JP2000304799A - Wiring diagnostic device - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To diagnose whether there is an abnormality in a conductor wiring or a circuit wiring. SOLUTION: Connectors 1 at both ends of a wire harness 14
A power source is connected to the connectors 6 and 18 via the connector 12, and a resistor R13 is connected to the connector 20. At this time, the voltage applied to the wire harness 14 is compared with the voltage when the transistor Q1 is off, that is, when the reference voltage is constant, by the comparator CMP3, and when the reference voltage is varied by turning on the transistor Q1. The voltage and the voltage applied to the wire harness 14 are compared by a comparator CMP3. When both levels are “H” and “L”, the voltage is normal. When both levels are “L”, the voltage is open. Are both "H", it is diagnosed that there is a short circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wiring diagnostic device, and more particularly to a wiring diagnostic device suitable for diagnosing the state of a conductor wiring such as a wire harness or a circuit wiring.

[0002]

2. Description of the Related Art Conventionally, when diagnosing the presence or absence of an abnormality in a conductor wiring such as a wire harness, both ends of the wire harness are connected to a power supply, and the wire harness is disconnected from the state of current or voltage drop flowing through the wire harness. A configuration is used to check whether or not there is.

[0003]

However, in the prior art, since a voltage is simply applied to the wire harness, it is possible to detect that the wire harness has been disconnected, but a short circuit occurs in the middle of the wire harness. Or when the line resistance of the wire harness indicates an abnormal value, it may not be possible to check for these abnormal states. Note that a configuration in which a microcomputer is used to diagnose an abnormality in the wire harness is also conceivable. However, since the microcomputer handles minute voltages and minute currents, it is possible to simply use a microcomputer to determine whether there is an abnormality in the wire harness that allows a large current to flow. It is difficult to check.

An object of the present invention is to provide a wiring diagnosis device capable of diagnosing the presence or absence of an abnormality in a conductor wiring or a circuit wiring.

[0005]

In order to achieve the above object, the present invention provides a driving means for outputting a driving signal and the driving means inserted into a power supply circuit connecting one end of a wiring to be diagnosed and a power supply. First switching means that is turned on by a signal to close the power supply circuit, a series resistor inserted in series in a power supply circuit connecting a reference potential side of the power supply to the other end of the wiring, A reference resistor for receiving a supply and generating a reference voltage; a second switching means inserted into a shunt circuit connecting the power supply and the reference resistor and conducting by the drive signal to close the shunt circuit; A diagnostic switching unit that is controlled to be conductive or non-conductive in response to an on / off signal, and that the reference voltage is connected in parallel to the reference resistor on condition that the diagnostic switching unit is conductive. An auxiliary reference resistor to Do, is obtained by forming the interconnection diagnostic device including a comparing means for outputting a comparison result of the comparison between the output voltage of said reference voltage and said switching means.

In configuring the wiring diagnostic device,
The following elements can be added:

There is provided display means for displaying the comparison result of the comparison means in association with the conduction / non-conduction state of the diagnostic switching means.

According to the above-described means, a voltage is applied to the wiring to be diagnosed, the voltage applied to the wiring to be diagnosed is compared with a voltage keeping the reference voltage constant, and the voltage is applied to the wiring to be diagnosed. The output voltage is compared with the voltage obtained when the reference voltage is changed, and the comparison result of the two is output. Therefore, the state of the wiring to be diagnosed can be determined from each comparison result, and the wiring to be diagnosed can be determined. It is possible to diagnose whether there is a short circuit or whether there is an abnormality in the resistance value. In particular, by displaying each comparison result, the state of the wiring to be diagnosed can be easily determined, and it is possible to easily diagnose whether there is a short circuit in the wiring to be diagnosed or whether there is an abnormality in the resistance value. Become.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the embodiments of the present invention, a basic configuration and a basic operation of a switching device having a current oscillation type interruption function to which the present invention is applied will be described.

As shown in FIG. 1, a switching device having a current oscillation type interruption function is a semiconductor integrated circuit (power I) in which various circuit elements are integrated on a semiconductor chip 110.
C), and the power supply terminal T1 is connected to the output voltage V
B (for example, +12 volts), the ground terminal T2 is grounded, and the output terminal T3 is connected to the load 102.

