JP2007072908A - Plc muting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform the accurate detection of abnormalities in a PLC muting device regardless of the power consumption of a muting lamp. <P>SOLUTION: This PLC muting device comprises: a field-effect transistor FET1 that is placed between a first terminal of a muting lamp L1 and a power terminal, and controls power supply to the lamp L1; a field-effect transistor FET2 that is placed between a second terminal of the lamp L1 and a ground terminal, and controls power supply; and a determination detection circuit comprising a failure detection current path CD and placed in parallel with the field-effect transistor FET1 where an electric current does not pass through with a determination detection signal in an off-state when the field-effect transistor FET1 is in an conduction state and the lamp L1 is applied with electricity while an electric current from the power terminal is made to bypass the lamp L1 and the determination detection signal is outputted in an on-state in response to the bypass current when the field-effect transistor FET1 is in a cut-off state. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

In the safety PLC in which an emergency stop is performed when the safety sensor detects the entry of a person or the like into the work area, when the work is carried into the work area, the muting sensor The present invention relates to a muting device that detects this and lights a muting lamp to notify an operator that the safety function is stopped and temporarily stops the safety function.
In particular, the present invention relates to a device with improved fault diagnosis function of the muting device.

  Conventionally, when a safety sensor such as a light curtain for detecting the worker's entry into the work area is detected and a worker's entry into the work area is detected, a work tool such as a machine tool or robot is used. There is known a safety PLC designed to make an emergency stop. On the other hand, when a work is automatically loaded into the work area, a muting function is provided so that the safety sensor does not detect an intrusion and stop the work equipment urgently every time a work is loaded. A muting device is known. This muting device is equipped with a muting sensor at a position where only a work, not a person, can be detected. When a work is detected by the muting sensor, the safety sensor is masked to detect the safety function. Is not working. At this time, the muting lamp is turned on to inform the operator that the muting period is when the safety function is stopped.

  In order to prevent the muting lamp from being disconnected and the lamp from being turned off in spite of the muting period, a circuit for detecting the disconnection of the lamp is provided. As shown in FIG. 4, this circuit inserts a resistor R in series with the muting lamp L and compares the potential at the connection point between the resistor R and the lamp L with a reference potential, thereby preventing the disconnection of the lamp. The presence or absence is detected. That is, when the lamp L is disconnected, there is almost no voltage drop across the resistor R, and when the lamp L is conducting, it is determined by the potential at the connection point that the voltage drop across the resistor R increases. I have to.

  However, the power consumption of the muting lamp L varies and varies, such as 20 W and 40 W. For this reason, the current flowing through the resistor R changes depending on the power consumption of the lamp L. Therefore, the voltage drop between the terminals of the resistor R changes according to the power consumption of the lamp L. It is necessary to change the reference voltage of the comparator or the value of the resistor R. Further, if the resistance R is reduced in order to reduce the power consumption at the resistance R, there is a problem that the voltage drop at the resistance R is reduced and the noise resistance is lowered. Conversely, when the value of the resistor R is increased, the voltage drop increases and the noise resistance improves, but heat generation at the resistor R becomes a problem, and there is a problem that the element cannot be reduced. In the case of a light-emitting diode lamp, the flowing current is small, the voltage drop at the resistor R is small, and it is difficult to determine the presence or absence of disconnection.

Accordingly, the present invention has been made to solve the above-described problem, and an object thereof is to detect a failure such as a disconnection of a muting lamp without using a voltage drop of a resistor. This reduces power consumption, eliminates the hassle of replacing resistors and resetting the reference voltage according to lamp power consumption, improving noise resistance, and improving the reliability of fault detection. is there.
Another object is to accurately detect a failure such as disconnection even when a lamp with low power consumption such as an LED lamp is used.

