JP2017005290A - Contact input circuit - Google Patents

Abstract

[PROBLEMS] To cope with multi-point input in a relatively wide input range while using a photocoupler with relatively low current consumption, and can stabilize the generation of glitches even when a signal with a slow rise from the outside is input. A contact input circuit that eliminates the need for a circuit drive power supply on the primary side by using a photocoupler without the need for a power supply, reduces heat consumption by reducing power consumption, and reduces costs, thereby reducing costs. To do. In a contact input circuit configured to read a contact open / close status via a photocoupler, a constant current circuit is connected in series with a light emitting element of the photocoupler. [Selection] Figure 1

Description

  The present invention relates to a contact input circuit, and more particularly, to an improvement of a contact input circuit for reading a switching status of a contact in a field device such as a PLC (Programmable Logic Controller) contact input module or a counter input module. Is.

  The device to which the present invention can be applied is not limited to the PLC, and the same effect as in the case of the PLC can be expected if a device other than the PLC has a contact input circuit.

  FIG. 4 is a circuit diagram showing an example of a conventional contact input circuit. In FIG. 4, a series circuit of resistors Ra and Rb is connected between both ends of the external input voltage source Vin, and a light emitting diode LED constituting the photocoupler PC is connected in parallel with the resistor Rb.

  FIG. 5 is a voltage-current characteristic example diagram of the circuit shown in FIG. A current If proportional to the input voltage Vin as shown by the characteristic line A in FIG. 5 flows through the photocoupler PC shown in FIG. Then, when the current If falls within the recommended on-current range B, the photocoupler PC is turned on.

  Focusing on this recommended on-current range B, for example, in a contact input module used in a PLC, a relatively wide range such as 8 to 26.4 V is required as an on-voltage range.

  In the case of a photocoupler that consumes a relatively large current, the recommended on-current range B is also relatively wide, so that it can accommodate a relatively wide range of on-voltages such as 8 to 26.4V.

  On the other hand, in the case of a photocoupler with a relatively low current consumption, the recommended on-current range B is relatively narrow, making it difficult to accommodate a wide range of on-voltages.

  In general, a Schmitt trigger circuit is mounted on a phototransistor constituting the photocoupler PC. However, some photocouplers do not include a Schmitt trigger circuit.

  When using a photocoupler PC with no Schmitt trigger circuit mounted on such a phototransistor as a component, when a DC voltage near the threshold level is applied or an input with a slow rise (falling) is entered, The on / off timing fluctuates and becomes unstable, causing glitches in the output.

  In the conventional circuit as shown in FIG. 4, a current If proportional to the input voltage Vin flows into the photocoupler PC, and thus a glitch is generated when a signal having a slow rise (falling) is input from the outside. It is inevitable.

  On the other hand, the most important thing to be aware of when trying to realize multi-point contact input using a PLC I / O module is heat generation. In some cases, it is difficult to realize multiple points due to heat generation, or even if the number of points can be increased, it is necessary to limit the number of points that can be turned on simultaneously depending on the state of heat generation.

  Patent Document 1 describes a technique for realizing a contact input circuit that minimizes power consumption in an input module and reduces heat generation.

JP 2005-64929 A

  The present invention solves these problems, and its purpose is to support a multipoint input with a relatively wide input range using a photocoupler with a relatively low current consumption, and to input a signal with a slow rise from the outside. In this case, it is possible to suppress the occurrence of glitches, so that a stabilized power supply is not required, and by using a photocoupler, the circuit drive power supply on the primary side can also be made unnecessary. The object is to realize a contact input circuit capable of reducing the number of parts and reducing the cost.

In order to achieve such an object, the invention described in claim 1 of the present invention is
In the contact input circuit configured to read the open / close status of the contact through the photocoupler,
A constant current circuit is connected in series with the light emitting element of the photocoupler.

The invention according to claim 2 is the contact input circuit according to claim 1,
A switch circuit is connected in series with the constant current circuit.

The invention according to claim 3 is the contact input circuit according to claim 2,
A comparator for driving the switch circuit on and off is connected.

  According to such a configuration, it is possible to realize a contact input circuit that can handle multi-point input in a relatively wide input range while having relatively low current consumption.

It is a block diagram which shows the outline | summary of the contact input circuit based on this invention. FIG. 2 is a voltage-current characteristic example diagram of the circuit shown in FIG. 1. FIG. 2 is a specific circuit diagram of FIG. 1. It is a circuit diagram which shows an example of the conventional contact input circuit. FIG. 5 is a voltage-current characteristic example diagram of the circuit shown in FIG. 4.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an outline of a contact input circuit according to the present invention, and parts common to FIG. In FIG. 1, a light emitting diode LED, a switch circuit SW, and a constant current circuit CC constituting a photocoupler PC are connected in series between both ends of an external input voltage source Vin, and a comparator circuit CMP is connected in parallel with these series circuits. Has been. The comparator circuit CMP is connected so as to drive the switch circuit SW on and off with the output signal.

  FIG. 2 is a voltage-current characteristic example diagram of the circuit shown in FIG. In FIG. 2, a thick broken line A shows the same voltage-current characteristic line as the voltage-current characteristic line in the conventional circuit shown in FIG. 5, and two thin broken lines show a recommended on-current range B of the photocoupler PC, which is a solid line. C and D show the state of change of the input current If of the photocoupler PC.

  As is clear from FIG. 2, the input current If of the photocoupler PC is made constant by the constant current circuit CC so as to be within a predetermined recommended on-current range B even if the input voltage Vin is increased.