On the semiconductor chip 110, as a semiconductor element (power device) having a thermal cutoff function, n
The FET QA with a built-in channel temperature sensor is integrated. The FET QA with a built-in temperature sensor has a drain electrode connected to a power supply 101 via a drain terminal D and a power supply terminal T1, a source electrode connected to a load 102 via a source terminal S and an output terminal T3, and a gate electrode connected to a gate terminal. It is connected to the drive circuit 111 via a TG and a resistor RG. The FET QA with a built-in temperature sensor is inserted into a power supply circuit connecting the power supply 101 and the load 102 and conducts (turns on) in response to a drive signal (on-pulse signal) input to the gate terminal TG to close the power supply circuit. It is configured as first switching means. An n-channel FET QB and an FET QC are integrated as reference devices in parallel with the temperature sensor built-in FET QA.

The FET QB has a drain electrode connected to a power supply 101 via a drain terminal D and a power supply terminal T1, a source electrode connected to a first reference resistor Rr1 via an output terminal T4, and a gate electrode connected to a gate terminal TG. Through the resistor R
Connected to G. The FET QC has a drain electrode connected to the power supply 101 via a drain terminal D and a power supply terminal T1, a source electrode connected to a second reference resistor Rr2 via an output terminal T5, and a gate electrode connected via a gate terminal TG. It is connected to a resistor RG. The FET QB is configured as second switching means that is turned on by a drive signal (on-pulse signal) input to the gate terminal TG to close a shunt circuit connecting the power supply terminal T1 and the first reference resistor Rr1. The FET QC is configured as third switching means that is turned on by a drive signal (on-pulse signal) input to the gate terminal TG and closes a shunt circuit connecting the power supply terminal T1 and the second reference resistor Rr2.

As the FETs QA, QB, and QC, for example, a power MOSFET having a DMOS structure, a VMOS structure, or a UMOS structure, and a MOSF having a similar structure to these.
ET can be used, and MOS composite devices such as EST and MCT, and other insulated gate power devices such as IGBT can be used. If the gate is always used with a reverse bias, the junction type FE
T, junction type SIT, SI thyristor, or the like can also be used. Further, FETs QA and Q used for a power IC
As B and QC, either an n-channel type or a p-channel type can be used.

In addition, FETs QA, Q
B and QC are configured using, for example, a power device having a multi-channel structure in which a plurality of unit cells (unit cells) are connected in parallel, and each FET is arranged adjacent to each other. The current capacity of the FETs QB and QC is F
It is set smaller than the current capacity of ETQA. This setting is adjusted by the number of unit cells connected in parallel that constitute the FETs QB and QC. For example, the number of the unit cells of the FET QB is 1 and the number of the unit cells of the FET QA is 1000.
The ratio of the channel width W of the ETQA is, for example, 1: 1000.

Further, the source terminal S of the FET QA is connected to the plus input terminals of the comparators CMP1 and CMP2, the source electrode of the FET QC is connected to the minus input terminal of the comparator CMP1, and the source electrode of the FET QC is connected to the comparator CMP2. Connected to negative input terminal. Comparator CM
The output terminal of P1 is connected to the drive circuit 111, and the output terminal of the comparator CMP2 is connected via the output terminal T6 of the semiconductor chip 110 to the undercurrent detection, lamp disconnection detection, and abnormality detection unit 501 that performs open detection. .
The source terminal S of the FET QA is connected to the drive circuit 111 via a Zener diode ZD1, and the Zener diode ZD1 is connected to the FET QA and the FET QA.
B, 1 between gate terminal TG and source terminal S of FETQC
The voltage is maintained at 2 volts, and when an overvoltage is applied to the gate terminal TG, the overvoltage is bypassed.

On the other hand, the power supply enable section 302 and the masking circuit 30
3. The ON / OFF counting circuit 304, the charge pump circuit 305, and the cutoff latch circuit 306 are integrated, the power enable unit 302 is connected to the terminal T7, and the masking circuit 303 is connected to the capacitor C11 via the terminal T8. , The ON / OFF counting circuit 304 is connected to the capacitor C12 via the terminal T9, the drive circuit 111 is connected to the switch SW1 and the resistor R11 via the input terminal T10, and the cutoff latch circuit 306 is connected via the output terminal T11. An output unit (diagnosis result output unit) 502 is connected.

