JP2002276524A - Failure detecting circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a failure detecting circuit having an inexpensive constitution capable of detecting a failure of a glow plug. SOLUTION: This failure detecting circuit 1 for detecting a failure of a glow plug driving circuit 11 comprises the glow plug 13 and a first resistor 33 connecting to a harness 15 is series. The failure detecting circuit has a transistor 55 between a first series circuit 31 comprising the first resistor 33, the harness 15, and the glow plug 13 and a power supply 17, a reference potential generating circuit for generating a reference potential (potential of 2-3 connecting point 44) V23 depending on the value of the voltage V of the power supply 17, and a comparing circuit for comparing the potential V1 H level of the 1-H connecting point 34 between the first resistor 33 and the harness 15 with the reference potential V23 level.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a failure detection circuit for detecting a failure of a glow plug which assists in starting an internal combustion engine.

[0002]

2. Description of the Related Art Generally, in a diesel vehicle or the like,
When the temperature of the outside air is low and the engine is cooled, the air in the cylinder is not compressed to the fuel ignition temperature even if the air is compressed. Therefore, a glow plug is attached for the purpose of assisting this. That is, after the glow plug is heated to a high temperature, the air is compressed and the fuel is blown, thereby igniting the fuel in the cylinder and assisting the start of the engine. Therefore, in order to improve the startability of the engine, it is necessary to heat the glow plug to a high temperature in a short time, so that a large load is applied to the glow plug. Therefore, when the engine is started repeatedly,
A failure such as disconnection or short circuit of the glow plug may occur. When such a failure occurs, a trouble such as deterioration of the startability of the engine occurs. Therefore, it is necessary to detect the failure.

Several proposals have been made to detect a failure of a glow plug. For example, Japanese Patent Publication No. 61-51152 discloses a disconnection detecting device that detects a disconnection of a glow plug by flowing a constant current through the glow plug and detecting a resistance value of the glow plug. Japanese Patent Publication No. 3-77384 discloses a disconnection detection device that detects a disconnection of a glow plug by a circuit using a current limiting resistor when the glow plug is energized. In addition, Japanese Unexamined Patent Application Publication No.
Japanese Patent Application Laid-Open No. 81578 discloses a failure diagnosis device that detects a failure such as disconnection or short circuit of a glow plug using a low-voltage power supply that is another power supply.

[0004]

However, all of the conventional devices have the following problems. That is, the disconnection detecting device disclosed in JP-B-61-51152 has a complicated circuit configuration and is expensive. In addition, the disconnection detecting device disclosed in Japanese Patent Publication No. 3-77384 detects a failure during energization of the glow plug, so that the loss due to the current limiting resistor is large. Further, it is impossible to detect a short circuit of the glow plug. Further, a problem is likely to occur due to the influence of variations in circuit components. Further, the failure diagnosis apparatus disclosed in Japanese Patent Application Laid-Open No. 4-81578 requires a low-voltage power supply and a circuit driven by the low-voltage power supply, and is expensive.

The present invention has been made in view of such circumstances, and can detect a failure of a glow plug.
Another object of the present invention is to provide an inexpensive fault detection circuit.

[0006]

Means for Solving the Problems, Action and Effect The solution is to provide one or a plurality of glow plugs connected in parallel and a first switch means for turning on / off the connection between the glow plug and a power supply. A fault detection circuit for detecting a fault in the glow plug drive circuit, comprising: a first resistor connected in series with the glow plug and in parallel with the first switch means; and a glow plug and a first resistor. A connection wiring to be connected; a second switch connected in parallel with the first switch to turn on / off a current flowing from the power supply to the first resistor, the connection wiring, and the glow plug; A reference potential generating circuit for generating a reference potential, and a comparing circuit for comparing the level of the potential at the 1-H connection point between the first resistor and the connection wiring with the reference potential. It is a detection circuit.

[0007] Since this failure detection circuit has the first resistor connected in series to the glow plug and the connection wire, the potential (monitor potential) at the 1-H connection point between the first resistor and the connection wire can be obtained. . On the other hand, an appropriate reference potential according to the power supply voltage can be generated by the reference potential generation circuit.
Then, the monitor potential (1-H connection point potential) and the reference potential can be compared by the comparison circuit. Therefore, when the reference potential is set to an appropriate value, when the second switch is turned on when the first switch is turned off, the magnitude judgment of the comparison circuit is inverted between when the glow plug is normal and when the glow plug is disconnected or short-circuited. I do. For this reason, it is possible to detect either a disconnection or a short circuit of the glow plug.

Further, since the reference potential is generated according to the magnitude of the power supply voltage, even if the power supply voltage fluctuates, both the 1-H connection point potential and the reference potential are generated according to the power supply voltage. As a result, disconnection or short circuit of the glow plug can be accurately detected. Specifically, the reference potential includes a reference potential proportional to the power supply voltage, such as a voltage obtained by dividing the power supply voltage by resistance. Note that it is preferable to generate the reference potential by the resistance voltage division because a separate power supply is unnecessary. Further, in the failure detection circuit according to the present invention, since there is no need to provide a current limiting resistor between the first switch means and the glow plug, loss of electricity to the glow plug when the first switch means is turned on is suppressed. be able to. Examples of the first and second switch means include, in addition to a relay switch, a switching element such as a transistor, a power MOSFET, a thyristor, and a GTO, or a switch circuit including these.

Another solution is a failure in a glow plug drive circuit having one or a plurality of glow plugs connected in parallel and first switch means for turning on / off the connection between the glow plug and a power supply. A first resistor connected in series with the glow plug and in parallel with the first switch means, a connection wire connecting the glow plug and the first resistor, A second series circuit in which a second resistor and a third resistor are connected in series, and a second series circuit in which the second resistor and the third resistor are connected in series; A second switch means for turning on / off a connection between a circuit including the first series circuit and the second series circuit and the power supply; a potential at a 1-H connection point between the first resistor and the connection wiring; Two If a fault detection circuit and a comparison circuit for comparing the level of the potential at 2-3 the connection point between the third resistor.

[0010] Since this failure detection circuit has the first resistor connected in series to the glow plug and the connection wiring, the potential (monitor potential) at the 1-H connection point between the first resistor and the connection wiring can be obtained. . On the other hand, since there is a second series circuit in which the second resistor and the third resistor are connected in series, the second resistor and the third resistor are connected in series.
The potential (reference potential) at the 2-3 connection point with the resistor can be taken. The monitor circuit (1-H connection point potential) and the reference potential (2-3 connection point potential) can be compared by the comparison circuit. Therefore, when the reference potential (2-3 connection point potential) is set to an appropriate value while the circuit is simple, when the second switch is turned on when the first switch is turned off, the case where the glow plug is normal is obtained. The magnitude judgment of the comparison circuit is reversed depending on the disconnection or short circuit. For this reason, it is possible to detect either a disconnection or a short circuit of the glow plug. In addition, since the power supply for driving the glow plug is used to generate the reference potential (2-3 connection point potential) used for determining the failure of the glow plug by the second resistor and the third resistor, the power supply is separately provided. It is unnecessary, and even if the power supply voltage fluctuates, the 1-H connection point potential and the 2-3 connection point potential are generated according to the power supply voltage, and disconnection or short-circuit of the glow plug can be accurately performed. Can be detected.

