 1 is a circuit diagram in which the present invention is applied to an AC two-wire proximity switch. In this figure, terminals 1 and 2
A diode bridge 3 is connected between them. The diode bridge 3 rectifies the alternating current voltage applied between the terminals 1 and 2, and an avalanche diode 4 for absorbing surge voltage is connected between its positive and negative output terminals, and a field effect for power is connected in parallel to it. A series transistor such as a MOSFET 5 and a resistor R6 for overcurrent detection are connected to the transistor, and a constant voltage circuit having a Zener diode 7 and a transistor 8 is further connected. A Zener diode 9 is connected between the gate of the FET 5 and the other end of the overcurrent detection resistor R6 to keep the gate voltage applied to the FET 5 constant. A transistor 10 for supplying current to the detection circuit is connected to the output terminal of the transistor 8 of the constant voltage circuit, and the output terminal is connected to the power input terminal of the sensor circuit 13 via a light emitting diode 11 and a diode 12 for display. Connected to Vcc.
The sensor circuit 13 is an IC circuit that has an internal oscillator, detects the proximity of an object based on changes in its oscillation state, and outputs a digital output, and uses a circuit that consumes as little power as possible. A bypass resistor R14 is provided in parallel with the transistor 10 and the light emitting diode 11, and a capacitor C15 for supplying voltage to the sensor circuit 13 is connected between the power input terminal Vcc of the sensor circuit 13 and the ground. Here, the resistor R14 constitutes a charging circuit that charges a capacitor C15, which is a power storage element of the sensor circuit 13. A resonant circuit including a detection coil 16 and a capacitor C17 is connected to the sensor circuit 13. The sensor circuit 13 is provided with an NH output that outputs an "H" level output that is almost the same as the power supply voltage when no object is detected, and an NL output that outputs an "L" level output that is almost the same as the ground terminal. A reset terminal is provided for inactivation. Resistors R18 and R19 for supplying base current to the transistor 10 are connected to the NH output terminal, and a thyristor 21 is connected to the NL output terminal via a resistor R20. The thyristor 21 is conductive when this switch is turned on, and its cathode is connected to the gate of the FET 5, which is an output switching element, and the gate of the thyristor 21 and the diode 1 are connected to each other.
A Zener diode 22 is connected between the common connection points of 1 and 12.
is connected. Here, the transistor 10 and the resistors R18 and R19 constitute a switching circuit that charges the capacitor C15 each time the voltage of the capacitor C15 decreases and the thyristor 21 becomes non-conductive when the sensor circuit 13 is turned on. The sensor circuit 13 is reset when the voltage applied to its reset terminal becomes lower than the reset voltage Vr, and a capacitor C23 and a transistor 24 are connected in parallel between the reset terminal and the ground terminal. Capacitor C23 is provided to provide timing to prevent erroneous output from occurring immediately after power is turned on. The transistor 24 operates to reset the sensor circuit 13 when an overcurrent is detected, and its base terminal is connected to the source of the FET 5 and the overcurrent detection resistor R6 via a resistor R25.
connected to a common connection point. Here, the transistor 24 and the resistor R25 are a reset circuit that detects the terminal voltage of the overcurrent detection resistor R6 and resets the sensor circuit 13 for a certain period of time by discharging the capacitor when the terminal voltage exceeds a predetermined value. It consists of Power supply voltage of sensor circuit 13
Vcc is approximately determined by the Zener diode 7 when the transistor 10 is off. Strictly speaking, the power supply voltage Vcc is (Zena voltage of Zener diode 7 - ON voltage of transistor 8 - Voltage drop of resistor R14 - ON voltage of diode 12). On the other hand, the power supply voltage Vcc in this case is lower than the voltage Va, which is the sum of the Zener voltages of Zener diodes 9 and 22.
The zener voltage Vb of the zener diode 9 is set higher than that of the zener diode 9. Also, the reset voltage Vr is the Zener voltage.
A voltage even lower than Vb is selected. In the following explanation regarding the operating voltage, it is necessary to take into account the forward voltage drop of the transistor, but to simplify the explanation, this voltage drop will be ignored.