As shown in FIG. 2, the drive circuit 111 includes a source transistor Q5 and a sink transistor Q6, and further includes a drive element for controlling the on / off of each transistor, and the transistors Q5 and Q6 are connected in series. It is connected. The collector of the source transistor Q5 is connected to the terminal of the potential VP, and the emitter is connected to the gate terminal TG via the resistor RG.
The sink transistor Q6 has a collector connected to the gate terminal TG via the resistor RG, and an emitter connected to the ground potential (GN).
D). The terminal of the potential VP is connected to the charge pump circuit 305, and the potential VP of this terminal is
Is output from the charge pump circuit 305,
01, for example, the voltage of the power supply 101 is 12
When V is set, it is set to 12V + 10V.

When the switch SW1 is turned on and the input terminal T10 is grounded via the switch SW1, the drive circuit 111 turns on the source transistor Q5 in response to a command signal from the input terminal T10, and the output terminal (The connection point between the transistor Q5 and the transistor Q6) is configured as a driving unit that outputs a high-level driving signal (on-pulse signal). On the other hand, when the switch SW1 is opened, the voltage of the power supply 101 is applied to the input terminal T10 via the resistor R11.
6 is turned on, and the level of the output terminal (the connection point between the transistor Q5 and the transistor Q6) is changed to a low level. Note that the driving circuit 111 is a CMOS FE instead of a bipolar transistor.
It is also possible to configure using T.

When a drive signal (on-pulse signal) from the drive circuit 111 having the above configuration is input to the gate terminal TG, each of the FETs QA, QB, and QC conducts, and the voltage between the drain and source electrodes of each FET becomes as shown in FIG. 2V
It falls below. At this time, when the load 102 is in a normal state, while the drive signal is output from the drive circuit 111, the voltage between the drain and source electrodes of each FET is maintained at 2 V or less, and the drain current 705 of the FET QA becomes constant.

On the other hand, when the load 102 is short-circuited,
2, a large current may flow, and the load 102 and the FET QA may be damaged.

Therefore, a configuration is adopted in which the source voltages of the FETs QA and QB are monitored by the comparator CMP1 and when the voltages of both are abnormal, the driving circuit 111 forcibly stops the output of the driving signal.

That is, the source voltage of the FET QA is input to the plus input terminal of the comparator CMP1, and the source voltage of the FET QB is input to the minus input terminal. The comparator CMP1 having a hysteresis characteristic compares the voltages input to the plus input terminal and the minus input terminal, and when the source voltage of the FET QA substantially matches the source voltage of the FET QB, or when the source voltage of the FET QA When the source voltage of the FET QA is higher than the reference voltage (FET QB
Out of the allowable value by the source voltage of
When a large current flows through the load 102 and the source voltage of the FET QA becomes lower than the reference voltage generated by the first reference resistor Rr1, it is determined that an abnormal current has flowed through the FET QA, and a low-level signal is output. 111. The driving circuit 111 includes a comparator CMP1
When an "H" level signal is input from the controller, the output of the drive signal is enabled, but when an "L" level signal is input, the output of the drive signal is forcibly stopped. ing. That is, the comparator CMP1
Are configured as drive stop means.

On the other hand, when the FET QA transitions from the on state to the off state, the transistor Q6 is turned on, and the diode D1 is turned on. As a result, the resistance R1,
A current flows through the path of the diode D1 and the comparator CM
The potential of the plus input terminal of P1 is lower than when the drive circuit 111 is performing on-control. Therefore, immediately after the transition to the off state, FE is maintained until a small specific drain-source voltage difference occurs, that is, until the source voltage of the FET QA becomes substantially equal to the source voltage of the FET QB.
TQA is kept off.

However, when the FET QA is turned off due to a short circuit in the wiring, the drain current increases due to the short circuit in the wiring, and the FET QA passes through the pinch-off region, for example, in the triode characteristic region. The state transits to the off state through the operation state of. As a result, after the elapse of a certain time, the potential of the plus input terminal of the comparator CMP1 increases, the output level of the comparator CMP1 changes from the “L” level to the “H” level, and the FET QA transitions to the ON state again. As shown in FIG. 3, the periodic transition of the drain-source voltage 703 of the FET QA at the time of an abnormality such as a short circuit of the load 102 is continued while the switch SW1 is closed. As a result, the drain current 707 of the FET QA fluctuates periodically. FET QA drain
The transition cycle of the source-to-source voltage 703 is determined by a time constant based on wiring inductance and wiring resistance, the capacitance of the FET QA, and the like.