Further, in the above failure detection circuit, the number of the glow plugs is n (n = 1, 2, 3,...)
The normal resistance value of the glow plug at normal time is Rg,
When the resistance value of the connection wiring is Rh, the resistance value of the first resistance is R1, the resistance value of the second resistance is R2, and the resistance value of the third resistance is R3, (Rh + (Rg /
n)) / (R1 + Rh + (Rg / n)) <R3 / (R2
+ R3) <(Rh + (Rg / (n-1))) / (R1 +
It is preferable to use a failure detection circuit that is Rh + (Rg / (n-1))).

In the failure detection circuit according to the present invention, first, the resistance value Rg of the glow plug and the resistance value Rh of the connection wiring are represented by (Rh + (Rg / n)) / (R1 + Rh + (Rg /
The resistance values R1, R2, and R3 of the first to third resistors are selected so that n)) is smaller than R3 / (R2 + R3). As a result, when any glow plug is normal without disconnection, the potential at the 1-H connection point serving as the monitor potential is
It becomes lower than the potential of the 2-3 connection point which is the reference potential. Further, in this circuit, (Rh + (Rg / (n-1)))
/ (R1 + Rh + (Rg / (n-1))) is R3 / (R
2 + R3), the resistances R1, R2, and R3 of the first to third resistors are selected. Accordingly, when any one of the glow plugs is disconnected, the monitor potential (1-H connection potential) becomes higher than the reference potential (2-3 connection potential). Therefore, when the second switch is turned on with the first switch turned off, the magnitude judgment of the comparison circuit is reversed when the glow plug is disconnected as compared with when the glow plug is normal. Disconnection can be detected.

Further, in the above failure detection circuit, the number of the glow plugs is n (n = 1, 2, 3,...)
The normal resistance value of the glow plug at normal time is Rg,
When the resistance value of the connection wiring is Rh, the resistance value of the first resistance is R1, the resistance value of the second resistance is R2, and the resistance value of the third resistance is R3, Rh / (R1 + R
h) <R3 / (R2 + R3) <(Rh + (Rg / n))
/ (R1 + Rh + (Rg / n)).

In the failure detection circuit according to the present invention, first, the resistance value Rg of the glow plug and the resistance value Rh of the connection wiring are represented by (Rh + (Rg / n)) / (R1 + Rh + (Rg /
The resistance values R1, R2, and R3 of the first to third resistors are selected so that n)) is larger than R3 / (R2 + R3). As a result, when any glow plug is not short-circuited and is normal, the monitor potential (1-H connection potential) becomes higher than the reference potential (2-3 connection potential). Further, in this circuit, Rh / (R1 + Rh) is equal to R3 / (R2 + R
The resistance values R1, R2, R3 of the first to third resistors are selected so as to be smaller than 3). Thereby, when any one of the glow plugs is short-circuited, the monitor potential (1)
−H connection potential) becomes lower than the reference potential (2-3 connection potential). Therefore, when the second switch is turned on while the first switch is turned off, when the glow plug is short-circuited, compared to when the glow plug is normal.
Since the magnitude judgment of the comparison circuit is reversed, a short circuit of the glow plug can be detected.

Another solution is a failure in a glow plug drive circuit having one or a plurality of glow plugs connected in parallel and first switch means for turning on / off the connection between the glow plug and a power supply. A first resistor connected in series with the glow plug and in parallel with the first switch means, a connection wire connecting the glow plug and the first resistor, A second series circuit connected in parallel with a first series circuit including a first resistor, a connection wiring, and a glow plug, and a second resistor, a third resistor, and a fourth resistor connected in series; A second switch means connected in parallel to turn on / off a connection between the circuit including the first series circuit and the second series circuit and the power supply; and a 1-H connection point between the first resistor and the connection wiring. Potential and A first comparator circuit for comparing the level of the potential at 2-3 the connection point between the serial second resistor and the third resistor,
A second comparison circuit for comparing the level of the potential at the 1-H connection point with the potential at the 3-4 connection point of the third resistance and the fourth resistance, wherein the number of the glow plugs is n (n
= 1, 2, 3,...), The standard resistance value of the glow plug at normal time is Rg, and the resistance value of the connection wiring is Rh.
When the resistance of the first resistor is R1, the resistance of the second resistor is R2, the resistance of the third resistor is R3, and the resistance of the fourth resistor is R4, (Rh +
(Rg / n)) / (R1 + Rh + (Rg / n)) <(R
3 + R4) / (R2 + R3 + R4) <(Rh + (Rg /
(N-1))) / (R1 + Rh + (Rg / (n-
1))) and Rh / (R1 + Rh) <R4 /
(R2 + R3 + R4) <(Rh + (Rg / n)) / (R
1 + Rh + (Rg / n)).

Since this failure detection circuit has the first resistor connected in series to the glow plug and the connection wiring, the potential (monitor potential) at the 1-H connection point between the first resistor and the connection wiring can be obtained. . On the other hand, since there is a second series circuit in which the second resistor and the third resistor are connected in series, the second resistor and the third resistor are connected in series.
The potential at the 2-3 connection point with the resistor (first reference potential) can be taken. Then, the first comparison circuit can compare the monitor potential (1-H connection potential) with the first reference potential (2-3 connection potential). In addition, in this failure detection circuit, first, the resistance value Rg of the glow plug and the resistance value Rh of the connection wiring are calculated by (Rh + (Rg / n)) / (R
1 + Rh + (Rg / n)) is (R3 + R4) / (R2 +
R3 + R4), the resistances R1, R2, R3, and R4 of the first to fourth resistors are selected. As a result, when any glow plug is normal without disconnection,
The monitor potential (1-H node potential) is changed to the first reference potential (2
−3 node potential). Further, in this circuit, (Rh + (Rg / (n-1))) / (R1 + Rh +
(Rg / (n-1)) is R3 + R4 / (R2 + R3 +
The resistances R1, R2, R3, and R4 of the first to fourth resistors are selected so as to be larger than R4). Thus, when any one of the glow plugs is disconnected, the monitor potential (1-H connection potential) becomes lower than the reference potential (2-3 connection potential). Therefore, when the second switch is turned on with the first switch turned off, the magnitude comparison of the first comparison circuit is reversed when the glow plug is disconnected compared to when the glow plug is normal. Disconnection of the plug can be detected.

