JPS60150318A - Electronic switch - Google Patents

Abstract

PURPOSE:To attain short-circuit protection with simple circuit constitution by using an FET for a constant voltage circuit given to a detection circuit and an output switch circuit at short circuit. CONSTITUTION:When an object approaches a proximity switch at the short-circuit state of a load L, an output of a sensor circuit 12 triggers a thyristor 8 via a transistor (TR) 14 and a large current flows to an FET6 and a thyristor 8 functioning as a switching element. Since a gate voltage of the FET6, however, is kept to a prescribed value by a Zener diode 5, the current flowing to the FET6 is suppressed to a value below a prescribed value with the constant current characteristic of the FET. A voltage across a resistor R9 for overcurrent detection is increased just after and a thyristor 7 is triggered and turned off. Then the gate potential of the FET6 is decreased to near zero voltage, the FET6 is turned off and the short-circuit current is applied with current limit and cut off momentarily.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to electronic devices such as two-wire photoelectric switches and proximity switches that are connected in series between a load and a power source and are configured to directly control the load based on a sensed output. It is related to switches.

Conventional technology and its problems Two-wire electronic switches such as photoelectric switches and proximity switches that are configured to directly control loads have a detection section and a switching element such as a thyristor, and switch based on the detection output. The device is configured to drive and control the supply of power to the load. By the way, if a load becomes internally short-circuited due to an accident, or by mistake, the power source may be connected directly to the electronic switch without connecting the load. In order to protect the internal circuit from such a short-circuit condition, a protection circuit for short-circuit protection is provided inside the electronic switch. However, if the short circuit protection circuit operates slowly,
There is a problem in that a large short-circuit current continues to flow through the switching elements and the like until the short-circuit protection is activated. In order to prevent such large currents from flowing, there are electronic switches that have a current-limiting resistor connected in series with the switching element, but the current-limiting resistor must have a large power capacity and its shape There was a problem in that the size of the electronic switch also increased, making it difficult to miniaturize the entire electronic switch.

Furthermore, conventional electronic switches equipped with short-circuit protection circuits have a complicated circuit configuration, resulting in an increase in price.

OBJECTS OF THE INVENTION The present invention solves the problems of conventional electronic switches, and is capable of interrupting short-circuit current without flowing a large current to the switching element even in the event of a short-circuit with a simple circuit configuration. The present invention provides an electronic switch that can protect internal circuits.

Structure and Effects of the Invention The present invention is an electronic switch that has a sensor circuit and an output switching element that is connected in series to a load and a power source and controls power supply to the load based on the output of the sensor circuit. a power field effect transistor connected in series to the output switching element and one end of which is connected to the power input terminal of the sensor circuit; and an overcurrent transistor connected in series to the output switching element. A detection resistor, a constant voltage circuit that keeps the gate terminal of the field effect transistor at a constant voltage, and a gate terminal connected between the gate terminal of the field effect transistor and the power supply terminal, and its gate 1- connected to the overcurrent detection resistor. The sensor is characterized by having a short-circuit protection risk that is triggered by a voltage rise during a short circuit and is maintained by the power source of the sensor circuit.

According to the present invention having such characteristics, power is supplied to the sensor circuit through the field effect type l-run risk whose gate potential is kept constant, so that the power supply voltage applied to the sensor circuit can be kept constant. becomes possible. Furthermore, even if a short circuit occurs, the current flowing through the switching element is limited to a constant value due to the constant current characteristics of the field effect transistor, and the short circuit protection circuit immediately conducts to cut off the field effect transistor. Therefore, it becomes possible to block the short circuit current and protect the internal circuit from now on.

In this way, the present invention uses field-effect type 1-run risk in both the constant voltage circuit that provides the detection circuit and the output switching circuit in the event of a short circuit, making it possible to provide short circuit protection with an extremely simple circuit configuration. . And - [If short-circuit protection is performed, the Cyrisk is kept in the on state by the holding current supplied from the sensor circuit, so the protective operation continues as it is, and reliable protection can be performed.

DESCRIPTION OF EMBODIMENTS FIG. 1 is a circuit diagram showing an embodiment of an AC two-wire proximity switch according to the present invention. In this figure, a diode bridge 3 is connected between the ζ terminals 1 and 2, and a constant voltage circuit consisting of a resistor R4 and a Zener diode 5 is provided between the positive and negative terminals of the diode bridge 3. Since current flows through the resistor R4 from the power supply regardless of the circuit state, it is preferable to have a high resistance to reduce current consumption. And a power field effect transistor (MOSFET) is connected to both ends of the resistor R4.
The drain of No. 6 and the gate No. 1- are connected in parallel, and the Zener diode 5 is connected with No. Iristaf for short circuit protection in parallel.