Therefore, the number of times the FET QA is turned on and off is counted, and when the counted value reaches the set value, the FET QA
A is forcibly shut off and this shut off state is maintained.

Specifically, an ON / OFF counting circuit 304 is used as a circuit for counting the ON / OFF state of the FET QA.
And a cutoff latch circuit 306 (Japanese Unexamined Patent Publication No.
See 244414. ).

As shown in FIG. 2, the ON / OFF counting circuit 304 includes bipolar transistors Q41, Q42, Q
43, n-channel FET Q44, diodes D41, D
42, D43, Zener diode ZD41, resistor R4
1 to R46.

The cathode side of the Zener diode ZD41 is connected to the source terminal S of the FET QA, and when the voltage of the source terminal S is in a normal state, the transistor Q
A forward bias voltage is applied to the base of the transistor 43, and the transistor Q43 is on. Therefore, the transistor Q
42 is also in the ON state. On the other hand, since the base of the transistor Q41 is connected to the output terminal of the drive circuit 111 via the resistor R41 and the diode D42, when the transistor Q5 is on, that is, when the FET QA is on, the transistor Q41 is off. is there.

On the other hand, when the transistor Q6 is turned on, that is, when the FET QA is turned off, the diode D42 is grounded via the transistor Q6, so that the transistor Q41 is turned on. Transistor Q
When the switch 41 is turned on, the current from the power supply 101 is supplied to the capacitor C1 via the transistors Q41 and Q42 and the resistor R44.
2 and the capacitor C12 is charged.

Next, when the transistor Q5 transitions from off to on, the transistor Q41 turns off, and the electric charge charged in the capacitor C12 is discharged via the resistor R46. Thereafter, when the transistor Q6 is turned on again and the transistor Q41 is turned on, the capacitor C12 is further charged.

In the process of repeating such an on / off operation, the electric charge charged in the capacitor C12 causes the FET
When the gate voltage of Q44 exceeds the threshold, FET Q4
4 turns on and the diode D42 conducts. As a result, both ends of the temperature sensor 121 are short-circuited via the diode D43, and a latch command signal is output to the cutoff latch circuit 306. That is, the ON / OFF counting circuit 304 is configured as a latch command unit. The time until the ON / OFF count reaches the set value is
It can be adjusted by the time constant of the resistor R46 and the capacitor C12.

The cut-off latch circuit 306 includes an n-channel FE
It comprises TQS, Q11, Q12, Q13, Q14, a temperature sensor 121, and resistors R31 to R35,
The drain electrode of the FET QS is the gate terminal T of the FET QA.
G, and the source electrode is connected to the source terminal S of the FET QA.
It is connected to the. The temperature sensor 121 is configured by connecting four diodes in series.
When the temperature of 10 exceeds the set temperature, the voltage at both ends is lower than the set voltage. That is, the voltage across the temperature sensor 121 is F
The threshold value is set higher than the threshold value between the source and gate electrodes of the ETQ11, and the FET Q11 is always kept on. When the FET Q11 is on,
FET Q14 is off, FET Q13 is on, FET
Q12 and the FET QS are kept off.

On the other hand, the FET Q44 is turned on and both ends of the temperature sensor 121 are short-circuited via the diode D43, or the temperature of the semiconductor chip 110 exceeds the set temperature and the voltage across the temperature sensor 121 becomes lower than the set voltage. If it drops, the FET Q11 turns off from on and the FET Q14 turns on. When the FET Q14 turns on, the FET Q13 turns on and the FET QS
Is turned on, and F is applied between the source and gate electrodes of the FET QA.
A short circuit occurs due to ETQS, and FET QA is turned off. This short-circuit state is caused by the FET Q1 constituting the latch circuit.
2, latched by Q13. That is, the cutoff latch circuit 306 turns ON / OFF of the ON / OFF counting circuit 304.
When the number of OFF times reaches a set value, or when the temperature sensor 121 detects that the temperature of the semiconductor chip 110 has exceeded the set temperature, the FET QA is turned off and the non-conductive state is set. It is configured as a breaking latch means for latching.

Next, the present embodiment will be described.

In this embodiment, the switching device having the current oscillation type interrupting function described above is mainly used. The output terminal T3 of the semiconductor chip 110 is connected to the plus input terminal of the comparator CMP3 and the connector 12. I have. The connector 12 is connected to a conductor wiring to be diagnosed, for example, a connector 16 at one end of a wire harness 14.
Connected through. A connector 18 is connected to the other end of the wire harness 14, and the connector 18 is grounded via a connector 20 and a resistor (series resistor) R13.