In the second series circuit of the failure detection circuit, since the fourth resistor is connected in series in addition to the second and third resistors, a 3-4 connection between the third resistor and the fourth resistor is provided. The potential of the point (second reference potential) can be obtained. Then, the monitor potential (1-H connection point potential) and the second reference potential (3-4 connection point potential) can be compared by the second comparison circuit. In addition, first, the resistance value Rg of the glow plug and the resistance value Rh of the connection wiring are represented by (Rh + (Rg / n)) /
(R1 + Rh + (Rg / n)) is R4 / (R2 + R3 +
The resistances R1, R2, R3, and R4 of the first to fourth resistors are selected so as to be larger than R4). As a result, when any glow plug is not short-circuited and is normal, the monitor potential (1-H node potential) becomes higher than the second reference potential (3-4 node potential). Further, in this circuit, Rh
/ (R1 + Rh) is smaller than R4 / (R2 + R3 + R4) so that the resistance values of the first to fourth resistors R1, R
2, R3 and R4 are selected. Thus, when one of the glow plugs is short-circuited, the monitor potential (1-
H connection point potential) becomes lower than the second reference potential (3-4 connection point potential). Therefore, when the second switch is turned on with the first switch turned off, the magnitude comparison of the second comparison circuit is reversed when the glow plug is disconnected compared to when the glow plug is normal. The short circuit of the plug can be detected.

As described above, the failure detection circuit of the present invention can detect two failure modes, that is, disconnection and short circuit of the glow plug, by using a single failure detection circuit while being a simple circuit. In addition, using a power source for driving the glow plug, the second resistor, the third resistor, and the fourth resistor are used to determine the reference potential used for determining the failure of the glow plug (2-3 connection point potential, 3-4 connection point). Potential), a separate power supply is not required, and even if the power supply voltage fluctuates, the 1-H connection point potential, the 2-3 connection point potential, and the 3-4 connection point according to the power supply voltage. A potential is generated, and disconnection and short circuit of the glow plug can be accurately detected.

Further, in the failure detecting circuit according to any one of the above, 0.6 <R1 / (Rh + Rg / n) <
It is preferable to use a failure detection circuit of 1.5.

The potential of the 1-H connection point of the first series circuit is the first
It is determined by the ratio between the resistance value R1 of the resistance and the combined resistance value Rh + Rg / n of the connection wiring and the glow plug. Here, the resistance value R1 of the first resistor is set to the combined resistance value R of the connection wiring and the glow plug.
If the value is extremely larger or smaller than h + Rg / n, the change in the potential at the 1-H connection point when the glow plug fails is reduced. Therefore, the resistance value Rg of the glow plug, the resistance value Rh of the connection wiring, and the resistance value R1 of the first resistor
In some cases, it is not possible to accurately detect a failure of the glow plug due to the influence of the variation of the glow plug or the offset variation of the comparison circuit.

On the other hand, according to the present invention, the resistance value R1 of the first resistor and the combined resistance value Rh +
Rg / n is substantially equal, specifically, 0.6 <R1
/(Rh+Rg/n)<1.5. Therefore, the potential of the 1-H connection point greatly changes depending on whether or not the glow plug has a failure. Therefore, the resistance value Rg of the glow plug,
Even if the variation in the resistance value Rh of the connection wiring and the resistance value R1 of the first resistor, the offset variation in the comparison circuit, etc. are considered,
Within the range of the above expression, disconnection or short circuit of the glow plug can be accurately detected.

Further, in the failure detecting circuit according to any of the above, between the power supply and the 2-3 connection point,
It is preferable that the failure detection circuit includes a third switch and a sixth resistor connected in series to the third switch.

Although rare, it is not a glow plug,
The first switch means may be short-circuit failure (constant conduction failure). According to the present invention, it is possible to determine this always-on failure. In other words, in both cases of disconnection of the glow plug and a normal conduction failure of the first switch means, the second
When only the switch means is turned on, the monitor potential (1
−H connection point potential) becomes higher than the reference potential (2-3 connection point potential), and the magnitude determination of the comparison circuit is reversed.

Then, the third switch means is further turned on to raise the reference potential (2-3 connection point potential) to a potential lower than the power supply potential by the resistance of the sixth resistor. Here, if the glow plug is disconnected, the monitor potential (1-H connection point potential) is raised to the reference potential (2-H).
3 (potential at three connection points), the magnitude judgment of the comparison circuit is again inverted and returns to the original state. On the other hand, in the case of a normal conduction failure of the first switch means, since the monitor potential (1-H connection potential) becomes higher than the raised reference potential (2-3 connection potential), the magnitude judgment of the comparison circuit is made. Maintain inversion. Therefore, the failure detection circuit according to the present invention can also determine whether the glow plug is disconnected or the first switch means is always on.

Further, the glow plug drive circuit may have an always-open failure in which the first switch means does not conduct normally, but according to the present invention, this always-open failure can be determined. That is, when the third switch is turned on after the first switch is turned on, if the first switch is normally conducted, the monitor potential (1-H connection point potential) is changed from the power supply potential to the sixth resistance. Therefore, the magnitude judgment of the comparison circuit does not change before and after turning on the third switch means. On the other hand, when the first switch means is always open failure, the monitor potential (1-H connection point potential) becomes lower than the reference potential, so before and after turning on the third switch means,
The magnitude judgment of the comparison circuit is inverted. Therefore, the failure detection circuit of the present invention can also detect the presence / absence of the always open failure of the first switch means.

Further, in the failure detecting circuit according to any one of the above, a fifth circuit connected in series to the first series circuit and the second series circuit connected in parallel and connected to the power supply.
It is preferable that the power supply includes a resistor, and the second switch means is a failure detection circuit that turns on / off a connection between the power supply and the circuit including the first series circuit, the second series circuit, and the fifth resistor.

According to the present invention, there is provided a fifth resistor connected in series to the first and second series circuits. The voltage applied to the first and second series circuits when the second switch is turned on can be reduced by the fifth resistor, so that the current (power) required for detecting a failure in the glow plug drive circuit is reduced. be able to.

Further, in the above failure detection circuit, it is preferable that the resistance value of the fifth resistor is larger than the combined resistance value of the first series circuit and the second series circuit.

Since the resistance value of the fifth resistor is larger than the combined resistance value of the first and second series circuits, the voltage applied to the first and second series circuits can be further reduced, and the glow plug drive circuit The current (power) required for failure detection can be further reduced. In particular, when the resistance value of the fifth resistor is
When the combined resistance value of the second series circuit is 10 times or more, the voltage applied to the first and second series circuits can be suppressed to about 1 V or less with respect to the battery voltage (for example, 12 V), The current (power) required for detecting the failure of the glow plug drive circuit can be sufficiently suppressed.

[0030]

(Embodiment 1) Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a circuit diagram of a failure detection circuit 1 of the present embodiment. The failure detection circuit 1 is a circuit that detects a failure of the glow plug drive circuit 11 that drives the glow plug 13. The glow plug drive circuit 11 includes two glow plugs 13 arranged in parallel.
(Normal standard resistance value Rg) is connected to power supply 17 (power supply voltage V). Further, a relay switch (first switch means) 19 for turning on / off the connection between the glow plug 13 and the power supply 17 is provided. Therefore, the relay switch 1
By turning on 9, the glow plug 13 can be heated to a high temperature.

