JPH07295608A - Input circuit - Google Patents

Abstract

PURPOSE:To provide the input circuit which is small in power consumption and generated heat and where a contact switch and a noncontact switch can be connected. CONSTITUTION:A switching transistor(TR) 3, an inductor 4, a current detection resistance 16, and a photocoupler 1 are connected in series between an external power terminal POW and an external input terminal IN, and a flywheel diode 13 is connected between an external common terminal COM and an inductor 14. When the switch TR 3 is ON, a current flows from a power source 21 for input to a detection switch 20 through the switching TR 3, inductor 14, current detection resistance 16, and photocoupler 1 and when OFF, a current flows by the self-induction of the inductor 14 from the external common terminal COM to the detection switch 20 through the flywheel diode 13, inductor 14, current detection resistance 16, and photocoupler 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an input circuit for inputting the state of an external switch to an internal circuit.

[0002]

2. Description of the Related Art A programmable controller is used to sequentially control the operation of controlled equipment. This programmable controller has an input circuit for inputting to an internal circuit the state of an external switch that indicates the operating status of the controlled device and the input operation.

FIG. 4 is a circuit diagram showing an example of an input circuit of a conventional programmable controller. Although an actual programmable controller is provided with a plurality of input circuits corresponding to a plurality of external switches, FIG. 4 shows only one input circuit 300.

In the input circuit 300, the external input terminal I
A series resistor 31 and a light emitting element 30a of the photocoupler 30 are connected in series between N and the external common terminal COM, and a parallel resistor 32 is connected in parallel to the photocoupler 30. The collector of the light receiving element 30b of the photocoupler 30 is connected to the input reading circuit of the internal circuit and is also connected to the internal power supply terminal via the pull-up resistor 33. The external input terminal IN of the input circuit 300 and the external common terminal CO
A detection switch 34 and an input power source 35 are connected in series with M.

When the contact of the detection switch 34 is closed, a current is shunted from the input power source 35 to the photodiode 30 and the parallel resistor 32, and further to the series resistor 31. In this case, a sufficient current flows through the light emitting element 30a of the photocoupler 30 and the light emitting element 30a emits light. As a result, the light receiving element 30b of the photocoupler 30 is turned on, and a current flows through the pull-up resistor 33. In this way, the input signal can be read by the internal circuit electrically insulated from the detection switch 34.

Assuming that the resistance value of the series resistor 31 is R1 and the resistance value of the parallel resistor 32 is R2, the resistance value R1
And the resistance value R2 is determined in consideration of the following.

First, the voltage E of the input power source 35 is usually 24
V, but the voltage has a tolerance range. The maximum value of this tolerance range is E _MAX , and the minimum value is E _MIN . Next, as the detection switch 34, a contact switch such as a limit switch and a non-contact switch such as a high frequency proximity switch are used.

The common property of the contact switch and the contactless switch is that they do not operate normally unless a current equal to or more than the minimum current I _ND specified at the time of turning on is supplied. The minimum current I _ND required for the detection switch 34 is usually 5 to 10 mA.
Is. On the other hand, the different properties are that the contact switch has a leakage current I _MIN at the off time and a residual voltage V _MIN at the on time.
Can be regarded as substantially zero, but in a non-contact switch, especially in a two-wire proximity switch, the off-state leakage current I _MIN and the on-state residual voltage V _MIN cannot be ignored.

That is, when the voltage of the input power source 35 is the minimum value E _MIN , the voltage applied to the input circuit 300 when the two-wire proximity switch is turned on is the on-state residual voltage V _MIN due to the two-wire proximity switch. Input circuit 3
00 must be able to correctly detect the ON state. Therefore, the resistance value R1 of the series resistor 31 is limited to a certain value or less so that a sufficient current flows through the input circuit 300. The resistance value R1 of the series resistor 31 is set to 4.3 KΩ, for example.

When the two-wire proximity switch is off,
Even if the leakage current I _MIN at the time of off is flowing, the input circuit 300
Must detect the off state. Therefore, the voltage drop of the parallel resistor 32 causes the light emitting element 30 of the photocoupler 30.
The resistance value R2 of the parallel resistor 32 is limited to a certain value or less so that it becomes lower than the voltage at which a current starts flowing in a. The voltage at which a current starts to flow in the light emitting element 30a of the photocoupler 30 is about 1 V, and the resistance value R2 of the parallel resistor 32 is, for example, 4V.
It is set to 30Ω.