Next, the operation of this electronic switch will be explained with reference to the waveform diagram in FIG. First, an AC power source 30 and a load L are connected in series between terminals 1 and 2 as shown in the figure. FIG. 2a shows the voltage waveform of the AC power supply 30. Then, a small current flows through the load L, and the AC voltage of the AC power supply 30 is rectified by the diode bridge 3, and converted into a DC power supply of a predetermined voltage by the constant voltage circuit of the Zener diode 7 and the transistor 8. Power is supplied to the sensor circuit 13 via the resistor R14 and the diode 12, and the capacitor C15 is charged. Then, the oscillator of the sensor circuit 13 oscillates due to the resonant circuit formed by the detection coil 16 and the capacitor C17 connected to the sensor circuit 13, and a state is reached in which the object can be detected. However, if no object is detected, the NH output is at the "H" level and the NL output is at the "L" level, so no base current flows to the base of the transistor 10, and the transistor 10 is in an off state, and the light emitting diode 11 It also doesn't light up. Furthermore, as mentioned above, the power supply voltage of the sensor circuit 13 is set to be lower than the voltage Va, which is the sum of the Zener diodes 9 and 22, so no gate current flows through the Zener diode 22 to the thyristor 21. Therefore, the thyristor 21 is also in an off state. Therefore, the FET 5 is in an off state and the load L is not driven, power is supplied to the sensor circuit 13 via the bypass resistor R14, and the sensor circuit 13 continues to operate and wait for an object. Then, when the oscillation state changes due to the proximity of an object at time t1,
The NH output of the sensor circuit 13 is at the "L" level as shown in FIG. 2d, and the NL output is at the "H" level, contrary to the NH output. Then, a base current flows through the transistor 10 through the resistors R19 and 18, turning it on, and the resistor R14 is short-circuited. Therefore, as the light emitting diode 11 lights up, the power input voltage of the sensor circuit 13 rises rapidly as shown in FIG. 2b. And if the voltage becomes more than the voltage Va of the sum of Zener diodes 9 and 22,
A gate current flows through the gate of the thyristor 21 and the thyristor 21 is turned on. Therefore, a gate voltage determined by the Zener diode 9 is applied to the gate of the FET 5, and the FET 5 is turned on and the load L is driven. If the current flowing through the load L is within the rated current value of this electronic switch, the voltage across the overcurrent detection resistor R6 is below a certain value, and the voltage across the proximity switch is as shown in Figure 2c. The residual voltage becomes a value close to zero, and there is no adverse effect on the load L. While the FET 5 is on, the sensor circuit 13 is supplied with current from the capacitor C15. At this time, since the FET 5 has a high input impedance, almost no current flows through the gate, and the amount of charge consumed by the capacitor C15 is determined by the current flowing through the sensor circuit 13 and the thyristor 21. Therefore, if the sensor circuit 13 is configured with a low power consumption type IC as described above,
The voltage of capacitor C15 will gradually decrease. Now, when the voltage of the capacitor C15 decreases and becomes lower than the voltage Vb of the Zener diode 9 at time t2 as shown in FIG. 2b, the thyristor 21 is turned off, and at the same time, the FET 5 is turned off. Then, a voltage approximately equal to that of the AC power supply 30 is applied between the terminals 1 and 2 as shown in FIG.
A large current flows through the capacitor C15, rapidly charging the capacitor C15, and as a result, the voltage across the capacitor C15 rises as shown in FIG. 2b. Then, at time t3 when the voltage value reaches the voltage Va determined by the Zener diodes 9 and 22, the thyristor 21 is turned off again as described above, the FET 5 becomes conductive, and from then on, the load current is similarly supplied to the load L. Ru. In this way, the capacitor C15 for the power supply of the sensor circuit 13
Since charging is performed and charge is supplied only when the voltage across the switch falls below a predetermined value, the moment of charging is extremely short compared to the time that the electronic switch is conducting, and the residual voltage at both ends of the electronic switch is extremely short. It becomes possible to reduce the voltage.

Now, if an overcurrent flows to the load L or if the load L is short-circuited, the FET 5 and the overcurrent detection resistor R
A large current flows through 6. However, the gate voltage of the FET 5 is limited to the Zener voltage of the Zener diode 9, and due to its constant current characteristics, the short circuit current is suppressed below a certain value. Now in Fig. 3, time t
4, the terminal voltage of resistor R6 increases, transistor 24 is turned on, and capacitor C23 is rapidly discharged. Then, as shown in FIG. 3b, the voltage at the reset terminal of the sensor circuit 13 drops instantaneously to the reset voltage Vr.
As a result, the sensor circuit 13 is reset and NL
The output becomes the original "L" level and the thyristor 21 is turned off. Therefore, FET5 is also turned off to prevent short circuit. Thereafter, as shown in FIG. 3b, the capacitor C23 connected to the reset terminal is gradually charged by supplying current from the power supply through a resistor (not shown) in the sensor circuit 13, but the voltage across the capacitor C23 reaches the reset voltage Vr. When the above conditions are met, the sensor circuit 13 starts operating, and when an object is detected, the NL output becomes "H" level, the thyristor 21 is triggered, and the FET 5 is also turned on at the same time. Therefore, at time t5, the short circuit current is detected by the overcurrent detection resistor R6 through which the short circuit current flows again, and the sensor circuit 13 is turned off again. In this way, the FET 5 is intermittently turned on and a large current flows. However, as shown in FIG. 3, the period in which the short circuit current flows is determined by the time constant for charging and discharging the capacitor C23, regardless of the period of the AC power supply. Therefore, by setting this time constant to a large value, the period of the current flowing through the FET 5 can be increased, and it is possible to prevent the FET 5 from generating heat. Furthermore, since the light emitting diode 11 lights up at the moment when a short circuit current flows through the FET 5, it is also possible to detect a short circuit state based on the blinking period.

