JPH09222448A - Safety circuit with fault-diagnosing function - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a safety circuit having a fault-diagnosing function and a simple structure, which can correctly diagnose cause of a failure with effectively protecting a circuit device from overcurrent and excessive temperature rise. SOLUTION: A switching means 14 such as an FET(field-effect transistor) 14 or the like is interposed between a buttery 10 as a power source and lamps 16A, 16B as loads and a PTC 15 (positive temperature coefficient thermister) is arranged in series with the switching means 14. At least either one of electric potentials at both terminals of the PTC (e.g. potential at point F) is inputted to a signal processing section 18 to diagnose cause of a failure on the basis of the potential when the potential is found to be abnormal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a safety circuit with a failure diagnosing function provided with a switch means between a power source and a load in an automobile or the like.

[0002]

2. Description of the Related Art Conventionally, in a circuit provided with a switch means such as a manual switch or a semiconductor switching device between a power source and a load, the presence or absence of a failure is automatically determined to perform a safe operation. There is a safety circuit with a fault diagnosis function.

An example is shown in FIG. In the figure, 90
Is a semiconductor switching device FET (field effect transistor), the source of which is connected to the positive terminal of a battery 92 which is a power source, the drain is connected to a load such as a lamp 94, and the drain and the lamp 94 are connected to each other. A resistor 98 is provided between the two. Control unit 96
Is a microcomputer or the like,
A gate signal is input to the gate of 0 to turn on / off the FET 90, and the potential of the X point on the power supply side of the resistor 98 and the potential of the Y point on the ground side are taken in, and the presence or absence of failure is determined based on these potentials. Play a role in. The contents of the judgment are as follows.

1) When the FET 90 is switched to the ON state (the source and the drain are electrically connected), but the potential difference between XY is 0 V, that is, when the current is not flowing, It is determined that the load of the lamp 94 or the like is broken. 2) When the FET 90 is in the ON state and the potential difference between XY becomes larger than the potential difference during normal use, that is, when an overcurrent flows, a short circuit of a load such as the lamp 94 or a voltage abnormality of the battery 92 occurs. The FET 90 is immediately turned off (that is, a safe operation is performed) when it is determined that it has been done.

[0005]

In recent years, the FET 90 described above is used.
In particular, the semiconductor switching device has been remarkably improved, and with the expansion of the allowable current range, the semiconductor switching device can be used in a wide range of fields such as lamp dimming and motor control. However, such a semiconductor switching device becomes uncontrollable even if it exceeds the permissible current range even a little or the device ambient temperature slightly exceeds its standard temperature, or in the worst case, the device itself. There is a risk that the product will be destroyed and cannot be reused, and countermeasures against it are very important. Even if such a semiconductor switching device is not used, it is still necessary to protect other circuit elements.

Here, in the circuit shown in FIG. 3, the control unit 96 takes in the electric potentials at points X and Y, determines whether or not an overcurrent is generated based on these potentials, and the overcurrent is generated. If it is determined that the control unit 96 switches the FET 90 to the off state, the FET 90 or the like will not be effectively protected if the control unit 96 fails or a false detection occurs. However, it has the drawback of being unreliable. In addition to the above-mentioned overcurrent, it is necessary to newly install an expensive device such as a temperature sensitive device in addition to the above resistor 98 in order to know whether or not an excessive temperature rise occurs in the circuit. As the cost increases, the control becomes complicated.

A fuse has been known as a means for avoiding the above-mentioned inconvenience due to overcurrent. However, once the fuse is blown by the occurrence of overcurrent, it cannot be re-energized unless replaced. However, there is a drawback that maintenance is very troublesome. Moreover, there is a drawback that the automatic failure diagnosis as described above cannot be performed.

In view of such circumstances, the present invention has a simple structure and can reliably protect circuit components such as a semiconductor switching device from an overcurrent and an excessive temperature rise, and can accurately diagnose the occurrence of a failure. It is intended to provide a safety circuit with a failure diagnosis function.