As described above, the voltage E of the input power source 35 is usually 24 V, and the minimum current I _ND required for the detection switch 34 is usually 5 to 10 mA. Therefore, the input circuit 3 of FIG.
The power loss generated inside 00 is about 120 to 240 mW
Is. Even in this state, a certain amount of heat is generated when the detection switch 34 is turned on. On the other hand, when the contact switch is used as the detection switch 34 and the voltage of the input power supply 35 has the maximum value E _MAX , the maximum voltage is applied to the series resistor 31 and the parallel resistor 32 when the contact switch is turned on. Will be applied, and more heat will be generated than in the above state.

Since a programmable controller usually has a large number of input circuits, natural air cooling is adopted for cost reduction and the like even though a conventional input circuit generates a large amount of heat, and a resin molded product is used. It is stored in a case using. Therefore, it is necessary to secure the case capacitance enough to keep the temperature rise within the allowable range by diffusing the heat generated from the resistance element and reducing the density. Therefore,
Although there are many demands for miniaturization of the device, it has its limits.

Therefore, in order to suppress the internal power consumption and the heat generation amount of the input circuit, the following various techniques have been proposed.

Japanese Unexamined Patent Publication No. 59-68007 discloses an input device in which a power supply switch circuit is provided between a power source and an external common terminal, and the power supply switch circuit is conducted for a fixed time. Japanese Unexamined Patent Publication No. 59-68009 discloses an input device in which a power supply switch circuit is provided between a power supply and an external common terminal, and the power supply switch circuit is intermittently conducted for a fixed time. Actual Kaisho 6
No. 1-159884 discloses an input-only power supply circuit for a sequencer, which constantly applies a voltage that guarantees an operation to an input circuit and intermittently applies a voltage that guarantees a contact to a switch.

Japanese Utility Model Laid-Open No. 63-81421 discloses an input circuit configured to interrupt an input current at high speed. Japanese Utility Model Application Laid-Open No. 63-147701 discloses an input circuit configured to supply an input current only at an input fetch timing using a switching element. Japanese Patent Application Laid-Open No. 5-173611 discloses an input circuit in which an opening / closing means for intermittently supplying a DC power supply is provided in a power supply path.

The common point of the above techniques is to intermittently supply power to the input circuit to switch the current flowing through the input circuit in two steps, that is, the rated current value and 0 or a low current value. As a result, the average power loss per unit time and the amount of heat generated in the input circuit are reduced, or the number of input points that can be turned on at the same time is increased.

[0017]

However, in any of the above-mentioned prior arts, only the contact switch is used as the detection switch connected to the input circuit, and the contactless switch such as the two-wire proximity switch is used. Is not considered.

The contactless switch is composed of an electronic circuit such as a detection circuit, and it is necessary to supply a predetermined current and voltage to this electronic circuit. The three-wire type solid state switch is
The electronic circuit has a dedicated power supply terminal for supplying electric power to the electronic circuit, and the electronic circuit is operated by a DC power supply different from the input power supply connected to the input circuit. However, since the two-wire type contactless switch does not have a dedicated power supply terminal for supplying power to the electronic circuit, the electronic circuit is operated by the input power supply connected to the input circuit. Therefore, when the two-wire non-contact switch is used as the detection switch in the above-mentioned conventional technique, the supplied current and voltage are intermittent, and the predetermined current and voltage are not supplied to the electronic circuit, which may cause malfunction. There is a nature.

Further, in the contactless switch, each part of the electronic circuit is in an unstable state for a while when the power is turned on. Therefore, during this unstable state, a detection output may be derived although it is not originally in the detection state. Therefore, there is a contactless switch equipped with a power-on reset circuit in order to prevent the detection output from being derived even in the non-detection state. A contactless switch having this power-on reset circuit cannot detect for a certain period of time after power-on.

When a two-wire non-contact switch having a power-on reset circuit is used as a detection switch, if the power supply to the input circuit is interrupted as in the prior art, the power-on reset circuit operates at each interruption. become. Therefore, the programmable controller cannot read the input state at high speed.

Therefore, it is impossible to construct a practical programmable controller capable of inputting the state of a two-wire type contactless switch such as a two-wire type proximity switch in the above-mentioned prior art.