On the other hand, a display 22 as a display means is connected to the output terminal of the comparator CMP3, and the minus input terminal of the comparator CMP3 is connected to the collector of the transistor Q1 via a resistor (auxiliary reference resistor) R12. And is connected to the first reference resistor Rr1. The transistor Q1 is configured as diagnostic switching means. The emitter is grounded, and a diagnostic on / off signal is input to the base. When the transistor Q1 is off, the reference voltage across the reference resistor Rr1 is kept constant by the conduction of the FET QB. However, when the transistor Q1 is turned on, the resistor 12 is connected in parallel to the reference resistor Rr1.
The reference voltage decreases.

Here, in the present embodiment, the resistance Rr
1, the resistance values of the resistors R12 and R13 are set as follows. For example, when the wire harness 14 is in a normal state and a current of, for example, 10 A flows, the resistance value of the resistor R13 is set according to the normal current. Also, the upper limit of the normal current is 12A,
When the lower limit is set to 8A, the resistance value of the reference resistor Rr1 is set so that a current of 12A flows through the wire harness 14 with the transistor Q1 turned off. Further, when the transistor Q1 is on, the resistance value of the resistor R12 is set such that a current of 8 A flows through the wire harness 14 by the combined resistance value of the resistor R12 and the reference resistance Rr1.

Next, a specific example of the resistance value of each resistor will be described. For example, when the resistance value of the wire harness 14 in a normal state is 0.4 ohm, the normal current of the wire harness 14 is 10 A, and when the output voltage of the power supply 101 is 12 V, the resistance value of the resistor R13 is 0.8 ohm. Once set, the combined resistance value of the wire harness 14 and the resistor R13 is 1.2 ohms.

In the above configuration, when diagnosing the presence / absence of a short circuit of the wire harness 14 and the abnormality of the resistance value, etc.
First, an ON signal is input to the base of the transistor Q1 to turn on the transistor Q1. When the transistor Q1 is turned on, the reference resistor Rr1 is connected in parallel with the resistor 12, and the reference voltage decreases. At this time, wire harness 14
When a current of 10 A flows through the comparator CMP
3 outputs an “H” signal. That is, the signal of "H" is output from the comparator CMP3 assuming that the current is higher than the lower limit of 8A.

Next, when the transistor Q1 is turned off,
That is, when a current of 10 A flows through the wire harness 14 when the reference voltage is at the original reference voltage, the upper limit of 12 A
As lower than A, the comparator CMP3 outputs “L”
Is output.

In this case, as shown in FIG.
Indicates the output voltage of the comparator CMP3 corresponding to the on / off state of the transistor Q1, and from this result it is diagnosed that the wire harness 14 is in a normal state.

Next, when the wire harness 14 is in the open state, for example, when the terminal of the connector 16 is disconnected, the transistor Q1 is turned on.
Since no current flows through the wire harness 14, an "L" signal is output from the comparator CMP3. on the other hand,
Even when the transistor Q1 is turned off, since no current flows through the wire harness 14, the comparator CMP3
Outputs an "L" signal. This result is displayed on display 2
When it is displayed as 2, the current in the on / off state of the transistor Q1 is lower than the lower limit value and the upper limit value, and it is diagnosed that the wire harness 14 is in the open state.

Next, when the resistance value of the wire harness 14 indicates an abnormal value, for example, when a short circuit occurs in the middle of the wire harness 14 and the internal resistance of the wire harness 14 decreases to 0.1 ohm, the transistor When Q1 is turned on, a current of 13 A flows through the wire harness 14. That is, the current i = 12 volts / (the internal resistance of the wire harness 14 is 0.1 ohm + the resistance of the resistor R3 is 0.1 .OMEGA.).
8) = 13A On the other hand, even when the transistor Q1 is turned off, the current of 13A flows through the wire harness 14, so that the comparator C
An “H” signal is output from MP3. Then, when this result is displayed on the display 22, it is determined that a current larger than the lower limit value and the upper limit value has flowed, and the wire harness 14
Is diagnosed as being in a short condition.