[0009]

As a means for solving the above-mentioned problems, the present invention is a circuit provided with a switch means for switching between an ON state for conducting a power source and a load and an OFF state for interrupting the load. A positive characteristic thermistor is arranged in series with the switch means, and a failure determining means for determining the presence or absence of a failure based on the potential of at least one of the potentials at both ends of the positive characteristic thermistor and outputting a determination signal is provided. is there.

In this circuit, if the temperature rises excessively in the circuit, the temperature of the positive temperature coefficient thermistor also rises, its electrical resistance rapidly increases, and the current is cut off. Also, when an overcurrent is generated due to a short circuit of a load or the like, the Joule heat is generated in the switch means or the like accompanying this and the temperature of the positive temperature coefficient thermistor is also raised, and the current is cut off. Therefore, it is automatically prevented that the circuit is continuously driven in the state of excessive temperature rise or excessive current, and each device is effectively protected. Moreover, the failure determination means takes in at least one of the potentials at both ends of the positive temperature coefficient thermistor, so that the failure determination means makes an appropriate failure diagnosis.

Generally, it is preferable to arrange the switch means and the positive temperature coefficient thermistor in series between the power source and the load. In this case, the power source side potential of the positive temperature coefficient thermistor is the positive voltage thermistor. Unless there is a relatively large resistance on the power supply side, there is almost no change between when the current flows in the positive temperature coefficient thermistor and when it does not flow, so it is more accurate to determine whether the current abnormality is due to a failure that shuts off the positive temperature coefficient thermistor. For the determination, it is preferable to take in at least the ground potential of the positive temperature coefficient thermistor.

More specifically, when the grounding potential of the PTC thermistor is substantially 0, even though the switch means is in the ON state, it is determined that the PTC thermistor breaks down. The positive-characteristic thermistor is cut off when the failure determination means is configured to output a signal or when the ground side potential of the positive-characteristic thermistor is substantially 0 and the power-source potential of the positive-characteristic thermistor is substantially equal to the power-supply voltage. It is preferable that the failure determination means is configured to output a determination signal indicating that a failure has occurred. In particular, in the latter case, there is an advantage that it is possible to accurately determine whether the positive characteristic thermistor itself cuts off and no current flows, or whether the current does not flow due to a failure on the power supply side of the positive characteristic thermistor.

Further, according to the present invention, when the switch means is in the ON state and the ground side potential of the PTC thermistor is substantially equal to the power supply voltage, that is, when the PTC thermistor is not electrically cut off but no current flows. The failure determination means may be configured to output a determination signal indicating that the load has been cut off.

The switch means may be manually opened and closed, for example, but when the switch means is a semiconductor switching device which can be switched between the on state and the off state by a control signal input to a control terminal. This makes it possible to directly protect this semiconductor switching device from the above-mentioned overcurrent and overheating.

Also in this case, it is preferable to arrange the switch means and the positive temperature coefficient thermistor in series between the power source and the load, and the positive temperature coefficient thermistor may be provided on the ground side of the semiconductor switching device. However, it may be provided on the power supply side.

In the former case, although the control signal for turning on the semiconductor switching device is input to the control terminal of the semiconductor switching device, the positive temperature coefficient thermistor and the semiconductor switching device are connected. When the potential between the semiconductor switching devices is approximately 0 V, it can be determined that the semiconductor switching device has a disconnection failure, and a control signal for turning off the semiconductor switching device is provided to the control terminal of the semiconductor switching device. If the ground side potential of the semiconductor switching device is equal to the potential when the semiconductor switching device is in the ON state despite the input of, it is possible to determine that the semiconductor switching device has a short circuit failure.