Therefore, an object of the present invention is to provide an input circuit which consumes less power and generates less heat and which can input the states of a contact switch and a non-contact switch.

[0023]

[Means for Solving the Problems]

(1) 1st invention The input circuit which concerns on 1st invention WHEREIN: In the input circuit which inputs the state of an external switch into an internal circuit via a coupling means,
The switching means and the inductive element are inserted in the current path from the power supply to the external switch via the coupling means, and the current detection means for detecting the current flowing through the coupling means and the switching based on the current detected by the current detection means The control means for controlling the on / off of the means is provided.

(2) Second Invention In the input circuit according to the second invention, in the configuration of the input circuit according to the first invention, the coupling means is connected to the light emitting portion and the internal circuit connected to the current path. And an inductive element is an inductor.

(3) Third Invention In the input circuit according to the third invention, in the configuration of the input circuit according to the first invention, the coupling means is connected to the primary winding and the internal circuit connected to the current path. And an inductive element is a primary winding of the inductive coupling means.

(4) Fourth Invention An input circuit according to a fourth invention is an input circuit for inputting the state of an external switch to an internal circuit, and includes a coupling means, a switching means, a control means and a power storage means. .

The coupling means transmits the state of the external switch to the internal circuit based on the current supplied from the power source. The switching means connects and disconnects a current path from the power supply to the external switch via the coupling means. The control means controls ON / OFF of the switching means so that the value of the current supplied to the coupling means becomes a predetermined value. The power storage means stores the power supplied from the power source when the switching means is on, and supplies the current to the coupling means by the stored power when the switching means is off.

[0028] As a preferred example, the coupling means is an optical coupling means composed of a light emitting portion inserted in the current path and a light receiving portion connected to the internal circuit, and the power storage means is an inductive element inserted in the current path. is there. As another preferred example,
The coupling means is an inductive coupling means composed of a primary winding inserted in a current path and a secondary winding connected to an internal circuit, and the power storage means is a primary winding of the inductive coupling means.

(5) Fifth Invention In the input circuit according to the fifth invention, in the configuration of the input circuit according to the fourth invention, when the switching means is off, a current including an external switch, a power storage means and a coupling means is provided. It further comprises a current path forming means for forming a path.

[0030]

In the input circuit according to the first to third inventions,
When the switching means is on, a current is supplied from the power supply to the coupling means via the inductive element, and when the switching means is off, the self-induction action of the inductive element causes current to continue to flow through the coupling means. As a result, a substantially constant DC current flows through the coupling means and the external switch.

In the input circuit according to the second aspect of the present invention, when the switching means is on, current is supplied from the power supply to the light emitting portion of the optical coupling means through the inductor, and when the switching means is off, the self-induction action of the inductor causes the current to flow. Current is subsequently applied to the light emitting portion of the optical coupling means. As a result, a substantially constant DC current flows through the light emitting portion of the optical coupling means, the light emitting portion emits light, and a substantially constant DC current also flows through the light receiving portion of the optical coupling means.

In the input circuit according to the third aspect of the invention, when the switching means is on, current is supplied from the power supply to the primary winding of the inductive coupling means, and when the switching means is off, the self-winding of the primary winding of the inductive coupling means. Due to the inductive action, a current continues to flow through the primary winding. A current also flows in the secondary winding due to the mutual induction action of the inductive coupling means.

In the input circuits according to the fourth and fifth aspects of the present invention, the switching means is turned on when the external switch is on, and the current is supplied from the power supply to the coupling means. At this time, a part of the power from the power source is stored in the power storage means. When the current flowing through the coupling means increases to a predetermined value, the switching means is turned off by the control means. As a result, the current path from the power source to the coupling means is cut off, and current is supplied to the coupling means by the electric power stored in the power storage means. When the current supplied to the coupling means decreases and drops below a predetermined value, the control means turns on the switching means. By repeating the above operation, a direct current is supplied to the coupling means and the external switch.

As described above, when the switching means is turned on, a current is supplied from the power source to the coupling means, and at the same time, a part of the electric power is stored in the power storage means, and when the switching means is turned off, it is stored in the power storage means. Since the electric current supplies the coupling means with electric power, most of the electric power supplied from the power source is effectively used for supplying the electric current to the coupling means, and almost no electric power is converted into heat. Therefore, the internal power consumption and the heat generation amount of the input circuit are reduced. Moreover, since a constant direct current is supplied to the external switch, not only a contact switch but also a contactless switch can be used as the external switch.