According to a first aspect of the present invention for solving the above-mentioned problem, in the PLC muting apparatus, the display means displays the muting period during which the safety function by the safety sensor is invalidated by the detection by the muting sensor. A first switch means that is disposed between the terminal and the power supply terminal and controls power supply to the display means, and is disposed between the second terminal of the display means and the ground terminal to control power supply to the display means. When the second switch means is arranged in parallel with the first switch means between the first terminal of the display means and the power supply terminal, and the first switch means is in a conductive state and current is flowing through the display means When the determination detection signal is turned off without current flowing and the first switch means is in the cut-off state, the current from the power supply terminal is bypassed to the display means, and the A muting device, characterized in that it comprises a determination detection circuit for outputting a determination detection signal as an on-state.
Here, the first switch means and the second switch means are transistors such as bipolar transistors and field effect transistors, and relays. The display means may be anything such as an incandescent lamp or LED as long as it gives an alarm to the worker. If the determination detection circuit operates as described above, the circuit configuration is arbitrary. In other words, a state in which a current can flow in the display means is configured, and any circuit may be used as long as the current flows in the display means via the determination detection circuit when the first switch means is in the cutoff state.

According to a second aspect of the present invention, in the first aspect of the invention, the determination detection circuit includes a series circuit of a Zener diode, a photocoupler that outputs a determination detection signal, and a resistor.
When the first switch means is in a conductive state, no large voltage is applied to the determination detection circuit, and the Zener diode is not in a conductive state. On the other hand, if the first switch means is in the cut-off state, a voltage that makes the Zener diode conductive is applied to the determination detection circuit if a current flows through the display means. Thereby, a current is supplied to the display means through the determination detection circuit. Depending on the level of this determination detection signal, it is possible to determine the abnormality of the display means.

  According to a third aspect of the present invention, there is provided a diode inserted between the first switch means and the first terminal of the display means in the forward direction, and connected between the anode terminal of the diode and the ground, so that the flowing current is reduced. In response to the first state determination circuit for outputting the first state determination signal, the second state determination signal is arranged between the power supply terminal and the second terminal of the display unit in parallel with the display unit and according to the flowing current. The muting device according to claim 1, further comprising: a second state determination circuit that outputs a second state determination circuit.

  According to a fourth aspect of the present invention, there is provided a determination unit for determining a disconnection failure of the display means when the determination detection signal is in the off state when the first switch means is in the cut-off state and the second switch means is in the conductive state. The muting device according to any one of claims 1 to 3, wherein the muting device is provided.

First, in the muting operation state, both the first switch means and the second switch means are controlled to be in a conductive state, and a current flows through the display means to light up. In this state, since a current flows through the first switch means, no current flows through the determination detection circuit. Therefore, the determination detection signal indicates an off state. Next, when the first switch means is turned off while the second switch means is in a conducting state, no current flows through the first switch means, so that the voltage across the terminals of the first switch means rises. A current flows through a series circuit of the determination detection circuit and the display means connected in parallel with each other. Therefore, the determination detection signal is turned on. As described above, when the determination detection signal changes in level between the OFF state and the ON state in response to the conduction / cutoff of the first switch means, the display means can determine that there is no disconnection. On the other hand, when the display means is disconnected, even if the first switch means is disconnected, no current flows through the determination detection circuit because the determination detection circuit and the display means are connected in series. Therefore, the determination detection signal remains off. The presence / absence of disconnection and the presence / absence of the display means can be detected by such a change in the state of the determination detection signal.
As a result, since no resistor is used as in the prior art, less power is consumed, and the voltage drop due to the resistor is not measured, so that noise resistance is improved and the accuracy of abnormality monitoring is improved.

  According to the second aspect of the present invention, the operating voltage of the Zener diode inserted in the determination detection circuit is set to a value sufficiently larger than the voltage drop during conduction of the first switch means, so that the first switch means is in the conductive state. Sometimes, the Zener diode does not operate and no current flows through the determination detection circuit. On the contrary, when the first switch means is in the cut-off state, the power supply voltage is applied to the series circuit of the determination detection circuit, the display means, and the second switch means, so that the Zener voltage is applied to the Zener diode. The diode becomes conductive, and the determination detection signal can be turned on. Thereby, it can be determined that the display means is normal. If the determination detection signal indicates an off state when the second switch means is in the conductive state and the first switch means is in the cut-off state, no voltage is applied to the determination detection circuit. Judgment can be made.