In the latter case, although the control signal for turning on the semiconductor switching device is input to the control terminal of the semiconductor switching device, the positive temperature coefficient thermistor and the semiconductor switching device are connected. When the potential between the two is equal to the power supply voltage, it can be determined that the semiconductor switching device has a cutoff failure, and a control signal for turning off the semiconductor switching device is input to the control terminal of the semiconductor switching device. Nevertheless, if the ground potential of the PTC thermistor is equal to the potential when the semiconductor switching device is in the ON state, it can be determined that the semiconductor switching device has a short circuit failure.

In each of the above devices, further, by providing the display means for displaying the determination content of the determination signal output from the failure determination means, it is possible to promptly inform the user or the like of the occurrence of the failure.

[0019]

FIG. 1 shows a first embodiment of the present invention.
It will be described based on. Here, a current control circuit for an automobile, which uses the battery 10 (voltage 12V) mounted on the automobile as a power source and uses the rear lamp 16A and the like mounted on the automobile as a load, is shown. However, it can be widely applied to other motor drive circuits.

In FIG. 1, the source of an FET (Field Effect Transistor) 14, which is a switching device, is connected to the positive terminal of the battery 10 via a fuse 12.
The positive temperature coefficient thermistor (Positive Temperature
Coefficient thermistor; hereinafter simply referred to as PTC. ) 15 via a pair of left and right rear lamps 16A, 16
One terminal of B is connected in parallel. The other terminals of the rear lamps 16A and 16B are connected to the negative terminal of the battery 10, and these terminals are grounded. The FET 14 is in a state where the source-drain is energized (ON state) only when the gate potential thereof is lower than the source potential by a predetermined value or more, and is otherwise in a state where the source-drain is not energized (OFF state). ).

When a semiconductor switching device is used in the present invention, the type thereof is not particularly limited, and
Besides ET, IGBT (Insulated Gate Bipolar Transis)
tor), BPT (Bipolar Transistor) and the like can be preferably used.

Both of the rear lamps 16A and 16B are also used as a brake lamp and a tail lamp of an automobile, and their lighting control is performed based on turning on / off of the brake switch S1 and the lamp switch S2.

The brake switch S1 is normally open, and is configured to be closed only when the brake of the automobile is depressed. One terminal of the brake switch S1 is connected to the plus terminal of the battery 10 via the fuse 12, and the other terminal is connected to the lamp 22A via a PTC 20A similar to the PTC 15. The lamp switch S2 is
It is provided in a position near the driver's seat in the vehicle and is opened / closed by a driver or the like, one terminal of which is connected to the plus terminal of the battery 10 via the fuse 12 and the other terminal is connected to the PTC 15 PT similar to
It is connected to the lamp 22B via C20B.

Incidentally, the above-mentioned fuse 12 is used for each PTC1.
There is a sufficient case where the fuse 12 is not cut and the PTC 15, 20A, 20B is cut off first, when a current sufficiently larger than the current for cutting off the 5, 20A, 20B flows. .

In this circuit, a signal processing section 18 is provided as means for performing the above-mentioned lighting control and the failure diagnosis which is a feature of the present invention. The signal processing unit 18 is composed of a microcomputer, a transistor, and the like, and outputs a potential at a point A between the brake switch S1 and the PTC 20A and a potential at a point C between the lamp switch S2 and the PTC 20B to each switch S1. , S2 is taken in as a lamp lighting command signal which changes depending on ON / OFF, and further, as a parameter for failure diagnosis, the potential at the point B between the PTC 20A and the lamp 22A and the point D between the PTC 20B and the lamp 22B are further detected. Potential, FET14 and PTC1
The electric potential of the point E between 5 and PTC15 and both rear lamps 1
It is configured to take in the potential at the point F between 6A and 16B.

The contents of the lamp lighting control operation and the failure determination operation performed by the signal processing unit 18 are as follows.