  In the circuit configuration of FIG. 1, the comparator circuit CMP monitors the output voltage of the external input voltage source Vin, and turns on the switch circuit SW when the output voltage of the external input voltage source Vin reaches a predetermined voltage. With the circuit configuration of FIG. 1, the voltage for turning on the switch circuit SW can be easily set.

  Until the switch circuit SW is turned on, no current flows through the light emitting diode LED provided on the primary side of the photocoupler PC, and when the switch circuit SW is turned on, the light emitting diode LED on the primary side of the photocoupler PC is turned on. Current flows through

  By introducing the combination of the comparator circuit CMP and the switch circuit SW, the rising of the input current If of the photocoupler PC is more rapid than the characteristic of the conventional circuit shown by the broken line A as shown by the solid line C in FIG. It becomes a gradient and can be accelerated.

  Since glitches occur near the threshold level of the photocoupler PC, the occurrence of such glitches can be reduced by increasing the rise of the input current of the photocoupler PC as shown in FIG.

  FIG. 3 is a specific circuit diagram of FIG. The comparator circuit CMP includes resistors R1 to R3, a Zener diode D, and a transistor Tr1.

  The Zener diode D and the resistor R1 are connected in series and connected to both ends of the external input voltage source Vin, and the resistors R2 and R3 are also connected in series to be connected in parallel to both ends of the external input voltage source Vin together with the series circuit of the Zener diode D and the resistor R1. It is connected.

  The anode of the Zener diode D is connected to one end of the external input voltage source Vin and is connected to the common potential point, and the cathode of the Zener diode D is connected to one end of the resistor R1. The other end of the resistor R1 is connected to the other end of the external input voltage source Vin and to one end of resistors R2 and R3 connected in series.

  The base of the transistor Tr1 is connected to the connection point between the cathode of the Zener diode D and one end of the resistor R1, the emitter is connected to the connection point of the resistors R2 and R3 connected in series, and the collector is connected to the base of the transistor Tr2. Yes.

  The other ends of the resistors R2 and R3 connected in series are connected to a common potential point together with the anode of the Zener diode D and one end of the external input voltage source Vin.

  The resistor R1 and the Zener diode D generate a reference voltage Vref, and the reference voltage Vref and the emitter voltage Ve1 of the transistor Tr1 are compared to drive the switch circuit SW on / off.

  The emitter voltage Ve1 of the transistor Tr1 is generated by dividing the external input voltage Vin by the resistors R2 and R3. The on-voltage is adjusted by changing the Zener voltage of the Zener diode D or by changing the voltage dividing ratio of the resistors R2 and R3 connected in series.

  The switch circuit SW is composed of a transistor Tr2. The switch circuit SW receives an output signal from the comparator circuit CMP provided in the preceding stage, and controls on / off of a current flowing through the light emitting diode LED provided on the primary side of the photocoupler PC.

  The constant current circuit CC includes resistors R1 and R5, a Zener diode D, and a transistor Tr2. The constant current circuit CC shares the reference voltage circuit of the comparator circuit CMP and also the transistor Tr2 that configures the switch circuit SW.

  The collector of the transistor Tr2 is connected to the cathode of the light emitting diode LED provided on the primary side of the photocoupler PC, the emitter is connected to the other end of the resistors R2 and R3 connected in series via the resistor R5, and the anode of the zener diode D And one end of the external input voltage source Vin is connected to the common potential point.

  The anode of the light emitting diode LED provided on the primary side of the photocoupler PC is connected to the other end of the resistor R1, the other end of the external input voltage source Vin, and one end of resistors R2 and R3 connected in series via a resistor R4. Connected to the connection point.

  When the output signal of the comparator circuit CMP is turned on, the voltage input to the base of the transistor Tr2 becomes a constant value substantially equal to the reference voltage Vref. Since the emitter voltage Ve2 of the transistor Tr2 also becomes a constant value, the current flowing through the collector of the transistor Tr2 becomes a constant value of Ve2 / R5 and operates as a constant current circuit.

  With such a circuit configuration, it is possible to realize a contact input circuit that can handle a relatively wide input voltage range while using a photocoupler with a low current consumption that has a relatively narrow recommended on-current range.

  Even if a signal with a relatively slow rise is input from the outside, no current flows through the photocoupler until the input voltage reaches the reference voltage, and the rise of the current input of the photocoupler becomes faster. The glitch generated in the output signal can be suppressed.

  Comparator circuit CMP that constitutes a contact input circuit according to the present invention using individual elements that can be obtained relatively inexpensively, such as transistors, without using a relatively expensive integrated circuit that requires a stable drive power supply, Since the switch circuit SW and the constant current circuit CC can be realized, it is not necessary to prepare a separate power supply circuit, and the cost can be reduced.

  Since the configuration based on the present invention is a low current consumption circuit, a multi-point input configuration is also possible, and the cost can be reduced by reducing heat countermeasure components.

  Note that when the photocoupler is integrated, a configuration of the contact input circuit according to the present invention may be incorporated and incorporated.

  As described above, according to the present invention, it is possible to realize a contact input circuit that can handle multi-point input in a relatively wide input range while having relatively low current consumption.

Vin External input voltage source CMP Comparator circuit PC Photocoupler LED Light emitting diode SW Switch circuit CC Constant current circuit

Claims (3)

In the contact input circuit configured to read the open / close status of the contact through the photocoupler,
A contact input circuit, wherein a constant current circuit is connected in series with the light emitting element of the photocoupler.   The contact input circuit according to claim 1, wherein a switch circuit is connected in series with the constant current circuit.   The contact input circuit according to claim 2, wherein a comparator for driving the switch circuit on and off is connected.

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