According to a third aspect of the present invention, the first state determination circuit outputs the first state determination signal of the on state / off state according to the conduction / cutoff state of the first switch means even when the display means is in a disconnected state. Circuit. When the second switch means is in the conductive state and the first switch means is in the cut-off state, a current flows through the display means via the determination detection circuit, so that a voltage is applied to the first terminal of the display means. The diode prevents the current from flowing back to the first state determination circuit due to this voltage, and when the first switch means is in the cut-off state, the first state determination signal is turned off. When the first terminal of the display means falls, the first state determination circuit may be configured so that no current flows through the first state determination circuit when the potential of the anode of the diode drops to near the ground potential. it can. In this case, even if the first switch means is in the on state, no current flows through the first state determination circuit existing between the anode terminal of the diode and the ground, so the first state determination signal indicates the off state. . Even if the first switch means is in the conductive state, if the first state determination signal is in the OFF state, it can be determined that the first terminal has a ground fault. Further, the ground failure of the first terminal can be determined by the determination detection signal not being turned off when both the first switch means and the second switch means are turned on. That is, the ground voltage of the first terminal applies a power supply voltage to the determination detection circuit, and the Zener diode becomes conductive, so that the determination detection signal is on. If the first terminal is not grounded, the determination detection signal is in the off state as described above.
On the other hand, the second state determination circuit is a circuit that outputs a second state determination signal of an on state / off state according to the conduction / cutoff state of the second switch means. When the second terminal of the display means falls, a current flows through the second state determination circuit regardless of whether the second switch means is on or off, and the second state determination signal is turned on. A ground fault of the second terminal can be determined by the second state determination signal.

  In the invention of claim 4, when the first switch means is in the cut-off state and the second switch means is in the conductive state, if the display means is normal, the determination detection circuit, the display means, the second switch means, Since a current flows through the ground terminal, the determination detection signal is turned on. However, if this determination detection signal is in the OFF state, it can be determined that the display means is not mounted between the first terminal and the second terminal or that the display means is disconnected. Since this apparatus does not use the voltage drop of the resistance as a detection signal in this way, it is possible to reliably detect an abnormality in the display means.

  Hereinafter, the present invention will be described based on specific examples. The present invention is not limited to the following examples.

  FIG. 1 shows an electric circuit of an input / output unit 10 for connecting a muting lamp L1 (hereinafter simply referred to as “lamp L1”) of a muting device according to an embodiment of the present invention. This apparatus has the same two systems of circuits, and the processing is duplicated. In the following description, only one system circuit will be described. The lamp L1 which is a display means is connected between an output terminal OUTB + which is a first terminal and an output terminal OUTB- which is a second terminal. The DC power supply is DC24V, and a series circuit of a Zener diode D2, a photocoupler PJ, and a resistor R1 is connected between the power supply terminal E and the output terminal OUTB +. In addition, a field effect transistor FET2 (hereinafter referred to as a field effect transistor is represented by "FET"), which is the second switch means, is connected between the output terminal OUTB- and the ground terminal EA. A current path including the power supply terminal E, the Zener diode D2, the photocoupler PJ, the resistor R1, the output terminal OUTB +, the lamp L1, the output terminal OUTB- terminal, the FET2, and the ground terminal EA is defined as a failure detection current path CD. The determination detection signal SJ output from the photocoupler PJ is input to the control unit 20. The determination detection circuit includes a series circuit of a Zener diode D2, a photocoupler PJ, and a resistor R1 connected between the power supply terminal E and the output terminal OUTB +. Accordingly, the current flowing through the determination detection circuit is the current flowing through the failure detection current path CD.

  Further, a direct circuit of FET1 and diode D1, which are first switch means, is connected between the power supply terminal E and the output terminal OUTB +. A current path composed of the power supply terminal E, FET1, diode D1, output terminal OUTB +, lamp L1, output terminal OUTB-, FET2 and ground terminal EA is defined as a main current path CM. The current flowing through the main current path CM is a current for lighting the lamp L1 during the muting operation.

  A resistor R2 and a photocoupler PO2 are connected between the power supply terminal E and the output terminal OUTB−. A current path including the power supply terminal E, the resistor R2, the photocoupler PO2, the output terminal OUTB-, the FET2, and the ground terminal EA is defined as a second state detection current path C2. The second state determination signal SO2 output from the photocoupler PO2 is input to the control unit 20. The second state detection current path C2 serves as a second state determination circuit.

  Further, a resistor R3 and a photocoupler PO1 are connected between the connection point a between the FET1 and the diode D1 and the ground terminal EA. A current path composed of the power supply terminal E, FET1, resistor R3, photocoupler PO1, and ground terminal EA is defined as a first state detection current path C1. The first state determination signal SO1 output from the photocoupler PO1 is input to the control unit 20. The first state detection current path C1 serves as a first state determination circuit.