A) Lamp lighting operation: The signal processing section 18
A signal having a duty ratio according to ON / OFF of the switches S1 and S2 is input to the gate of the FET 14 as a gate signal (control signal). The duty ratio here is
The duty ratio is set as follows, which is a value obtained by dividing the time during which the gate signal has a level for turning on the FET 14 in one pulse cycle by the pulse cycle.

[0028]

[Table 1]

B) Fault diagnosis operation: The signal processing unit 18
~ The electric potential at the point F is monitored, and when these electric potentials are different from the electric potentials in the normal state, it is judged that a failure has occurred, and a judgment signal of a type corresponding to the electric potential is output to a display device (display means) 24 such as a CRT. Then, the determination content is displayed. The relationship between each potential and the content of the determination signal is as follows.

[0030]

[Table 2]

Next, the operation of this circuit will be described.

First, during normal driving, the following effects are obtained.

When the brake is depressed: The brake switch S1 is turned on, and the corresponding signal is input to the control device 18. Control device 1 which received this
Numeral 8 inputs a signal having a duty ratio of 100% (first duty ratio) (that is, a signal for keeping the FET 14 always in the ON state) to the gate of the FET 14 as a gate signal regardless of whether the lamp switch S2 is on or off.
The source-drain of No. 4 is always electrically connected. Therefore, in this case, regardless of whether the lamp switch S2 is on or off (that is, regardless of day or night), the rear lamps 16A and 16A are
B lights up with high brightness. At the same time, the lamp 22A is energized and lit.

When the brake is not depressed but the lamp switch S2 is turned on: the control unit 1
8 inputs a square wave signal having a duty ratio of X% (second duty ratio) to the gate of the FET 14 as a gate signal. As a result, the source and drain of the FET 14 are intermittently conducted only during the period corresponding to the duty ratio of X%. Therefore, in this case, both the rear lamps 16A and 16B light up with a lower brightness than when the brake pedal is depressed, and these rear lamps 16A and 16B function as tail lamps. Further, the lamp 22B is energized and turned on, and the driver is informed that the lamp switch S2 has actually been turned on.

When the brake is not depressed and the lamp switch S2 is off: The control device 18 controls the FET 14
Signal with a duty ratio of 0% (that is, always F
A signal for keeping the ET 14 in the off state) is output. Therefore, the source and drain of the FET 14 do not conduct, and the rear lamps 16A and 16B do not light at all. Also, the lamps 22A and 22B are not turned on.

Therefore, in this circuit, the so-called P that changes the duty ratio of the gate signal input to the FET 14 is used.
The brightness of both rear lamps 16A, 16B is adjusted by the WM control, and the rear lamps 16A, 16B are distinguished by distinguishing the brightness.
16B is used as both a brake lamp and a tail lamp.

Next, the failure diagnosis operation performed when an abnormality occurs in the circuit will be sequentially described based on the case numbers shown in Table 2 above.

Case 1 (when the brake switch S1 is on and the potential difference between AB is the battery voltage 12V): In this case, the PTC 20A cuts off the current, so that an overcurrent has flowed to the PTC 20A. is expected. As a cause of the overcurrent flowing through the PTC 20A, a short circuit of the lamp 22A, which is a load, and an abnormality in the battery voltage are considered. However, if the potentials at the other points C to F are normal, there is no abnormality in the battery voltage. Therefore,
In this case, a determination signal indicating that the lamp 22A is short-circuited is output.

Case 2 (when the brake switch S1 is on and the potential at the point B is the battery voltage 12V): In this case, the current is cut off on the ground side of the PTC 20A, but the cutoff is made by the lamp 22A. The probability of disconnection is extremely high. Therefore, a determination signal indicating that the lamp 22A is disconnected is output.

Case 3 (when the lamp switch S2 is on and the potential difference between the CDs is the battery voltage 12V):
In this case, since the current is cut off by the PTC 20B, it is expected that an overcurrent has flowed to this PTC 20B. As in case 1, if the potentials at the other points A, B, E, F are normal, there is no abnormality in the battery voltage, and therefore a determination signal indicating that the lamp 22B is short-circuited is output.

