JPH053766B2 - - 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 opened 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 people and surrounding objects. Ta.

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. Therefore, there was a problem in that the configuration of the protection circuit 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. As described above, even when driving a load, the switching element is turned on and off every half cycle, and a constant voltage remains at both ends of the electronic switch, causing 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 conventional electronic switches, and is aimed at increasing the charging cycle 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 light-emitting element is connected in series with a field-effect transistor for output switching, whose gate potential is limited to a certain value or less, and a voltage drop element provided between the output end of the detection circuit and the gate of the field-effect transistor. a photocoupler connected to the photocoupler, whose output short-circuits the voltage drop element to drive the field effect transistor, and which stops the light emitting element from emitting light due to a drop in the voltage applied to the detection circuit; A charging circuit is connected between the type 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 charging circuit is connected in parallel with the charging circuit, and the voltage of the storage element drops when the detection circuit section is turned on. The photocoupler is characterized in that it includes a switching circuit that charges the power storage element each time the photocoupler stops emitting light and becomes non-conductive.

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 that is connected in series with the load and whose gate potential is limited to a certain value or less, and a voltage drop element provided between the output end of the detection circuit and the gate of the field effect transistor. A light emitting element is connected in series with the light emitting element, and its output short-circuits the voltage drop element to drive the field effect transistor, and the light emitting element stops emitting light due to a drop in the voltage applied to the detection circuit section. a charging circuit that is connected between the field effect transistor and the power input terminal of the detection circuit section and charges the power storage element when the detection circuit section is turned off;
A switching circuit is connected in parallel with the charging circuit, and charges the storage element each time the voltage of the storage element drops when the detection circuit turns on, and the photocoupler stops emitting light and becomes non-conducting. The present invention is characterized by comprising an overcurrent detection resistor connected thereto, and a reset circuit that detects the terminal voltage of the overcurrent detection resistor and inhibits the output of the detection circuit section for a predetermined period of time.

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 photocoupler lights up, 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 photocoupler is turned off and the power 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 connection body of a transistor such as a MOSFET 5 and a resistor R6 for overcurrent detection is connected, 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 vicinity of an object based on changes in the oscillator state, and outputs a digital output, and uses a circuit that consumes as little power as possible. transistor 1
0 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. The sensor circuit 13 includes a detection coil 1.
6 and a resonant circuit of capacitor C17 are connected. The sensor circuit 13 outputs an “H” level output that is almost the same as the power supply voltage when no object is detected.
An NL output that outputs an "L" level output that is substantially the same as the NH output and the ground terminal is provided, and a reset terminal that disables the sensor circuit 13 is also provided.
Resistors R18 and R19 that provide base current to the transistor 10 are connected to the NH output terminal.
A light emitting diode 21L, which is a light emitting part of the photocoupler 21, is connected to the NL output terminal via a resistor R20, and a Zener diode 22 and a transistor 21T of the photocoupler 21 are connected in parallel to its cathode. The photo coupler 21 is conductive when the switch is turned on, and its emitter end is connected to the gate of the FET 5, which is an output switching element. Here, the transistor 10 and the resistors R18 and R19 are connected to the capacitor C1 every time the voltage of the capacitor C15 decreases and the photo coupler 21 stops lighting when the sensor circuit 13 is turned on.
This constitutes a switching circuit that charges the battery.
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. transistor 24
is operated when overcurrent is detected to reset the sensor circuit 13, and its base end is connected to the resistor R2.
5 to a common connection point between the source of the FET 5 and the overcurrent detection resistor R6. Here, the transistor 24 and the resistor R25 detect the terminal voltage of the overcurrent detection resistor R6, and reset the sensor circuit 13 for a certain period of time by discharging the capacitor C23 when the terminal voltage exceeds a predetermined value. It constitutes a reset circuit. Sensor circuit 1
The power supply voltage of 3 is almost determined by Zener diode 7, and the NH and NL output levels are also determined based on it, but the "H" level voltage is the sum of the Zener voltages of Zener diodes 9 and 22.
It is assumed that a voltage is selected that is slightly lower than Va and higher than the Zener voltage Vb of the Zener diode 9. Further, the reset voltage Vr is selected to be even lower than the Zener voltage Vb. In the following explanation regarding the operating voltage, it is necessary to take into account the voltage drop across 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 the off state, and the light emitting diode 11 It also doesn't light up. Also, since the NL output is at the "L" level, the photo coupler 21
The light emitting diode 21L does not light up, and the gate signal is not applied to the FET 5, so it is in an off state. Therefore, the sensor circuit 13 is connected via the bypass resistor R14.
Power is supplied to the sensor circuit 13, and the sensor circuit 13 operates and continues in a standby state waiting for an object. and time t0
When the oscillation state changes due to the proximity of an object, the NH output of the sensor circuit 13 becomes "L" level,
The NL output goes to the "H" level as shown in FIG. 2b, 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, the light emitting diode 11 lights up and the capacitor C15 is rapidly charged, and the sensor circuit 1
Power supply voltage given to 3 and “H” of NL output
Level voltage increases. Then, at time t1 when the "H" level voltage of the NL output exceeds the voltage Va, which is the sum of the Zener diodes 9 and 22, the light emitting diode 21L of the photo coupler 21 lights up, the transistor 21T becomes conductive, and the Zener diode 2
2 is shorted. Therefore, from now on, the current is the sensor circuit's current.
From NL output to resistor R20, light emitting diode 21
L, transistor 21T and Zener diode 9
A current flows through the FET 5, a gate voltage determined by the Zener diode 9 is applied to the gate of the FET 5, 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 input impedance of FET 5 is high, almost no current flows through the gate, and the amount of charge consumed by capacitor C15 is determined by the current flowing through sensor circuit 13 and photocoupler 21. Therefore, if the sensor circuit 13 is constructed of a low power consumption type IC as described above, the voltage of the capacitor C15 will gradually decrease. Now, if the voltage of the capacitor C15 decreases, the "H" level voltage of the NL output of the sensor circuit 13 will also decrease as shown in Fig. 2b.
When the voltage of the Zener diode 9 drops below Vb at time t2, the current no longer flows through the photocoupler 21, the transistor 21T turns off, and the NL output of the sensor circuit 13 has two Zener diodes 22 and 9. is now connected. Therefore, no gate voltage is applied to FET5.
FET is turned off. Then, as shown in FIG. 2c, a voltage approximately equal to that of the AC power source 30 is applied between terminals 1 and 2, and since the transistor 10 is in the on state, a large current flows to the capacitor C15, causing rapid charging. , resulting in capacitor C15
The voltage across the sensor circuit 13 increases, and the power supply voltage applied to the sensor circuit 13 increases, and accordingly, the NL output voltage also increases as shown in FIG. 2b. And that
At time t3 when the NL output voltage reaches the voltage Va determined by the Zener diodes 9 and 22, the light emitting diode 21L of the photo coupler 21 is activated as described above.
lights up again, shorting Zener diode 22.
After the FET 5 becomes conductive, a load current is similarly supplied to the load L. In this way, sensor circuit 1
The power supply capacitor C15 in step 3 is charged and supplied with charge only when the voltage across it falls below a predetermined value, so the moment of charging is extremely short compared to the time when the electronic switch is conducting. This makes it possible to reduce the residual voltage across the electronic switch.