  Next, in order to control the gate of FET1, resistors R4 and R5 and a photocoupler PC1 are connected between the power supply terminal E and the ground terminal EA, and the connection point of the resistors R4 and R5 is connected to the gate of FET1. It is connected. A control signal CS1 is input to the photocoupler PC1 from the control unit 20, and the FET1 is controlled to be turned on / off by the control signal CS1.

  Next, in order to control the gate of FET2, a series circuit of resistor R6 and photocoupler PC2 is connected between the gate of FET2 and ground terminal EA, and resistor R7 is connected between the output terminal of photocoupler PC2 and ground terminal EA. Is connected. A control signal CS2 is input from the control unit 20 to the photocoupler PC2, and the FET 2 is controlled to be turned on and off by the control signal CS2.

  The control signals CS1 and CS2 turn on the lamp L1 at the time of muting and become control signals when executing a test related to the lamp L1. On the other hand, the output signals SJ, SO1, and SO2 are signals for detecting the on / off states of the currents in the failure detection current path CD, the first state detection current path C1, and the second state detection current path C2 in the input / output unit 10, respectively. .

Next, the operation of this apparatus will be described.
1) Checking operation of the lamp L1 at the time of turning on the power By setting the control signal CS1 to the off level and the control signal CS2 to the on level, the FET 1 is turned off and the FET 2 is turned on. At this time, if the lamp L1 is normal, the FET 1 is cut off, but current flows through the failure detection current path CD for the reason described later. As a result, a current flows through the photocoupler PJ, and the determination detection signal SJ is turned on. If the determination detection signal SJ is turned on in response to setting the control signal CS1 to the off level, it can be seen that the lamp L1 is normally mounted and there is no disconnection. Conversely, if the determination detection signal SJ is at the off level, it can be seen that the lamp L1 itself is not attached or is disconnected, and the control unit 20 outputs an alarm signal to the main unit. At this time, in the current value flowing through the failure detection current path CD, even if the lamp L1 is normal, the value of the resistor R1 is set to be large so that the lamp L1 does not light up. L1 does not light up.

2) Normal muting operation When the control unit 20 receives a muting signal from a muting sensor (not shown), it outputs a signal invalidating the detection signal of the safety sensor to the main unit of the safety PLC. At the same time, the control unit 20 outputs the control signals CS1 and CS2 to the photocouplers PC1 and PC2, respectively, so that the FET1 and FET2 are turned on. As a result, current flows through the main current path CM, the lamp L1 is lit, and the operator is informed that the device is in the muting state. At this time, since the operating voltage of the Zener diode D2 is set to be larger than the voltage drop of the FET1 and the diode D1 connected in series, no current flows through the Zener diode D2, and no current flows through the failure detection current path CD. No current flows. Therefore, since no current flows through the photocoupler PJ, the determination detection signal SJ is turned off.

  On the other hand, since the potential at the connection point a is a potential due to a voltage drop in the series connection of the diode D1, the lamp L1, and the FET2, an operating voltage is applied to the photocoupler PO1, and a current is applied to the photocoupler PO1. Flows, and a current flows through the first state detection current path C1. Therefore, the first state determination signal SO1 is on level. Further, since the FET 2 is on, a current flows through the photocoupler PO2, and accordingly, a current flows through the second state detection current path C2. Therefore, the second state determination signal SO2 is on level. When the first and second state determination signals SO1 and SO2 are at the on level, it can be detected that the FET1 and FET2 are in a conductive state.

3) Lamp check operation during muting By setting the control signal CS1 to the off level for a very short time, the FET 1 is cut off for a very short time and the current in the main current path CM is cut off for a very short time. Then, the potential of the output terminal OUTB + is lowered, a bias voltage is applied to the Zener diode D2 in the forward direction, a current flows through the failure detection current path CD, and a current also flows through the photocoupler PJ for a very short time. As a result, the determination detection signal SJ is on level for a very short time.