Case 4 (when the lamp switch S2 is on and the potential at the point D is the battery voltage 12V): In this case, the current is cut off on the ground side of the PTC 20B, but the cutoff is made by the lamp 22B. The probability of disconnection is extremely high. Therefore, a determination signal indicating that the lamp 22B is disconnected is output.

Case 5 (when the gate signal for turning on the FET 14 is output and the potential difference between the EFs is a battery voltage of 12 V): In this case, since the current is cut off by the PTC 15, an overcurrent flows in the PTC 15. Is expected to have flowed. For the same reason as in Case 1, since there is no abnormality in the battery voltage, a determination signal indicating that at least one of the lamp 16A and the lamp 16B is short-circuited is output.

By cutting off the current, it is possible to prevent a state in which an overcurrent flows for a long time, so that the FET 14 is effectively protected from this overcurrent.

Case 6 (when the gate signal for turning on the FET 14 is output and the potential at the point F is higher than the normal potential): In this case, the voltage drop between the EFs is lower than in the normal state. That is, since the current flowing through the PTC 15 is small, it is expected that a failure occurs in which the overall circuit resistance decreases. As the failure in which the circuit resistance increases, it is considered that the resistance of the PTC 15 itself increases and one of the lamps 16A and 16B connected in parallel is disconnected, but in the case of, the potential at the point F rises. It does not fall under this case, since it never happens. Therefore, a determination signal indicating that the lamp 16A or the lamp 16B is disconnected is output.

Case 7 (Even though the gate signal for turning on the FET 14 is output, the potential at the point E is 0V.
In this case): In this case, the source-drain of the FET 14 is not always electrically connected, and therefore, the determination signal indicating that the FET 14 has the interruption failure is output.

Case 8 (Even though the gate signal for turning on the FET 14 is not output, the potential at the point E is 12
V)): In this case, the source of FET 14
Since it means that there is always continuity between the drains,
A determination signal indicating that the FET 14 has a short circuit fault is output.

Case 9 (Brake switch S1 is on, the potential difference between AB is battery voltage 12V, Lamp switch S2 is on, the potential difference between CD is battery voltage 12V, and FET14 is on. When the gate signal is output and the potential difference between the EFs is a battery voltage of 12V): In this case, all PTs
It is expected that overcurrent or overheating occurred at C15, 20A, and 20B, and it was cut off.
It is virtually impossible for 16B, 22A and 22B to be short-circuited at the same time. Therefore, when the battery voltage temporarily rises and an overcurrent flows, or the ambient temperature rises excessively, all the PTCs 15, 20A, 20
It is reasonable to judge that B has blocked. Therefore, a determination signal to that effect is output.

Case 10 (Brake switch S1 is on, potential at point A is 0V, and lamp switch S
2 is on, the potential at point C is 0 V, and FET1
When the potential of the point E is 0 V in the state where the gate signal for turning on 4 is output): In this case, the switches S1 and S
It is expected that disconnection occurred on the power source side of FET 2 and FET 14. Therefore, a determination signal indicating that the fuse 12 is blown or other disconnection fault is output.

Therefore, according to this circuit, the PTC15,
By utilizing the resistance characteristics of 20A and 20B, circuit devices such as the FET 14 can be effectively protected from overcurrent and excessive temperature rise, and the potential changes on the power supply side and the ground side of the PTC 15, 20A and 20B can be monitored. It is possible to make an accurate failure judgment.

The second embodiment is shown in FIG. Here, the power source side of the FET 14 (that is, the fuse 12 and the FE
PTC15 is arranged between (T14). In addition, the signal processing unit 18 takes in the potential at the point G between the PTC 15 and the FET 14 and the potential at the point H between the FET 14 and the lamps 16A and 16B, and creates and displays the following determination signal. It is configured to output to the device 24.