FIG. 4 is a circuit diagram showing another embodiment of the electronic switch according to the present invention. In this embodiment, the sensor circuit 1
3 is a circuit diagram showing a configuration when only the NL output is used, and the same parts as in the above-mentioned embodiment are given the same reference numerals. In this embodiment, since the transistor 10 is operated using the NL output, the resistor R18 and R
This embodiment is the same as the embodiment shown in FIG. 1 except that a transistor 31 is connected between the transistors 19 and 19 to supply a base current to the base of the transistor 10. In this embodiment, when the sensor circuit 13 detects an object, the transistor 31 is turned on by the NL output, the transistor 10 is driven, and the capacitor C15 is rapidly charged. It is also possible to perform the gate circuit of the FET 5 using only the NL output of the sensor circuit 13 and using the thyristor 32 and the Zener diode 33 connected between its gate and anode, as shown in the schematic diagram in FIG. be. In this case as well, when the total voltage of the Zener diode 33 and the Zener diode 9 is reached, the thyristor 32 is triggered and the FET 5 is turned off.

Although the present embodiment has been described as an AC two-wire proximity switch, it goes without saying that the present invention can be applied to other two-wire electronic switches such as photoelectric switches. Further, although this embodiment uses an AC power source as the power source, it is also possible to apply the present invention to an electronic switch using a DC power source. In this case as well, the power supply capacitor C15 is rapidly charged intermittently by a time constant determined by the overcurrent inside the electronic switch to supply power to the sensor circuit 13, and in the event of a short circuit, the capacitor C23 is charged and discharged in a charging/discharging cycle. It is possible to make the light emitting diode 11 light up to indicate the short circuit condition.

In the embodiment described here, a capacitor is used as a power source for the sensor circuit, but a rechargeable small-capacity secondary battery may also be used. This allows the sensor circuit to operate for a longer period of time than when using a capacitor, making it possible to significantly lengthen the charging interval and further lowering the residual voltage when the electronic switch is operating. becomes possible.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the electronic switch according to the present invention, Fig. 2 is a waveform diagram showing the waveforms of each part during normal operation, and Fig. 3 is a waveform diagram showing the waveforms of each part during short circuit. , FIG. 4 is a circuit diagram showing another embodiment of the present invention, and FIG. 5 is a circuit diagram showing the main part of still another embodiment. 1, 2... terminal, 5... FET, 7, 9, 20,
33... Zener diode, R6... Resistor for overcurrent detection, 8, 10, 24, 31... Transistor, 11... Light emitting diode, C15, C17,
C23... Capacitor, 21... Thyristor, 3
0...AC power supply.

Claims (1)

[Scope of Claims] 1. A detection circuit section, a storage element that supplies power to the detection circuit section, and a field effect transistor for output switching, which is connected in series to a load and whose gate potential is limited to a certain value or less. and connected between the output terminal of the detection circuit section and the gate of the field effect transistor, conducts by the output of the detection circuit section to drive the field effect transistor, and a voltage applied to the detection circuit section. a thyristor that becomes non-conductive due to a decrease in the voltage; a charging circuit that is connected between the field effect transistor and the power input terminal of the detection circuit section and charges the storage element when the detection circuit section is turned off; The switching circuit is connected in parallel with the circuit and charges the power storage element each time the voltage of the power storage element decreases and the thyristor becomes non-conductive when the detection circuit part is turned on. Electronic switch. 2. The electronic switch according to claim 1, wherein the electricity storage element is a capacitor. 3. The electronic switch according to claim 1, wherein the electricity storage element is a rechargeable secondary battery. 4 Claims characterized in that the gate potential of the field effect transistor is limited to a certain value or less by a Zener diode provided between its gate and the other end of the overcurrent detection resistor. The electronic switch according to item 1. 5 a detection circuit section having a reset terminal that is reset by a voltage signal; a storage element that supplies power to the detection circuit section;
a field-effect transistor for output switching, which is connected in series to a load and whose gate potential is limited to a certain value or less; a thyristor that is rendered conductive by the output of the section to drive the field effect transistor and rendered non-conductive by a drop in the voltage applied to the detection circuit section; a power input for the field effect transistor and the detection circuit section; a charging circuit that is connected between the terminals and charges the electricity storage element when the detection circuit section is off; and a charging circuit that is connected in parallel with the charging circuit and that reduces the voltage of the electricity storage element when the detection circuit section is turned on; a switching circuit that charges the power storage element each time it becomes non-conductive; an overcurrent detection resistor connected in series with the field effect transistor; and a switching circuit that detects a terminal voltage of the overcurrent detection resistor; An electronic switch comprising: a reset circuit that applies a signal to a reset terminal of the detection circuit section to inhibit output of the detection circuit section for a predetermined period of time. 6. The electronic switch according to claim 5, wherein the electricity storage element is a capacitor. 7. The electronic switch according to claim 5, wherein the power storage element is a rechargeable secondary battery. 8 Claims characterized in that the gate potential of the field effect transistor is limited to a certain value or less by a Zener diode provided between its gate and the other end of the overcurrent detection resistor. The electronic switch according to item 5.