Electronic control unit including discharging circuit with plurality of resistors connected in series

1. An electronic control unit mounted on a vehicle comprising:

a capacitor that smoothes voltage of a DC power source electrically isolated from a body of the vehicle;

a resistor circuit including a plurality of resistors connected in series, which is connected in parallel to the capacitor to discharge electric charge stored in the capacitor; and

a fault judging circuit that judges whether or not the resistor circuit is in a fault condition by comparing the voltage across a resistor included in the resistor circuit with a fault threshold value, wherein the fault judging circuit changes the fault threshold value based on the voltage of the DC power source; wherein:

the resistor circuit includes at least three resistors,

the fault judging circuit includes an operational amplifier, a non-inverting input side resistor connected between a first series-connected point at a resistor included in the resistor circuit and a non-inverting input terminal of the operational amplifier, and an inverting-input side resistor connected between a second series-connected point of which potential is lower than the first series-connected point and an inverting input terminal of the operational amplifier so as to constitute a differential amplifier circuit that amplifies voltage across the resistor in the resistor circuit, and

among the resistors included in the resistor circuit, a resistance value of the resistor disposed at the higher potential side of the first series-connected point and a resistance value of the resistor disposed at the lower potential side of the second series-connected point are set to be identical, and a resistance value of the non-inverting input side resistor and a resistance value of the inverting-input side resistor are set to be identical.

2. The electronic control unit according to claim 1 , wherein the resistor circuit and the differential amplifier circuit are mounted on the same circuit board.

3. The electronic control unit according to claim 1 , further comprising an overvoltage detection circuit that detects whether or not voltage of the DC power source is overvoltage based on an output of the differential amplifier and stops operation in the electronic control unit when the overvoltage detection circuit detects the overvoltage of the DC power source.

4. The electronic control unit according to claim 3 , wherein among the resistor circuit, a resistance value of the resistor disposed at higher potential side than the first series-connected point and a resistance value of the resistor disposed at lower potential side than the second series-connected point are set to be lower than a resistance value of the resistor disposed between the first series-connected point and the second series-connected point.

5. The electronic control unit according to claim 3 , wherein the overvoltage detection circuit is configured by a hardware circuit.

6. The electronic control unit according to claim 1 , further comprising a monitoring device for monitoring a voltage of the DC power source, and the fault judging circuit is configured to change the fault threshold value based on the voltage of the DC power source detected by the monitoring device.

7. The electronic control unit according to claim 1 , further comprising an alert circuit configured by a hardware circuit, wherein the alert circuit judges whether or not the resistor circuit is in a fault condition based on an output of the differential amplifier and outputs an alert towards outside the electronic control unit when the alert unit judges that the resistor circuit is in a fault condition.

8. The electronic control unit according to claim 1 , wherein the electronic control unit is configured to control a motor used for driving the vehicle.