[0051]

[Table 3]

Since Cases 1 to 4 in this table are exactly the same as Cases 1 to 4 in Table 2, the description thereof is omitted here, and Cases 15 to 20 will be described below.

Case 15 (when the gate signal for turning on the FET 14 is output and the potential at the point G is 0 V):
In this case, since the current is cut off by the PTC 15, it is expected that an overcurrent has flowed to this PTC 15. Here, when the power supply side potentials (potentials at points A and C) of the other PTCs 20A and 20B are normal, there is no abnormality in the battery voltage.
A determination signal indicating that at least one of B is short-circuited is output.

Also in this case, the interruption of the current prevents the state in which the overcurrent flows from lasting for a long time, so that the FET 14 is effectively protected from this overcurrent.

Case 16 (when the potential at the point G is higher than the normal potential when the gate signal for turning on the FET 14 is output): In this case, the voltage drop at the PTC 15 is lower than that in the normal state. That is, since the current flowing through the PTC 15 is small, it is expected that a failure occurs in which the overall circuit resistance decreases. As a fault in which the circuit resistance increases, an increase in the resistance of the PTC 15 itself and a disconnection of either one of the lamps 16A and 16B connected in parallel can be considered. In the case of, the potential at the point G rises. It does not fall under this case, since it never happens. Therefore, a determination signal indicating that the lamp 16A or the lamp 16B is disconnected is output.

The disconnection failure can also be determined by monitoring the potential at the H point.

Case 17 (Even though the gate signal for turning on the FET 14 is output, the potential at the H point is 0
In the case where V remains): This case means that the source and drain of the FET 14 are not always conducted. Therefore, a determination signal indicating that the FET 14 has a cutoff fault is output.

Case 18 (when the potential at the H point is sufficiently higher than 0V even though the gate signal for turning on the FET 14 is not output): In this case, the FET is
Since the source-drain of 14 is always electrically connected, a determination signal indicating that the FET 14 has a short-circuit fault is output.

Case 19 (Brake switch S1 is on, the potential difference between AB is battery voltage 12V, Lamp switch S2 is on, the potential difference between CD is battery voltage 12V, and FET14 is on. When the potential difference between the GH and the battery voltage is 12 V with the gate signal being output): In this case, for the same reason as in case 9 above, a determination signal indicating that the battery voltage is abnormal or excessive temperature rise is generated. Output.

Case 20 (Brake switch S1 is on, potential at point A is 0V, and lamp switch S
When 2 is on, the potential at point C is 0V and at point G is 0V
Case): In this case, for the same reason as the case 10, the determination signal indicating that the fuse 12 is blown or other disconnection fault is output.

As described above, in the present invention, the PTC is replaced by the FET.
The switch means such as 14 may be provided on either the power supply side or the ground side. Further, in FIG. 2, if the power supply side potential of the PTC 15 is also taken in, more accurate failure diagnosis can be performed. That is, when the G point is 0 V, it is possible to determine whether the PTC 15 has a failure to be cut off or the fuse 12 is blown or the like without monitoring the potentials at the A point and the C point. it can.

The embodiment of the present invention is not limited to the above. For example, the following embodiment may be adopted.

(1) In each of the above-mentioned embodiments, P is connected in series with all the switching means (switches S1, S2 and FET 14).
Although the TC is provided, the present invention is intended to include at least one switch means in which the PTC is provided in series. However, arranging the PTC in series with the semiconductor switching device such as the FET 14 has an advantage that the semiconductor switching device can be directly protected.

(2) In the present invention, it is also possible to take in the power-source-side potential of the PTC and perform failure diagnosis. However, in this case, unless there is a relatively large resistance on the power source side of the PTC, there is almost no change in the above-mentioned potential between when the current flows through the PTC and when it does not flow, and it is difficult to determine. Therefore, P
In order to more accurately determine whether or not a failure such that the TC is cut off, it is preferable to take in at least the ground side potential of the PTC.