  Further, since the FET 2 continues to be in a conductive state, the second state determination signal SO2 maintains the on level. Further, when the FET 1 is cut off for a very short time, the current flowing through the first state detection current path C1 is cut off and no current flows through the photocoupler PO1, so the first state determination signal SO1 is turned off for a short time. It becomes. At this time, a current flows through the failure detection current path CD, but the current toward the photocoupler PO1 is blocked by the presence of the diode D1. If the control unit 20 detects that the determination detection signal SJ is turned on when the control signal CS1 is turned off, the control unit 20 can determine that the lamp L1 is normally lit. Since the time for keeping the FET 1 in the cut-off state is very short, the lamp L1 appears to be continuously lit and does not give misunderstanding to the operator.

  On the other hand, when the lamp L1 is disconnected, only the determination detection signal SJ is different from the normal state described above, and even if the control signal CS1 is turned off, no current flows through the lamp L1, and therefore failure detection is performed. Since no current flows through the current path CD, the determination detection signal SJ remains at the off level. With this state, it can be determined that the lamp L1 is disconnected.

4) Operation when the output terminal OUTB + and the output terminal OUTB- are short-circuited When the muting operation is performed, the control signals CS1 and CS2 are turned on, and both the FET1 and FET2 are turned on. At this time, a large current flows through the circuit, and the fuse inserted in the power supply circuit of the DC 24V power supply is blown, so that the power supply is stopped and a safe state is established, and the control unit 20 sends an abnormal signal to the main unit. Is output.

5) Operation when the output terminal OUTB + is grounded When the muting operation is performed, the control signals CS1 and CS2 are turned on, and both the FET1 and FET2 are turned on. At this time, a large current flows through the circuit, and the fuse inserted in the power supply circuit of the DC 24V power supply is blown, so that the power supply is stopped and a safe state is established, and the control unit 20 sends an abnormal signal to the main unit. Is output.

  Even if the fuse does not blow, since the output terminal OUTB + is at the ground potential, the potential at the connection point a, which is the anode of the diode D1, drops to near the ground potential, so that no current flows through the photocoupler PO1. Not flowing. Therefore, even if the FET 1 is turned on, the first state determination signal SO1 does not become the on level. Further, since a power supply voltage of approximately 24 V is applied between the power supply terminal E and the output terminal OUTB + of the failure detection current path CD, a current also flows through the Zener diode D2, and a current flows through the photowrapper PJ. . Therefore, the determination detection signal SJ is turned on. That is, it is possible to determine that the output terminal OUTB + is ground because the control signal SJ does not become the off level even though the FET 1 is turned on.

6) Operation when the output terminal OUTB- is grounded During the muting operation, even if the output terminal OUTB- is grounded, the FET1 and FET2 are turned on, and a current flows through the lamp L1. Since the determination detection signal SJ is also turned off and the second state determination signal SO2 is also turned on, it cannot be determined whether or not the output terminal OUTB− is grounded depending on the conduction state of the FET1 and FET2.
At the time of muting, only FET2 is cut off for a very short time. If the output terminal OUTB- is not grounded and is normal, the minute state current does not flow in the second state detection current path C2, that is, the photocoupler PO2, so that the second state determination signal SO2 is small time. Only off level. However, when the output terminal OUTB- is grounded, a current flows through the second state detection current path C2 regardless of whether the FET2 is turned on or off, so that the second state determination signal SO2 maintains the on level. It will be in the state. Thereby, it is possible to determine the ground level of the output terminal OUTB-. Further, when both the FET1 and FE2 are in the cut-off state, if the output terminal OUTB- is not grounded, no current flows through the failure detection current path CD. Therefore, the determination detection signal SJ should be off level, but when the output terminal OUTB- is grounded, the determination detection signal SJ is on level. Also from this fact, it can be detected that the output terminal OUTB- has fallen.
FIG. 2 shows the states of the control signals SC1 and SC2, the determination detection signal SJ, the first state determination signal SO1, the second state determination signal SO2, the FET1, and the FET2 in each operation mode.

7) Operation of control unit The control unit 20 includes a computer system having a CPU, a ROM, and a RAM as main components. The CPU of the control unit 20 executes the processing procedure shown in FIG. In step 100, it is determined whether or not the power is turned on. If the power is turned on, the control signal CS1 is turned off and CS2 is turned on in step 102, and the output signals SO1, SO2, and SJ are inputted in step 104. In step 106, it is determined whether or not the determination detection signal SJ is off. If the determination detection signal SJ is off, it is determined that the mode 5 in FIG. 2 is abnormal, that is, disconnection, and abnormal processing such as emergency stop or abnormality notification in step 108 is performed. Is executed.