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 time t4 in Figure 3
If a short circuit occurs, 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 instantly drops below the reset voltage Vr, and the sensor circuit 13 is reset, and the NL output returns to the original "L" level and the photo coupler 21
The light emitting diode 21L is turned off. accordingly
FET5 is also turned off to prevent short circuits. Then the capacitor C connected to the reset terminal
23 is gradually charged by supplying current from the power source through a resistor (not shown) in the sensor circuit 13 as shown in FIG. starts, and if an object is detected, the NL output becomes "H" level, the light emitting diode 21L of the photo coupler 21 lights up, and at the same time the FET 5 is turned on. Therefore, at time t5, the short circuit current flows again and is detected by the overcurrent detection resistor R6, turning off the sensor circuit 13 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.

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.

Furthermore, in this embodiment, the Zener diode 22 for applying a constant voltage is short-circuited by the photocoupler transistor, but since the current flowing through the Zener diode 9 is almost the same when the NL output is "H", Instead of the diode 22, a resistor with a resistance value that causes a voltage drop equal to the diode 22 can also be used.

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. It is. 1, 2... terminal, 5... FET, 7, 9, 22
...Zena diode, R6... Resistor for overcurrent detection, 8, 10, 24... Transistor, 11...
Light emitting diode, C15, C17, C23... Capacitor, 21... Photo coupling.

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. A light emitting element is connected in series with a voltage drop element provided between the output end of the detection circuit section and the gate of the field effect transistor, and the output of the light emitting element short-circuits the voltage drop element to reduce the voltage drop of the field effect transistor. a photocoupler that drives the transistor and stops the light emitting element from emitting light due to a drop in the voltage applied to the detection circuit; a photocoupler connected between the field effect transistor and the power input terminal of the detection circuit; a charging circuit that charges the power storage element when the detection circuit section is off; and a charging circuit that is connected in parallel with the charging circuit, and when the detection circuit section is turned on, the voltage of the power storage element decreases, and the photocoupler stops emitting light and becomes non-conductive. a switching circuit that charges the electricity storage element every time
An electronic switch comprising: 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. The electronic switch according to claim 1, wherein the voltage drop element connected in series to the light emitting element of the photocoupler is a Zener diode. 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 that is connected in series to a load and whose gate potential is limited to a certain value or less; and a voltage drop element that connects the output end of the detection circuit section to a voltage-dropping element that is provided between the gates of the field-effect transistor. Light emitting elements are connected in series, and their output short-circuits the voltage drop element to drive the field effect transistor, and a drop in voltage applied to the detection circuit causes the light emitting elements to emit light. a photo coupler to be stopped; a charging circuit connected between the field effect transistor and the power input terminal of the detection circuit unit and charging the electricity storage element when the detection circuit unit is off; a switching circuit that charges the power storage element each time the voltage of the power storage element decreases and the photocoupler stops emitting light and becomes non-conductive when the detection circuit section is turned on; and a switching circuit connected in series to the field effect transistor. an overcurrent detection resistor; and a reset circuit that detects a terminal voltage of the overcurrent detection resistor and applies the detection signal to a reset terminal of the detection circuit section to inhibit output of the detection circuit section for a predetermined period of time. An electronic switch characterized by comprising: 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. 9. The electronic switch according to claim 5, wherein the voltage drop element connected in series to the light emitting element of the photocoupler is a Zener diode.