(3) The present invention is not limited to the one in which the switch means and the PTC are provided between the power source and the load, but is, for example, the one in which the switch means and the PTC are provided between the load and the ground. Can also be applied. In this case, PT
By taking in the potential on the side close to the load at C,
It is possible to detect a short circuit or disconnection of the load.

[0066]

As described above, according to the present invention, the positive characteristic thermistor is arranged in series with the switch means for switching the conduction state between the power source and the load, and based on at least one of the potentials at both ends of the positive characteristic thermistor. Since it is provided with a failure determination means that determines the presence or absence of a failure and outputs a determination signal, the positive characteristic thermistor effectively protects each circuit device from overcurrent and excessive temperature rise, and There is an effect that an accurate failure diagnosis can be performed from the change of the potential corresponding to the characteristic.

Here, when the switch means and the positive temperature coefficient thermistor are arranged in series between the power source and the load, if at least the ground side potential of the positive temperature coefficient thermistor is taken in, the current abnormality is corrected. It is possible to more accurately determine whether or not it is due to a failure that shuts off the characteristic thermistor.

More specifically, when the ground side potential of the PTC thermistor is substantially 0 even though the switch means is in the ON state, or the ground side potential of the PTC thermistor is almost 0, and When the potential on the power source side of the positive temperature coefficient thermistor is substantially equal to the power source voltage, it can be determined that a failure that interrupts the positive temperature coefficient thermistor has occurred. If the ground potential of the positive temperature coefficient thermistor is substantially equal to the power supply voltage when the switch means is in the ON state,
That is, when the positive temperature coefficient thermistor is not electrically cut off, but no current flows, it can be determined that the load is cut off.

In the case where the switch means is provided with a semiconductor switching device which can be switched between the on state and the off state by a control signal inputted to a control terminal, the semiconductor switching device is connected to the positive temperature coefficient thermistor. It is possible to directly protect from an overcurrent or an excessive temperature rise.

By taking in the potential between the positive temperature coefficient thermistor and the semiconductor switching device or the ground side potential of the semiconductor switching device, it is possible to obtain an effect of diagnosing a short-circuit fault or a breaking fault of the semiconductor switching device itself. .

Further, each of the above-mentioned devices is provided with the display means for displaying the judgment contents of the judgment signal outputted from the above-mentioned failure judgment means, so that the user or the like can be promptly notified of the occurrence of the failure.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention.

FIG. 3 is a circuit diagram showing an example of a conventional safety circuit with a failure diagnosis function.

[Explanation of symbols]

 10 Battery (Power Supply) 14 FET (Semiconductor Switching Device) 15, 20A, 20B PTC (Positive Characteristic Thermistor) 16A, 16B Rear Lamp (Load) 18 Signal Processor (Failure Judgment Means) 22A, 22B Lamp (Load) 24 Display Device ( Display means) S1 Brake switch (switch means) S2 Lamp switch (switch means)

Front Page Continuation (72) Inventor Yukinobu Tabata 1-7-10 Kikuzumi, Minami-ku, Aichi Prefecture Haruna Research Institute Ltd. (72) Inventor Takashi Hoshino 1-7-10, Kikuzumi, Minami-ku, Aichi Prefecture No. Harness Research Institute Co., Ltd.