The failure detection circuit 1 includes two glow plugs 1
3 and a first resistor 33 (resistance value R1) connected in series and in parallel with the relay switch 19.
Harness (connection wiring) 1 between 3 and glow plug 13
5 (resistance value Rh). A second resistor 43 (resistance R2) is connected in parallel with the first series circuit 31 including the first resistor 33, the harness 15, and the two glow plugs 13.
And a third resistor 45 (resistance R3) connected in series.
It has a series circuit 41. Further, a fifth resistor 51 (resistance value R) connected in series to the first series circuit 31 and the second series circuit 41 connected in parallel and connected to the power supply 17 on the other side.
5). The circuit including the first series circuit 31, the second series circuit 41 and the fifth resistor 51 includes a transistor 53 together with a diode 53 having the anode on the power supply 17 side in order to turn on and off the connection between the circuit and the power supply 17. (Second switch means) 55 are connected in series. In addition,
The diode 53 is inserted to prevent a failure when the power supply 17 (battery) is reversely connected. Further, a Zener diode 57 is connected in parallel with the transistor 55 for protecting the transistor 55.

Further, the failure detection circuit 1 calculates the potential (monitor potential) V _{1H at} the 1-H connection point 34 between the first resistor 33 and the harness 15, and the potential of the second resistor 43 and the third resistor 45.
3 a comparator 61 for comparing the magnitude of the potential (reference potential) V ₂₃ at the connection point 44. That is, one terminal (non-inverting input terminal) of the comparator 61
Is connected to a 1-H connection point 34 to detect a monitor potential (1-H connection point potential) V _1H , and the other terminal (inverting input terminal) is connected to a reference potential (2-3 connection point potential) V _23. Is connected to the 2-3 connection point 44 in order to detect The output terminal of the comparator 61 is connected to a controller 71 that controls the failure detection circuit 1 and performs predetermined control such as turning on a lamp that warns the driver of a failure using the output of the comparator 61. .

In this embodiment, the glow plug 13
The standard resistance value Rg in the normal state is 0.5Ω, and the harness 15
Is 0.2Ω. The resistance value R1 of the first resistor 33 is 0.5Ω, and the resistance value R2 of the second resistor 43 is 1Ω.
kΩ, the resistance R3 of the third resistor 45 is 1.15 kΩ,
The resistance value R5 of the resistor 51 is set to 24Ω.

Such a failure detection circuit 1 operates as follows. That is, when the relay switch 19 is turned on while the transistor 55 is off, only the glow plug drive circuit 11 operates. That is, current flows from the power supply 17 to each glow plug 13 via the relay switch 19, and each glow plug 13 is heated. Therefore, it can be used for assisting ignition at the time of starting the engine.

On the other hand, when the transistor 55 is turned on with the relay switch 19 turned off, the fault detection circuit 1 operates with the glow plug drive circuit 11 stopped. That is, a current flows from the power supply 17 to the fifth resistor 51 via the transistor 55. And on the other hand, the first resistor 33,
Harness 15 and glow plug 13 (first series circuit 3
A current flows through 1), and on the other hand, a current flows through the second resistor 43 and the third resistor 45 (the second series circuit 41). At this time, the first and second series circuits 31,
The voltage applied to 41 is reduced. In particular, the resistance value R5 (24Ω) of the fifth resistor 51 is larger than the combined resistance value (0.95Ω) of the first series circuit 31 and the second series circuit 41,
Specifically, since the resistance is set to 10 times or more of the combined resistance value,
The voltage applied to the first and second series circuits 31, 41 can be particularly reduced to about 0.457V with respect to the power supply voltage V (about 12V). Therefore, the current (power) required for detecting the failure of the glow plug drive circuit 11 can be sufficiently reduced.

When the transistor 55 is turned on, the comparator 61 compares the monitor potential (1−H connection potential) V _1H with the reference potential (2-3 connection potential) V ₂₃ . In the present embodiment, the normal resistance value Rg of the glow plug 13 in a normal state is set.
(0.5Ω) and the resistance value Rh (0.2
Ω), the resistance values R1 (0.5Ω), R2 (1 kΩ), and R3 (1.15) of the first to third resistors 33, 43, and 45.
kΩ) is the first equation (Rh + (Rg / n)) /
(R1 + Rh + (Rg / n)) <R3 / (R2 + R3)
<(Rh + (Rg / (n-1))) / (R1 + Rh +
(Rg / (n-1))). Therefore, the presence or absence of disconnection of the glow plug 13 can be detected. Note that n is the number of glow plugs (n = 2 in the present embodiment).

That is, (Rh + (Rg / n)) / (R1
Since + Rh + (Rg / n)) is smaller than R3 / (R2 + R3), when none of the glow plugs 13 is disconnected, the monitor potential (1-H connection point potential) V _1H is equal to the reference potential (2-3 connection). (Point potential) V ₂₃ . Therefore, the output of the comparator 61 becomes low. On the other hand, (Rh + (Rg / (n-1))) / (R1 + Rh +
Since (Rg / (n-1))) is larger than R3 / (R2 + R3), when at least one of the glow plugs 13 is disconnected, the monitor potential V _1H becomes the reference potential V _23.
Higher than. Therefore, the output of the comparator 61 becomes high (Hi). As described above, when the glow plug 13 is disconnected, the output of the comparator 61 is inverted as compared with a normal case, so that the disconnection of the glow plug 13 can be detected.

Further, the normal resistance value Rg (0.5 Ω) of the glow plug 13 and the resistance value R
h (0.2Ω), the resistance value R1 of the first resistor 33
(0.5Ω) is the second equation 0.6 <R1 / (Rh
+ Rg / n) <1.5. For this reason, the monitor potential (1-H connection point potential) V _1H greatly changes depending on whether or not the glow plug 13 is disconnected. Therefore, the resistance value Rg of the glow plug 13, the resistance value Rh of the harness 15, and the first
The glow plug 13 is not affected by the variation of the resistance value R1 of the resistor 33 and the offset variation of the comparator 61.
Disconnection can be accurately detected.

As described above, the failure detection circuit 1 of the present embodiment
While having a simple circuit, a reference potential (2-3 connection point potential) V ₂₃ by an appropriate value, the relay switch 1
If the glow plug 13 is disconnected when the transistor 55 is turned on when the switch 9 is turned off, the output of the comparator 61 is inverted as compared with the normal state, and the disconnection of the glow plug 13 can be detected. Further, the second resistor 43 and the third resistor 4
The reference potential generating circuit including a 5, since the generated power supply voltage reference potential (2-3 connection point potential) V ₂₃ proportional to the magnitude of V, does not need a separate power source, variations in the power supply voltage V is Even if there is, 1-H in proportion to the magnitude of the power supply voltage V
The connection point potential V _1H and the 2-3 connection point potential V ₂₃ are generated, and the disconnection of the glow plug 13 can be accurately detected.

In the above-described failure detection circuit 1, the case where the number of glow plugs 13 is two has been described. However, as shown by a broken line in FIG.
It may be a book or more. In the case of three, n = 3 is substituted into the above-mentioned first equation, and the resistance values R1, R1 of the first to third resistors 33, 43, 45 are set so as to satisfy this equation.
When 2, R3 is selected, disconnection of the glow plug 13 can be detected. For example, R1 = 0.39Ω, R2 =
It is preferable that 1 kΩ and R3 = 1.05 kΩ. Further, if the resistance is selected in this manner, the value of.
Since 6 <R1 / (Rh + Rg / n) <1.5 is also satisfied,
The monitor potential (1-H connection point potential) V _1H can be greatly changed depending on the presence / absence of disconnection of the glow plug 13, and the disconnection of the glow plug 13 can be detected more accurately.