Particularly, in the input circuit according to the fifth aspect of the present invention, when the switching means is off, a current path including the external switch, the power storage means and the coupling means is formed, so that the current supplied to the external switch is constant. It becomes the direct current of.

[0036]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a circuit diagram showing the configuration of an input circuit according to an embodiment of the present invention.

In the input circuit 100 of FIG. 1, the light emitting element 1a of the photocoupler 1 is connected between the external input terminal IN and the node N1, and the parallel resistor 2 is connected in parallel to the light emitting element 1a. The collector of the light receiving element 1b of the photocoupler 1 is connected to the input reading circuit of the internal circuit and also connected to the internal power supply terminal via the pull-up resistor 19.

On the other hand, a switching transistor 3 is connected between the external power supply terminal POW and the node N2, and a base-emitter resistor 4 and a base resistor 5 are connected to the switching transistor 3. The base resistor 5 is connected to the external common terminal COM via the transistor 6. The smoothing capacitor 7 is connected between the external input terminal POW and the external common terminal COM.

A starting resistor 8 is connected between the external power supply terminal POW and the base of the transistor 6, a transistor 9 is connected between the base and the external common terminal COM, and the base of the transistor 9 and the transistor 6 are connected. A feedback resistor 10 is connected between the collector and the collector. A base / emitter resistor 11 and a base resistor 12 are connected to the transistor 9.

The flywheel diode 13 is connected between the node N2 and the external common terminal COM, and the flywheel diode 13 is connected to the node N2.
Inductor 14 is connected between node N3 and node N3, and smoothing capacitor 1 is connected between node N3 and external common terminal COM.
5 is connected. The node N3 is connected to the emitter of the current detection transistor 17 and also has the current detection resistor 16
Is connected to the node N1 via. The base of the current detection transistor 17 is connected to the node N via the base resistor 18.
1 and the collector is connected to the base resistor 12 of the transistor 9.

A detection switch 20 including a contact switch such as a limit switch or a non-contact switch such as a two-wire proximity switch is connected between the external input terminal IN and the external common terminal COM, and is connected to the external power supply terminal POW and the outside. The input power supply 21 is connected to the common terminal COM.

In this embodiment, the photocoupler 1 functions as a coupling means, and the current detection resistor 16 and the current detection transistor 18 function as a control means. Further, the inductor 14 functions as a power storage means, and the flywheel diode 13 functions as a current path forming means.

Next, the operation of the input circuit 100 of FIG. 1 will be described with reference to FIG.
This will be described with reference to the waveform chart of FIG. In FIG. 2, I _C
Represents a current flowing through the switching transistor 3, and I
_D represents the current flowing through the flywheel diode 13. V _R represents the voltage across the current detection resistor 16, V
_B represents the base voltage of the switching transistor 3 and V _N represents the voltage of the node N2. I _S represents the current flowing through the detection switch 20.

First, a current flows from the input power source 21 to the base of the transistor 6 via the starting resistor 8. As a result, the transistor 6 turns on and the switching transistor 3 also turns on. As a result, a current I _C flows from the emitter of the switching transistor 3 to the collector of the switching transistor 3 and charges the smoothing capacitor 15 through the inductor 14. As a result, the voltage of the smoothing capacitor 15 increases.

At this time, if the detection switch 20 is turned on, the current I _C flowing through the switching transistor 3 flows through the inductor 14, the current detection resistor 16 and the light emitting element 1a of the photocoupler 1 into the detection switch 20.
Due to the voltage drop of the current detection resistor 16, the voltage V _R across it is
Becomes higher than the base-emitter voltage V _BE of the current detection transistor 17, a current flows from the emitter of the current detection transistor 17 to the collector, and the transistor 9 is turned on. As a result, the transistor 6 is turned off and the switching transistor 3 is also turned off.

Even when the switching transistor 3 is turned off, the inductor 14 tries to maintain the current that has been flowing until then due to the self-induction effect. In this case, the node N2
Voltage V _N of the external common terminal CO drops below the ground voltage.
A current I _D flows from M through the flywheel diode 13 to the inductor 14 while decreasing. This current I _D
Is the current detection resistor 16 and the light emitting element 1 of the photocoupler 1.
It flows to the detection switch 20 through a.