Next, when the resistance value of the wire harness 14 increases due to poor contact of the connector 16 or 18 connected to the wire harness 14, for example, the resistance value becomes 1.2
When the transistor Q1 is turned on when the current increases to ohms, only a current of 5 A flows through the wire harness 14, so that an "L" signal is output from the comparator CMP3. Also, when the transistor Q1 is turned off, only a current of 5 A flows through the wire harness 14, so that an "L" signal is output from the comparator CMP3. Then, when this result is displayed on the display unit 22, as an abnormality caused by the increase in the resistance value of the wire harness 14,
It is diagnosed that the wire harness 14 is in the open state.

According to the present embodiment, the voltage from the output terminal T3 is applied to the wire harness 14, and the wire harness 1
Since the voltage applied to the wire harness 14 is compared with the voltage applied to the wire harness 14 and the voltage applied when the reference voltage is varied, the voltage applied to the wire harness 14 is compared with the voltage applied when the reference voltage is kept constant. Presence or absence of short circuit of the harness 14,
An abnormal state of the resistance value can be diagnosed.

[0046]

As described above, according to the present invention,
When a voltage is applied to the wiring to be diagnosed, the voltage applied to the wiring to be diagnosed is compared with a voltage that keeps the reference voltage constant, and the voltage applied to the wiring to be diagnosed and the reference voltage are changed. And output the result of comparison between the two, so that the state of the wiring to be diagnosed can be determined from each of the comparison results. The presence or absence can be diagnosed.
In particular, by displaying each comparison result, the state of the wiring to be diagnosed can be easily determined, and it is possible to easily diagnose whether there is a short circuit in the wiring to be diagnosed or whether there is an abnormality in the resistance value. Become.

[Brief description of the drawings]

FIG. 1 is a block diagram of a switching device having a current oscillation type interruption function, which is a basis of the present invention.

FIG. 2 is a main part circuit configuration diagram of the switching device shown in FIG. 1;

FIG. 3 is a waveform chart for explaining the operation of the switching device shown in FIG. 1;

FIG. 4 is a configuration diagram of a main part of a wiring diagnostic apparatus according to an embodiment of the present invention.

FIG. 5 is a diagram for explaining a diagnosis result of the wiring diagnosis device.

[Explanation of symbols]

 12, 16, 18, 20 Connector 14 Wire harness 22 Indicator 110 Semiconductor chip 111 Drive circuit 304 ON / OF counting circuit 305 Charge pump circuit 306 Cut-off latch circuit QA, QB, QC n-channel FET Rr1 First reference resistor Rr2 First 2 reference resistance CMP1, CMP2, CMP3 Comparator

Claims (2)

[Claims] 1. A driving means for outputting a driving signal, and a first switching means inserted into a power supply circuit connecting one end of a wiring to be diagnosed and a power supply, and turned on by the driving signal to close the power supply circuit. A series resistor inserted in series in a power supply circuit connecting a reference potential side of the power supply and the other end of the wiring, a reference resistance receiving a current supplied from the power supply to generate a reference voltage, and the power supply A second switching means inserted into a shunt circuit connecting the reference resistor and closed by the drive signal to close the shunt circuit; and a diagnostic switch controlled to a conductive or non-conductive state in response to a diagnostic on / off signal. A switching unit, an auxiliary reference resistor connected in parallel to the reference resistor to reduce the reference voltage on condition that the diagnostic switching unit is conductive, the reference voltage and the switch A wiring diagnosis device comprising: a comparison unit that compares an output voltage of the switching unit and outputs a comparison result. 2. A driving means for outputting a driving signal, and a first switching means inserted into a power supply circuit connecting one end of a wiring to be diagnosed and a power supply, and turned on by the driving signal to close the power supply circuit. A series resistor inserted in series in a power supply circuit connecting a reference potential side of the power supply and the other end of the wiring, a reference resistance receiving a current supplied from the power supply to generate a reference voltage, and the power supply A second switching means inserted into a shunt circuit connecting the reference resistor and closed by the drive signal to close the shunt circuit; and a diagnostic switch controlled to a conductive or non-conductive state in response to a diagnostic on / off signal. A switching unit, an auxiliary reference resistor connected in parallel to the reference resistor to reduce the reference voltage on condition that the diagnostic switching unit is conductive, the reference voltage and the switch And a display unit for displaying the comparison result of the comparison unit in association with the conduction / non-conduction state of the diagnostic switching unit. Diagnostic device.