(Modification 1) Next, a modification of Embodiment 1 will be described. The description of the same parts as in the first embodiment will be omitted or simplified. The failure detection circuit 101 of the first modification, whose circuit diagram is shown in FIG. 2, has the same circuit configuration as the failure detection circuit 1 of the first embodiment. However, the ratio between the resistance values R2 and R3 of the second resistor 143 and the third resistor 145 is different from that of the first embodiment. Thus, in the first modification, it is possible to detect a short circuit of the glow plug 13. Other portions are the same as those in the first embodiment, and have the same functions and effects.

When the transistor 55 is turned on with the relay switch 19 turned off, the fault detection circuit 101 operates with the glow plug drive circuit 11 stopped. Therefore, the comparator 61 compares the monitor potential (1−H connection potential) V _1H with the reference potential (2-3 connection potential) V ₂₃ . Here, in the present modification, the standard resistance value Rg (0.5Ω) of the glow plug 13 at normal time, the resistance value Rh (0.2Ω) of the harness 15, and the resistance value R1 of the first resistor 33 are set.
(0.5Ω) and the resistance value R5 of the fifth resistor 51 (24
Ω) are the same as those in the first embodiment, whereas the resistance R2 of the second resistor 143 is 1 kΩ and the third resistor 1
45 has a resistance value R3 of 649Ω, and its ratio is different from that of the first embodiment.

As a result, the first to third resistors 33, 1 are compared with the normal resistance value Rg (0.5Ω) of the glow plug 13 and the resistance value Rh (0.2Ω) of the harness 15 in a normal state.
43, 145 resistance values R1 (0.5Ω), R2 (1k
Ω) and R3 (649 Ω) are Rh / (R
1 + Rh) <R3 / (R2 + R3) <(Rh + (Rg /
n)) / (R1 + Rh + (Rg / n)). Therefore, the presence or absence of the short circuit of the glow plug 13 can be detected.

That is, (Rh + (Rg / n)) / (R1
Since + Rh + (Rg / n)) is larger than R3 / (R2 + R3), when none of the glow plugs 13 is short-circuited, the monitor potential (1-H connection point potential) V _1H is equal to the reference potential (2-3 connection). It is higher than the point potential) V _23. Therefore, the output of the comparator 61 becomes high (Hi). On the other hand, since Rh / (R1 + Rh) is R3 / (R2 + R3) is smaller than, when one of the glow plug 13 is short-circuited is lower than the reference potential V ₂₃ monitor potential V _IH is. Therefore, the output of the comparator 61 is low (Lo).
w). As described above, when the glow plug 13 is short-circuited, the output of the comparator 61 is inverted as compared with a normal case, so that the short-circuit of the glow plug 13 can be detected.

Further, as in the first embodiment, the standard resistance value Rg (0.5Ω) of the glow plug 13 in normal operation and the resistance value Rh (0.2Ω) of the harness 15
The resistance value R1 (0.5Ω) of the resistor 33 is equal to that of the first embodiment.
0.6 <R1 / (Rh + Rg /
n) <1.5 is satisfied. Therefore, glow plug 1
Monitor potential (1−H connection point potential) V _1H
Greatly changes, the short circuit of the glow plug 13 can be detected more accurately.

As described above, the failure detection circuit 101 according to the first modification is a simple circuit, but has the reference potential (2-3).
The connection point potential) V ₂₃ by an appropriate value, when turning on the transistor 55 when off the relay switch 19, if the glow plug 13 is shorted, the output of the comparator 61 is inverted compared to the normal, The short circuit of the glow plug 13 can be detected.

In this failure detection circuit 101, as shown by the broken line in FIG.
Three may be added, or three or more. In the case of three, n = 3 is substituted into the above-mentioned third equation, and the resistance values R1, R2, R3 of the first to third resistors 33, 143, 145 are selected so as to satisfy this equation. , Glow plug 1
3 can be detected. For example, R1 = 0.
39Ω, R2 = 1 kΩ, and R3 = 739Ω.
Furthermore, if the resistance is selected in this manner, the second equation 0.6 <R1 / (Rh + Rg / n) <1.5 is also satisfied.
The potential of the H connection point) V _1H can be largely changed, and the short circuit of the glow plug 13 can be detected more accurately.

(Embodiment 2) Next, a second embodiment will be described. The description of the same parts as in the first embodiment will be omitted or simplified. The failure detection circuit 201 of the second embodiment whose circuit diagram is shown in FIG.
In the failure detection circuit 1 of FIG.
4 is connected to a power supply 17. Further, a second Zener diode 267 is connected in parallel with the second transistor 265 to protect the second transistor 265. further,
The sixth resistor 269 (resistance value R6) is connected to the second transistor 26
5 and 2-3 connection point 44 are connected. When the second transistor 265 is turned on, a voltage lower than the power supply voltage V by a voltage drop of the sixth resistor 269 is applied to the 2-3 connection point 44. Other portions are the same as those in the first embodiment, and have the same functions and effects.

The failure detection circuit 201 can determine the failure of the relay switch 19 in addition to the failure detection (disconnection detection) of the glow plug drive circuit 11 that can be performed by the failure detection circuit 1 of the first embodiment. it can. That is, although rare, the relay switch 19 may have a short-circuit failure (constant conduction failure). In such a case, in the first embodiment, even if only the transistor 55 is intended to be turned on, the monitor potential (1-H connection point potential) V _1H becomes high because the relay switch 19 is conducting, so that the glow plug 13 Is the same as when the line is disconnected.
The output of 1 is inverted from the normal case. Therefore, it is not possible to determine whether the relay switch 19 has a continuous conduction failure or the glow plug 13 is disconnected.

However, in the second embodiment, with the transistor 55 turned on, the second transistor 265
The turn on, pulling the reference potential (2-3 connection point potential) V ₂₃ from the power supply voltage V to a potential just dropped resistance R6 minutes sixth resistor 269. Then, when the glow plug 13 is disconnected, the monitor potential (1-H connection point potential)
V _1H is raised to the reference potential (2-3 connection point potential) V
_Since it is lower than ₂₃ , the magnitude judgment of the comparator 61 changes before and after the second transistor 265 is turned on, and the output becomes low (Low).

On the other hand, if the relay switch 19 is in a continuous conduction failure, the monitor potential (1-H connection potential) V _1H is high, and the reference potential (2-3 connection potential) V ₂₃ is raised. Therefore, the magnitude determination of the comparator 61 does not change before and after the second transistor 265 is turned on, and the output remains high (Hi). Therefore, the second
When the transistor 265 is turned on, it is possible to determine from the outputs of the comparators 61 before and after whether a continuous conduction failure of the relay switch 19 or a disconnection of the glow plug 13 has occurred. As described above, the failure detection circuit 201 of the second embodiment
Can be detected separately from the disconnection of the glow plug 13 and the continuous conduction failure of the relay switch 19.