Claims (15)

[Claims] 1. A circuit provided with switch means for switching between an on state for conducting a power supply and a load and an off state for interrupting the load, wherein a positive characteristic thermistor is arranged in series with the switch means, and the positive characteristic thermistor is provided. 2. A safety circuit with a failure diagnosis function, comprising: failure determination means for determining the presence / absence of a failure based on at least one of the potentials at both ends of the. 2. The safety circuit with a failure diagnosis function according to claim 1, wherein the switch means and the positive temperature coefficient thermistor are arranged in series between the power supply and the load. . 3. The safety circuit with a failure diagnosis function according to claim 2, wherein the failure determination means is configured to determine the presence or absence of a failure based on at least the ground potential of the positive temperature coefficient thermistor and output a determination signal. A safety circuit with a failure diagnosis function. 4. The safety circuit with a failure diagnosis function according to claim 3, wherein the positive temperature coefficient thermistor is provided when the ground side potential of the positive temperature coefficient thermistor is substantially 0 even though the switch means is in an ON state. A safety circuit with a failure diagnosing function, characterized in that the failure determining means is configured to output a determination signal indicating that a failure to be shut off has occurred. 5. The safety circuit with a failure diagnosing function according to claim 4, wherein the positive characteristic thermistor has a ground side potential of substantially 0 and the positive characteristic thermistor has a power source side potential substantially equal to the power source voltage. A safety circuit with a failure diagnosis function, characterized in that the failure determination means is configured so as to output a determination signal indicating that a failure that shuts off is output. 6. The safety circuit with a failure diagnosis function according to claim 3, wherein the load is shut off when the switch means is on and the ground side potential of the positive temperature coefficient thermistor is substantially equal to the power supply voltage. A safety circuit with a failure diagnosing function, characterized in that the failure determining means is configured to output a determination signal to the effect. 7. The safety circuit with a failure diagnosis function according to claim 1, wherein the switch means includes a semiconductor switching device that can be switched between the on state and the off state by a control signal input to a control terminal. A safety circuit with a characteristic failure diagnosis function. 8. The safety circuit with a failure diagnosis function according to claim 2, wherein the switch means is:
A safety circuit with a failure diagnosis function, comprising a semiconductor switching device that is switched between the on state and the off state by a control signal input to a control terminal. 9. The safety circuit with a failure diagnosis function according to claim 8, wherein the positive temperature coefficient thermistor is provided on the ground side of the semiconductor switching device. 10. The safety circuit with a failure diagnosing function according to claim 9, wherein the control signal for turning on the semiconductor switching device is input to a control terminal of the semiconductor switching device. A failure diagnosis characterized in that the failure determination means is configured to output a determination signal indicating that the semiconductor switching device has a cutoff failure when the potential between the characteristic thermistor and the semiconductor switching device is approximately 0V. Safety circuit with functions. 11. The safety circuit with a failure diagnosis function according to claim 9, wherein a control signal for turning off the semiconductor switching device is input to a control terminal of the semiconductor switching device. When the ground potential of the semiconductor switching device is equal to the potential when the semiconductor switching device is in the ON state, the failure determination means is configured to output a determination signal indicating that the semiconductor switching device has a short circuit failure. A safety circuit with a characteristic failure diagnosis function. 12. The safety circuit with a failure diagnosis function according to claim 8, wherein the positive temperature coefficient thermistor is provided on the power supply side of the semiconductor switching device. 13. The safety circuit with a failure diagnosis function according to claim 12, wherein the control signal for turning on the semiconductor switching device is input to a control terminal of the semiconductor switching device. Failure diagnosis characterized in that the failure determination means is configured to output a determination signal indicating that the semiconductor switching device has a cutoff failure when the potential between the characteristic thermistor and the semiconductor switching device is equal to the power supply voltage. Safety circuit with functions. 14. The safety circuit with a failure diagnosis function according to claim 12, wherein a control signal for turning off the semiconductor switching device is input to a control terminal of the semiconductor switching device. When the earth side potential of the positive temperature coefficient thermistor is equal to the potential when the semiconductor switching device is in the ON state, the failure determination means is configured to output a determination signal indicating that the semiconductor switching device has a short circuit failure. A safety circuit with a characteristic failure diagnosis function. 15. The safety circuit with a failure diagnosis function according to any one of claims 1 to 14, further comprising display means for displaying the determination content of the determination signal output from the failure determination means. Safety circuit with failure diagnosis function.