  If the signal SJ is ON in step 106, the device is normal (mode 4 in FIG. 2). Therefore, in step 110, it is determined whether or not a muting command signal has been input. In 112, output signals SO1, SO2, and SJ are input. In step 114, the determination detection signal SJ is on (mode 8 in FIG. 2), or the second state determination signal SO2 is on (mode 10 in FIG. 2). Is determined. If the determination result is YES, it means that there is a mode 8 abnormality (OUTB + terminal ground) or a mode 10 abnormality (OUTB- terminal ground), so an abnormality process is executed in step 116. The above is the abnormality determination process when not muting.

If the muting state is determined in step 110, the control signal CS1 is turned on in step 118, CS2 is turned on, the lamp L1 is energized, the lamp L1 is lit, and in step 120, the output signals SO1, SO2, SJ are turned on. input. Next, in step 122, it is determined whether or not the determination detection signal SJ is off. If it is in the on state, it is determined that mode 7 is abnormal (OUTB + terminal ground), and abnormality processing is executed in step 124. Is done. If the signal SJ is off at step 122, it is normal (mode 1), so it is determined at step 126 whether or not it is a check timing. Off, the control signal CS2 continues to be on, and the output signals SO1, SO2, SJ are input at step 130, and whether or not the determination detection signal SJ remains off at step 132 (does not change to on for a short time) Whether or not) is determined. If the OFF state is continued, it is a mode 3 abnormality (disconnection), so in step 134 an abnormality process is executed. If it is detected in step 132 that the determination detection signal SJ changes to on for a short time, it is normal (mode 2), and thus the apparatus ends normally.
The above processing is executed at small time intervals such as 5-minute intervals.

The determination of the above output signals is normal by using all three output signals SO1, SO2 and SJ as shown in FIG. 2 to determine whether or not their state is the state shown in FIG. You may make it determine abnormality.
With such a circuit configuration and a check method, it is possible to determine abnormality or normality of the muting device including non-mounting of the lamp L1 or disconnection without using a resistor. Therefore, highly accurate abnormality detection can be performed regardless of the power consumption of the lamp. Further, in the present apparatus, even when the lamp is driven with a low current, it is possible to detect an abnormality with high accuracy, so that the LED lamp can be used as a muting lamp.

  INDUSTRIAL APPLICABILITY The present invention can be used for a PLC, in particular, a safety PLC, and can be a device that reliably detects abnormality of a muting device.

The circuit diagram which showed the structure of the muting apparatus which concerns on the specific Example of this invention. The figure which showed the relationship of the state of the control signal for determining normality and abnormality of this apparatus, FET, and an output signal. The flowchart which showed the process sequence of the control part of a present Example apparatus. The circuit diagram which showed the structure of the conventional muting apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Input-output part 20 ... Control part SJ ... Determination detection signal SO1 ... 1st state determination signal SO2 ... 2nd state determination signal CS1, CS2 ... Control signal

Claims (4)

In the PLC muting device that indicates the muting period in which the safety function by the safety sensor is invalidated by the detection by the muting sensor,
A first switch means disposed between a first terminal of the display means and a power supply terminal, for controlling power feeding to the display means;
A second switch means disposed between the second terminal of the display means and a ground terminal, for controlling power feeding to the display means;
Between the first terminal of the display means and the power supply terminal, it is arranged in parallel with the first switch means, and when the first switch means is in a conductive state and current is flowing through the display means, When the first switch means is in the cut-off state, the current from the power supply terminal is bypassed to the display means, and the determination detection signal is turned on according to the bypass current. A muting device comprising: a determination detection circuit that outputs as   The muting device according to claim 1, wherein the determination detection circuit includes a series circuit of a Zener diode, a photocoupler that outputs the determination detection signal, and a resistor. A diode inserted in a forward direction between the first switch means and the first terminal of the display means;
A first state determination circuit connected between the anode terminal of the diode and the ground and outputting a first state determination signal in accordance with a flowing current;
A second state determination circuit disposed in parallel with the display unit and outputting a second state determination signal according to a flowing current between the power supply terminal and the second terminal of the display unit; The muting device according to claim 1 or 2. When the first switch means is in a cut-off state and the second switch means is in a conductive state, it has a determination unit that determines that the display means is broken when the determination detection signal is in an off state. The muting device according to any one of claims 1 to 3.

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