[0052] Note that the resistance value R6 of the sixth resistor 269, while on the transistor 55 and a second transistor 265, about the reference potential (2-3 connection point potential) V ₂₃ of the power supply potential 20-80 % Is preferably selected. In the present embodiment, for example, the resistance value R6
= 1 kΩ. The reason why the glow plug 13 is disconnected when the glow plug 13 is disconnected is about 20% or more.
So as to be lower than the H connection point potential) V _IH is pulled reference potential (2-3 connection point potential) V _23, like reference potential (2-3 connection point potential) V ₂₃ was pulled to a high enough value Because. The resistance value R5 of the fifth resistor 51 is set in the first series circuit 3
This takes into account that the combined resistance value of the first and second series circuits 41 is 10 times or more. On the other hand, the reason why the voltage is set to about 80% or less is that the monitor potential (1-H connection point potential) V _1H is raised from the reference potential (2-3 connection point potential) V ₂₃ when the relay switch 19 has a continuous conduction failure. Is raised so that the reference potential (2-3 connection point potential) is further increased.
This is because it is desired to set _V23 to a sufficiently low value. This takes into account the fact that the conduction loss of the glow plug 13 when the relay switch 19 is turned on is normally set to 10% or less.

(Modification 2) Next, a modification of Embodiment 2 will be described. In each of the first and second embodiments,
The description of the same parts as those of any one of the second embodiment and the first modification will be omitted or simplified. Modification 2 whose circuit diagram is shown in FIG.
The failure detection circuit 301 has the same circuit configuration as the failure detection circuit 201 of the second embodiment. However, unlike the second embodiment, the ratio of the second resistor 143 to the third resistor 145 is the same as that of the first modification. This allows
In the second modification, a short circuit of the glow plug 13 can be detected, and a normally open failure in which the relay switch 19 does not normally conduct can be detected separately from the short circuit of the glow plug 13. Other parts are the same as those in the second embodiment.
And the like, and have the same functions and effects.

As a failure of the glow plug drive circuit 11,
There may be an always open failure where the relay switch 19 does not conduct normally. In the second modification, in the case of the first modification in which the short circuit of the glow plug 13 can be detected, the second transistor 265 is turned on after the relay switch 19 is turned on.
Is turned on, the monitor potential (1−H connection potential) V _1H is higher than the reference potential (2-3 connection potential) V ₂₃ if the relay switch 19 is normally conducting. The output remains high (Hi) unless the glow plug 13 is short-circuited, and does not change before and after the second transistor 265 is turned on.

On the other hand, when the relay switch 19 always has an open failure, the monitor potential (1-H connection potential) V _1H becomes lower than the reference potential (2-3 connection potential) V ₂₃ .
When the output of the comparator 61 becomes low (Low) and the glow plug 13 is not short-circuited, the output of the comparator 61 is inverted before and after the second transistor 265 is turned on. Therefore, it is also possible to detect a constantly open failure of the relay switch 19.

(Embodiment 3) Next, a third embodiment will be described. The description of the same parts as those in the first embodiment or the first modification will be omitted or simplified. FIG. 5 is a circuit diagram showing a failure detection circuit 401 according to the third embodiment.
The second embodiment differs from the first embodiment and the first modification in that two comparators 61 and 463 are provided and one circuit can detect both disconnection and short circuit of the glow plug 13. Other parts are the same as those of the first embodiment or the first modification.
It has the same operation and effect.

The failure detection circuit 401 has a first resistor 433 connected in parallel to the relay switch 19, as in the first embodiment. Then, the first series circuit 431
(The first resistor 433, the harness 15, and the glow plug 13
In parallel with the second resistor 443 and the third resistor 4
45 and a second series circuit 441 including a fourth resistor 447 are connected. That is, the second series circuit 4 of the present embodiment
41 is connected to a fourth resistor 447 in addition to the second and third resistors 443 and 445. The first series circuit 431 and the second series circuit 44 connected in parallel
1 is connected to a fifth resistor 51 as in the first embodiment and the like.
The Zener diode 57 is connected.

Further, the failure detecting circuit 401 has a 1-H connection point potential (monitor potential) as in the first embodiment.
There is provided a comparator 61 for comparing the level of V _1H with the 2-3 connection point potential (first reference potential) V ₂₃ . further,
In the third embodiment, the 1-H connection point potential (monitor potential) V
And _1H, The second comparator 463 for comparing the magnitude of the potential (second reference potential) V ₃₄ at 3-4 connection point 446 between the third resistor 445 and fourth resistor 447 having. One terminal (non-inverting input terminal) of the second comparator 463 is connected to the 1-H connection point 34 for detecting the monitor potential (1-H connection point potential) V _1H , and the other terminal (inverting input terminal). ) Is the second reference potential (3-4 connection point potential) V ₃₄
Is connected to a 3-4 connection point 446 in order to detect.
The output terminal of the second comparator 463 is connected to the controller 71.

In the third embodiment, the glow plug 1
The standard resistance value Rg in the normal state of No. 3 is 0.5Ω, and the harness 1
The resistance value Rh of No. 5 is 0.2Ω. Also, the first resistor 43
3 has a resistance R1 of 0.5Ω and the second resistor 443 has a resistance R
2 is 1 kΩ, the resistance value R3 of the third resistor 445 is 301Ω,
The resistance value R4 of the fourth resistor 447 is 845Ω, and the fifth resistor 51
Is set to 24Ω.

Such a failure detection circuit 401 operates as follows. That is, when the transistor 55 is turned on while the relay switch 19 is off, the failure detection circuit 401
Only the transistor operates, and current flows from the power supply 17 to the fifth resistor 51 via the transistor 55. On the other hand, current flows through the first resistor 433, the harness 15, and the glow plug 13 (the first series circuit 431), and on the other hand, the second resistor 443, the third resistor 445, and the fourth resistor 447 (the second series circuit 447). 441). At this time, the voltage applied to the first and second series circuits 431 and 441 is reduced by the fifth resistor 51. In particular, the resistance value R5 (2
4Ω) are the first series circuit 431 and the second series circuit 441.
Is larger than the combined resistance value (0.95Ω), specifically, 10 times or more of the combined resistance value, the voltage applied to the first and second series circuits 431 and 441 is changed to the power supply voltage V
(Approximately 0.457 V) with respect to (approximately 12 V). Therefore, the current (power) required for detecting the failure of the glow plug drive circuit 11 can be sufficiently reduced.

When the failure detection circuit 401 is driven, on the one hand, the monitor potential (1−H connection potential) V _1H and the first reference potential (2-3 connection potential) V ₂₃ are compared by the comparator 61, and And the monitor potential (1−H connection point potential) V _1H and the second
Reference potential (3-4 connection point potential) V ₃₄ is compared by the second comparator 463. In the third embodiment, the first to fourth resistors 433, 443, and 445 are compared with the normal resistance Rg (0.5Ω) of the glow plug 13 and the resistance Rh (0.2Ω) of the harness 15 in a normal state. , 447
(0.5Ω), R2 (1 kΩ), R3 (301Ω), R
4 (845Ω) is the fourth equation (Rh + (Rg /
n)) / (R1 + Rh + (Rg / n)) <(R3 + R
4) / (R2 + R3 + R4) <(Rh + (Rg / (n-
1))) / (R1 + Rh + (Rg / (n-1))). Therefore, the presence or absence of disconnection of the glow plug 13 can be detected.