For the constant voltage provided by the Zener diode 5, a gate voltage having a value such that the rated current of this electronic switch is equal to the constant current characteristic of the FET 6 shall be selected. Between the source of FIET 6 and the negative terminal of diode bridge 3, there is Sirisk 8, which is a switching element for output switching, and resistor R9 for overcurrent detection.
is connected. A common connection end of the thyrisk 8 and the resistor R9 is connected to the gate terminal of the thyrisk 7. Furthermore, FET6
The source of is connected to the emitter of the quick charging transistor 10, and its collector is connected to the sensor circuit 1 through a diode 11. &12 power supply human power 631 V cc. The sensor circuit 12 is formed as T and C and has an oscillation circuit to detect the proximity of an object, and a smoothing and power supply capacitor CI3 is provided between its power terminals. The output terminal of the sensor circuit 12 is then amplified with current and a trigger signal is provided to the thyristor 8.
The base of transistor 140 is connected, its emitter is connected to the negative terminal of diode bridge 3 via resistor R15, and the common connection end of resistor R15 and the emitter of 1-run resistor 14 is connected to the gate end of thyristor 8. ing. Further, the collector of transistor 14 is connected to the heath of transistor 10 via Zener diode 16. The connection between the collector of the transistor 10 and the eminook is bypassed by a resistor R17 that limits the rush current when the power is turned on, and the positive terminal of the power supply capacitor C13 is connected to a resistor R18 and a diode 19 that provide a holding current to the thyristor 7 during short-circuit protection operation. The anode of the thyristor 7 is connected to the anode of the thyristor 7 via. Furthermore, a resonance circuit including a detection coil 20 and a capacitor c21 is connected to the sensor circuit 12.

An AC power supply 3° and a load are connected in series between terminals 1 and 2 as shown in the figure. Then, a small current flows through the load I4, and the AC voltage of the AC power supply 3o is rectified by the diode bridge 3, and the gate of FET6 is kept at a constant voltage by the constant voltage circuit made of resistor R4゜zener diode 5, turning on FET6. . Then, its source voltage also becomes constant, and FET6. Power is supplied to the sensor circuit 12 via the resistor R17 and the diode 11, and the capacitor C13 is charged. At this time, the oscillator of the sensor circuit 12 oscillates due to the resonance circuit formed by the detection coil 2o and the capacitor C21 connected to the sensor circuit 12, and the sensor circuit 12 is in a standby state waiting for an object to approach. In the standby state, thyristors 7 and 8 are off, and only FET 6 is on.

If an object approaches and the oscillation state of the sensor circuit 12 changes, the sensor circuit 12 outputs an "H" level output from its output terminal, making the transistor 14 conductive. Then, a current flows through the resistor R15, and the emitter voltage of the transistor 14 rises, causing the risk to rise. Therefore, the thyristor 8 is turned on, current is supplied to the load via the diode bridge 3 and the FET 6, and the load is driven. In each subsequent cycle, the thyristor 8 is temporarily turned off after passing the zero cross point.
Transistors 10 and 14 are immediately turned on and capacitor C13 is rapidly charged, and when the Zener voltage of Zener diode 16 is reached, transistor 10 is turned off and thyristor 8 is turned on. If the current flowing through the load is within the rated current, the thyristor 7 will not be triggered because a sufficient voltage will not be generated across the overcurrent detection resistor R9. In this way, power is supplied to the sensor 9 and circuit 12, and the phase of the thyristor 8 is controlled.

Now, the operation when an overcurrent flows through the load or when the load is short-circuited will be explained. Suppose that an object approaches the proximity switch while the load is short-circuited.

Then the sensor circuit 12 outputs an output and the transistor 1
Since thyristor 8 is triggered via FET 6
A large current flows through the iris register 8. However, FET6
Since the gate voltage of is kept at a constant value by the Zener diode 5, the current flowing through the FET 6 is also held below a constant value by the constant current characteristic of the FET. Immediately after that, the voltage across the overcurrent detection resistor R9 rises by 2', and the thyristor 7 is triggered and turned on. Therefore FET
The gate potential of FET 6 drops to around zero volts, turning off FET 6. Therefore, the short circuit current is limited and instantaneously cut off. Since the holding current is thereafter supplied to the thyristor 7 from the capacitor C13 via the resistor R18 and the diode 19, the thyristor 7 does not return even at the AC zero cross point and continues to be in the on state, and the FET 6 continues to cut off the short-circuit current. Therefore, the protection circuit continues to operate until the power is turned on again, except when the load is short-circuited, and protection can be ensured.

Although the present embodiment has been described with reference to 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 a power source, it is also possible to apply the present invention to an electronic switch using a DC power source, and in that case, the holding circuit for the XIRISK 7 can be omitted.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an AC two-wire proximity switch according to the present invention. 1, 2---one terminal 4- da iode brine 6
--FET 7,8---Zyristor 10°14
-----1-Landisk l 2----Sensor circuit C
13, C21 --- Capacitor R9 --- Resistor for transient current detection R4, R15, R17, RlB --- Resistor Patent applicant Tateishi Dentsuki Co., Ltd. agent Patent attorney Okamoto Ginomiya (1 other person)

Claims (2)

[Claims] (1) Connected in series to the sensor circuit, load and power supply,
An electronic switch having an output switching element that controls power supply to a load based on the output of the sensor circuit, the electronic switch being connected in series to the output switching element and having one end connected to the output switching element of the sensor circuit. A power field effect type l-run risk connected to the power supply input terminal, an overcurrent detection resistor connected in series to the output switching element, and a gate terminal of the field effect transistor maintained at a constant voltage. a constant voltage circuit, which is connected between the gate terminal of the field effect transistor and the power supply terminal, the gate 1- of which is connected to the overcurrent detection resistor, and is triggered by a voltage increase at the time of a short circuit; An electronic switch comprising: a short-circuit protection cyrisk held by a power source; (2) The electronic device according to claim 1, wherein the constant voltage circuit includes a Zener diode connected in parallel to the sirisk, and a resistor connected between the sirisk and the other end of the power supply. switch.