Since the current flowing through the current detection switch 16 decreases, the voltage V _R across the current detection resistor 16 changes to the base-emitter voltage V _{BE of the} current detection transistor 17.
In the following cases, the current detection transistor 17 turns off.

By repeating the above operation, a substantially constant current flows through the current detection resistor 16 so that a voltage drop of about the base-emitter voltage V _BE of the switching transistor 17 occurs across the current detection resistor 16. . As a result, a substantially constant current I _S also flows through the detection switch 20.

Next, when the detection switch 20 is off, a small current flows through the input circuit 100 due to the leakage current when the detection switch 20 is off. As a result, a slight voltage drop occurs in the parallel resistor 2, but the voltage across the light emitting element 1a of the photocoupler 1 does not reach a voltage enough to turn on the light emitting element 1a. Further, the voltage drop due to the current detection resistor 16 becomes a small value, and the current detection transistor 17
Is off. Therefore, the transistor 9 is also turned off,
The transistor 6 and the switching transistor 3 are on. Therefore, the voltage of the smoothing capacitor 15 is almost equal to the voltage of the input power supply 21.

Next, the internal power loss in the input circuit 100 of FIG. 1 will be calculated. Here, the voltage of the input power supply 21 is set to 24V.

The internal power loss of the input circuit 100 when the detection switch 20 is on is due to the forward voltage of the light emitting element 1a of the photocoupler 1 and the base voltage of the current detection transistor 17.
It has a value obtained by multiplying the sum of the emitter voltage V _BE and the current I _S flowing through the detection switch 20.

Since the forward voltage of the light emitting element 1a of the photocoupler 1 is usually about 1.1V and the base-emitter voltage V _BE of the current detection transistor 17 is usually about 0.6V, the sum of them is 1. It is about 7V. The current I _S flowing through the detection switch 20 is usually set to 5 to 10 mA. Therefore, the power loss generated in the input circuit 100 of FIG. 1 is about 8.5 to 17 mW. However, since the efficiency of a switching circuit including a switching transistor or the like is usually 70 to 85%, in the worst case, the power loss is increased by about 1.5 times to about 13 to 26 mW.

Therefore, the input circuit 100 of this embodiment is
In comparison with the internal power loss of 120 to 240 mW of the conventional input circuit 300 of FIG. 4, the internal power loss of 1 is reduced to 1/9 in the worst case, and the heat generation amount is also reduced accordingly.

When a two-wire proximity switch is used as the detection switch 20, the off-state leakage current is usually 1 m.
It is A or less. If the resistance value of the parallel resistor 2 is 430Ω and the resistance value of the current detection resistor 16 is 120Ω, the power loss is 0.55 mW or less. Therefore,
When the detection switch 20 is off, the input circuit 1
The internal power loss of 00 is a small value of 1 mW or less.

As described above, the input circuit 10 of this embodiment.
At 0, the power supplied from the input power supply 21 is hardly consumed by the resistance, so that the conventional input circuit 300 of FIG.
The calorific value is considerably smaller than that of. Therefore, even if the same number of input circuits are provided, the case can be downsized by using the input circuit 100 of this embodiment. Further, since the current supplied from the input circuit 100 to the detection switch 20 becomes almost complete direct current, a two-wire non-contact switch such as a two-wire proximity switch can be connected as the detection switch 20.

Although the number of elements forming the input circuit 100 is increased as compared with the input circuit 300 of FIG. 4, the elements other than the photocoupler 1, the inductor 14 and the smoothing capacitor 15 are bipolar monolithic ICs. It is possible to easily form an integrated circuit by such a technique. As a result, the number of components can be reduced, and an inexpensive and highly reliable input circuit can be obtained.

FIG. 3 is a circuit diagram showing the structure of an input circuit according to another embodiment of the present invention. The input circuit 200 of FIG. 3 differs from the input circuit 100 of FIG. 1 in the following points.