That is, (Rh + (Rg / n)) / (R1
+ Rh + (Rg / n)) is (R3 + R4) / (R2 + R
3 + R4), any glow plug 1
When the line 3 is not disconnected, the monitor potential (1−H connection potential) V _1H becomes the first reference potential (2-3 connection potential) V _23.
Lower than. Therefore, the output of the comparator 61 becomes low. On the other hand, (Rh + (Rg / (n−
1))) / (R1 + Rh + (Rg / (n-1))) is larger than (R3 + R4) / (R2 + R3 + R4). Therefore, when at least one of the glow plugs 13 is disconnected, the monitor potential V _1H is equal to the first potential. It is higher than first reference voltage V _23. Therefore, the output of the comparator 61 is high (H
i). As described above, when the glow plug 13 is disconnected, the output of the comparator 61 is inverted as compared with a normal case, so that the disconnection of the glow plug 13 can be detected.

In the third embodiment, the fifth equation, Rh / (R1 + Rh) <R4 / (R2 + R3 + R4) <
(Rh + (Rg / n)) / (R1 + Rh + (Rg /
Since n)) is also satisfied, the presence or absence of a short circuit in the glow plug 13 can also be detected. That is, (Rh + (Rg
/ N)) / (R1 + Rh + (Rg / n)) is R4 / (R
2 + R3 + R4), so that when none of the glow plugs 13 is short-circuited, the monitor potential (1-
H connection point potential) V _1H becomes higher than the second reference potential (3-4 connection point potential) V ₃₄ . Therefore, the second comparator 4
The output of 63 goes high (Hi). On the other hand, Rh / (R1 +
Since Rh) is R4 / (R2 + R3 + R4 ) is smaller than, when one of the glow plugs 13 are shorting, monitor potential V _IH is lower than the second reference potential V _34. Therefore, the output of the second comparator 463 is low (Lo).
w). As described above, when the glow plug 13 is short-circuited, the output of the second comparator 463 is inverted as compared with a normal case, so that the short-circuit of the glow plug 13 can be detected.

Further, in the third embodiment as well,
Similarly to the first embodiment and the like, the above-described second equation, 0.6 <R1
/(Rh+Rg/n)<1.5.
Therefore, the glow plug 13 is monitored for disconnection or short circuit.
Potential (1-H node potential) V _1HChanges greatly,
It is possible to accurately detect disconnection and short circuit of the glow plug 13.
it can.

As described above, the failure detection circuit 401 of the third embodiment is a simple circuit, but has the first reference potential (2
By setting the potential of the (−3 connection point) V ₂₃ and the second reference potential (3-4 connection potential) V ₃₄ to appropriate values, the relay switch 19
When the glow plug 13 is disconnected when the transistor 55 is turned on when the switch 61 is off, the comparator 61
Is inverted, and this disconnection can be detected. When the glow plug is short-circuited, the output of the second comparator 463 is inverted, and this short-circuit can be detected. In other words, one failure detection circuit can detect two failure modes, that is, disconnection and short circuit of the glow plug 13. Also, the second resistor 443, the third resistor 445, and the fourth
The reference voltage generation circuit including the resistor 447 allows the power supply voltage V
Reference potential (2-3 connection point potential) proportional to the magnitude of
Since the V ₂₃ is generating the second reference potential (3-4 connection point potential) V _34, does not need a separate power supply, even if fluctuations in the power supply voltage V, proportional to the magnitude of the supply voltage V Then 1-H
The connection point potential V _1H , the 2-3 connection point potential V ₂₃ and the 3-4 connection point potential V ₃₄ are generated, and the glow plug 13
Disconnection or short-circuit of can be accurately detected.

The above-described failure detection circuit 401 has been described in connection with the case where the number of glow plugs 13 is two.
As shown by a broken line therein, one glow plug 13 may be added to provide three or more glow plugs. In the case of three, the resistance values R1, R2, R3, and R4 of the first to fourth resistors 433, 443, 445, and 447 are, for example, R1 = 0.
39Ω, R2 = 1kΩ, R3 = 182Ω, R4 = 866
If Ω is satisfied, the above-described fourth and fifth equations are satisfied, so that disconnection and short circuit of the glow plug 13 can be detected. Further, at this time, 0.6 <
R1 / (Rh + Rg / n) <1.5 will also be satisfied. Therefore, the monitor potential (1-H connection point potential) V _1H greatly changes depending on the presence or absence of the failure of the glow plug 13, so that the failure of the glow plug 13 can be accurately detected.

(Embodiment 4) Next, a fourth embodiment will be described. The description of the same parts as those in any of the above-described first to third embodiments and the first and second modifications will be omitted or simplified. Failure detection circuit 501 of the fourth embodiment
In the failure detection circuit 401 of the third embodiment, the 2-3 connection point 44 is connected to the power supply 17 via a second transistor (third switch means) 265 as in the second embodiment. Also, the second transistor 265
, A second Zener diode 267 is connected in parallel with the second transistor 265. Further, the sixth resistor 269 is connected to the second transistor 265 and the 2-3 connection point 44.
And are connected. When the second transistor 265 is turned on, a voltage lower than the power supply voltage V by the voltage drop of the sixth resistor 269 is applied to the 2-3 connection point 44.
The other portions are the same as those in any of the above-described first to third embodiments and the first and second modifications, and have the same functions and effects.

The failure detection circuit 501 determines whether the glow plug drive circuit 11 has the failure detected by the failure detection circuit 401 according to the third embodiment, and also determines whether the relay switch 19 is always on and always open. be able to. That is, in the third embodiment, when the normal conduction failure of the relay switch 19 occurs, when only the transistor 55 is turned on, the monitor potential (1-H connection point potential) is obtained in the same manner as when the glow plug 13 is disconnected. V _IH is higher than the first reference potential (2-3 connection point potential) V _23, the output of the comparator 61 is inverted. Therefore, it is not possible to determine whether the relay switch 19 has a continuous conduction failure or the glow plug 13 is disconnected.

However, in the fourth embodiment, when the transistor 55 is turned on, the second transistor 265
Is turned on, and the first reference potential (2-3 connection point potential) V ₂₃
Is raised from the power supply voltage V to a potential lowered by the resistance value R6 of the sixth resistor 269, in the case of disconnection of the glow plug 13, the output of the comparator 61 changes before and after the second transistor 265 is turned on. On the other hand, when the relay switch 19 has a continuous conduction failure, since the output of the comparator 61 does not change before and after the second transistor 265 is turned on, it is possible to determine that the relay switch 19 is a constant conduction failure.

Further, it is possible to determine the case where the relay switch 19 is always open failure. That is, when the second transistor 265 is turned on after the relay switch 19 is turned on, if the relay switch 19 is normally conducting, the monitor potential (1-H connection point potential) V _1H becomes the second reference potential ( since 3-4 is higher than the potential at the connection point) V _34, second
The output of the comparator 463 is the second transistor 265
It does not change before and after turning on. However, relay switch 1
In the case where 9 is an always open fault, the monitor potential (1−H connection point potential) V _1H becomes the second reference potential (3-4 connection point potential) V _34.
Therefore, the output of the second comparator 463 is inverted before and after the second transistor 265 is turned on.
Therefore, it is also possible to detect a constantly open failure of the relay switch 19.

As described above, in the failure detection circuit 501 of the fourth embodiment, the four failure modes that occur in the glow plug drive circuit 11, namely, the disconnection of the glow plug 13, the short circuit of the glow plug 13, and the continuous conduction of the relay switch 19 Malfunction,
And a normally open fault of the relay switch 19 can be separately detected by one fault detection circuit.

In the above, the present invention has been described with reference to the embodiments. However, the present invention is not limited to each of the above-described first to fourth embodiments and each of the first and second modifications, and a scope which does not depart from the gist of the present invention is described. It goes without saying that the present invention can be applied with appropriate changes. For example, in each of the first to fourth embodiments, the relay switch 19 is used as the first switch, and the transistors 55 and 265 are used as the second switch and the third switch. However, each switch is not limited to these. Absent. For example, a switching element such as a power MOS, an FET, or a GTO, or a switching circuit including these elements can be used.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a failure detection circuit according to a first embodiment.

FIG. 2 is a circuit diagram of a failure detection circuit according to a first modification.

FIG. 3 is a circuit diagram of a failure detection circuit according to a second embodiment.

FIG. 4 is a circuit diagram of a failure detection circuit according to a second modification.

FIG. 5 is a circuit diagram of a failure detection circuit according to a third embodiment.

FIG. 6 is a circuit diagram of a failure detection circuit according to a fourth embodiment.

[Explanation of symbols]

1, 101, 201, 301, 401, 501 Failure detection circuit 11 Glow plug drive circuit 13 Glow plug 15 Harness 17 Power supply 19 Relay switch (first switch means) 31, 431 First series circuit 41, 141, 441 Second series Circuit 33,433 First resistor 43,143,443 Second resistor 45,145,445 Third resistor 447 Fourth resistor 51 Fifth resistor 269 Sixth resistor 34 1-H connection point 44 2-3 Connection point 446 3- 4 connection point 53 diode 55 transistor (second switch means) 265 second transistor (third switch means) 61 comparator 463 second comparator

Claims (9)

[Claims] 1. A failure detection circuit for detecting a failure in a glow plug drive circuit having one or a plurality of glow plugs connected in parallel and a first switch means for turning on / off the connection between the glow plug and a power supply. A first resistor connected in series with the glow plug and in parallel with the first switch means, a connection wire connecting the glow plug and the first resistor, and a parallel connection with the first switch means And the first resistor,
A second switch for turning on / off a current flowing from the power supply to the connection wiring and the glow plug; a reference potential generation circuit for generating a reference potential according to a voltage of the power supply; A failure detection circuit comprising: a comparison circuit that compares the level of the potential at the H connection point with the reference potential. 2. A failure detection circuit for detecting a failure in a glow plug drive circuit having one or a plurality of glow plugs connected in parallel and a first switch means for turning on / off a connection between the glow plug and a power supply. A first resistor connected in series with the glow plug and in parallel with the first switch means; a connection wire connecting the glow plug and the first resistor; a first resistor and a connection wire And the first consisting of a glow plug
A second series circuit connected in parallel with the series circuit and having a second resistor and a third resistor connected in series; and a first series circuit and a second series circuit connected in parallel with the first switch means. A second switch for turning on and off a connection between a circuit and the power supply; a potential at a 1-H connection point between the first resistance and the connection wiring; and a 2-3 connection point between the second resistance and the third resistance. And a comparison circuit for comparing the level of the potential with the potential of the fault detection circuit. 3. The failure detection circuit according to claim 2, wherein the number of said glow plugs is n (n = 1, 2, 3,...), And a standard resistance value of said glow plugs in a normal state. Rg, the resistance of the connection wiring is Rh, the resistance of the first resistor is R1, the resistance of the second resistor is R2, and the resistance of the third resistor is R3, (Rh + (Rg / n)) / (R1 + Rh + (Rg /
n)) <R3 / (R2 + R3) <(Rh + (Rg / (n
-1))) / (R1 + Rh + (Rg / (n-1))). 4. The failure detection circuit according to claim 2, wherein the number of said glow plugs is n (n = 1, 2, 3,...), And a standard resistance value of said glow plugs in a normal state. Rg, the resistance value of the connection wiring is Rh, the resistance value of the first resistance is R1, the resistance value of the second resistance is R2, and the resistance value of the third resistance is R3. (R1 + Rh) <R3 / (R2 + R3) <(Rh
+ (Rg / n)) / (R1 + Rh + (Rg / n)). 5. A failure detection circuit for detecting a failure in a glow plug drive circuit having one or a plurality of glow plugs connected in parallel and a first switch means for turning on / off a connection between the glow plug and a power supply. A first resistor connected in series with the glow plug and in parallel with the first switch means; a connection wire connecting the glow plug and the first resistor; a first resistor and a connection wire And the first consisting of a glow plug
A second series circuit connected in parallel with the series circuit, and a second resistor, a third resistor, and a fourth resistor connected in series; a second series circuit connected in parallel with the first switch means; A second switch for turning on and off a connection between a circuit including a series circuit and the power supply; a potential at a 1-H connection point between the first resistance and the connection wiring; and a second resistance including a second resistance and a third resistance. A first comparing circuit for comparing the level of the potential at the -3 connection point, and comparing the level of the potential at the 1-H connection point with the potential at the 3-4 connection point of the third resistor and the fourth resistor. A second comparison circuit, wherein the number of the glow plugs is n (n = 1, 2, 3,...), The normal resistance value of the glow plugs in a normal state is Rg, and the resistance value of the connection wiring is Let Rh be the resistance value of the first resistor, and let R1 be the resistance value of the second resistor. Was a R2, the resistance value of the third resistor and R3, when the resistance value of the fourth resistor and R4, (Rh + (Rg / n)) / (R1 + Rh + (Rg /
n)) <(R3 + R4) / (R2 + R3 + R4) <(R
h + (Rg / (n-1))) / (R1 + Rh + (Rg /
(N-1))) and Rh / (R1 + Rh) <R4 / (R2 + R3 + R4) <
(Rh + (Rg / n)) / (R1 + Rh + (Rg /
n)) a fault detection circuit. 6. The fault detection circuit according to claim 3, wherein 0.6 <R1 / (Rh + Rg / n) <1.5. 7. The failure detection circuit according to claim 2, wherein a third switch is connected between the power supply and the 2-3 connection point in series with the third switch. A failure detection circuit including a sixth resistor; 8. The failure detection circuit according to claim 2, wherein the failure detection circuit is connected in series to the first series circuit and the second series circuit connected in parallel, and is connected to the power supply. A failure detection circuit for turning on / off a connection between the power supply and a circuit including the first series circuit, the second series circuit, and a fifth resistor. 9. The fault detection circuit according to claim 8, wherein a resistance value of the fifth resistor is larger than a combined resistance value of the first series circuit and the second series circuit.