A transformer 22 is provided as the coupling means instead of the photocoupler 1, and a primary winding 22a of the transformer 22 is used as the power storage means instead of the inductor 14. The primary winding 22a of the transformer 22 is connected between the node N2 and the node N3, and the secondary winding 22b of the transformer 22 includes a diode 23, a capacitor 24, and a resistor 2.
5 is connected to the rectifying / smoothing circuit 26. Also,
The node N1 is directly connected to the external input terminal IN.
The configuration of the other parts is similar to that shown in FIG.

In the input circuit 200 of FIG. 3, when the switching transistor 3 is turned on, a current flows from the input power supply 21 to the detection switch 20 through the switching transistor 3, the primary winding 22a of the transformer 22 and the current detection resistor 16. When the switching transistor 3 is off, a current flows from the external common terminal COM to the detection switch 20 through the flywheel diode 13, the primary winding 22a of the transformer 22, and the current detection resistor 16 by the self-induction action of the primary winding 22a of the transformer 22.

The smoothing capacitor 15 smoothes the current, and a substantially constant DC current flows through the detection switch 20. In this case, the voltage of the primary winding 22a of the transformer 22 becomes an intermittent rectangular wave due to the switching transistor 3 being turned on and off, so that the secondary winding 22b of the transformer 22 is similar to the voltage due to the mutual induction action of the transformer 22. A voltage appears, and the voltage is smoothed by the rectifying / smoothing circuit 26 into a constant DC voltage.

In the input circuit 200 of FIG. 2 as well, similar to the input circuit 100 of FIG. 1, the power supplied from the input power supply 21 is hardly consumed by the resistance. Therefore, compared with the conventional input circuit 300 of FIG. The amount of heat generated is considerably reduced. Further, since a DC current is supplied from the input circuit 100 to the detection switch 20, a two-wire non-contact switch such as a two-wire proximity switch can be connected as the detection switch 20.

The present invention can be applied not only to the input circuit of the programmable controller but also to various input circuits for inputting the state of the external switch to the internal circuit. In particular, the present invention is effective when applied to an input circuit having multiple input terminals.

[0063]

According to the first to fifth aspects of the invention, the internal power loss and the heat generation amount are reduced, and moreover, a substantially constant DC current is supplied to the external switch, so that the case can be downsized and A contact switch and a non-contact switch can be used as the external switch.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of an input circuit according to an embodiment of the present invention.

FIG. 2 is a current and voltage waveform diagram of each part of the input circuit of FIG.

FIG. 3 is a circuit diagram showing a configuration of an input circuit according to another embodiment of the present invention.

FIG. 4 is a circuit diagram showing a configuration of a conventional input circuit.

[Explanation of symbols]

 1 Photocoupler 1a Light emitting element 1b Light receiving element 2 Parallel resistance 3 Switching transistor 13 Flywheel diode 14 Inductor 16 Current detection resistor 17 Current detection transistor 20 Detection switch 21 Input power supply 22 Transformer 22a Primary winding 22b Secondary winding 100, 200 Input circuit IN External input terminal COM External common terminal POW External power supply terminal In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (5)

[Claims] 1. In an input circuit for inputting the state of an external switch to an internal circuit via a coupling means, a switching means and an inductive element are inserted in a current path from a power source to an external switch via the coupling means, and An input circuit comprising: current detecting means for detecting a current flowing through the coupling means; and control means for controlling ON / OFF of the switching means based on the current detected by the current detecting means. 2. The coupling means is an optical coupling means including a light emitting portion connected to the current path and a light receiving portion connected to the internal circuit, and the inductive element is an inductor. The input circuit according to claim 1. 3. The coupling means is an inductive coupling means composed of a primary winding connected to the current path and a secondary winding connected to the internal circuit, and the inductive element is the inductive coupling means of the inductive coupling means. The input circuit according to claim 1, wherein the input circuit is the primary winding. 4. An input circuit for inputting the state of an external switch to an internal circuit, the coupling circuit transmitting the state of the external switch to the internal circuit based on a current supplied from a power source; Switching means for connecting and disconnecting a current path to the external switch via coupling means, and control means for controlling on / off of the switching means so that the value of the current supplied to the coupling means becomes a predetermined value. When the switching means is turned on, the power supplied from the power source is stored, and when the switching means is turned off,
And an electric power storage means for supplying a current to the coupling means by the stored electric power. 5. The input circuit according to claim 4, further comprising a current path forming unit that forms a current path including the external switch, the power storage unit and the coupling unit when the switching unit is turned off.