Load controller

1. A load controller, comprising:

a first input circuit which detects that a drive instruction signal by an operation of a drive instructing unit is less or equal to a first input threshold value;

a first constant current source activated in accordance with a detection by the first input circuit;

a PWM signal supply unit that is activated by the first constant current source and supplies a PWM signal having a prescribed frequency and a duty ratio;

a constant control signal supply unit that includes a failure detecting unit that detects failure of the first input circuit or the first constant current source, and that supplies a constant control signal during the failure of the first input circuit or the first constant current source which the failure detecting unit detects;

a drive control unit that generates a PWM drive control signal in accordance with the PWM signal supplied from the PWM signal supply unit when the failure detecting unit does not detect the failure and generates a constant drive control signal in accordance with the constant control signal supplied from the constant control signal supply unit when the failure detecting unit detects the failure; and

a load driving element that is controlled by the PWM drive control signal supplied from the drive control unit so as to drive a load by PWM control when the failure detecting unit does not detect the failure, and controlled by the constant drive control signal supplied from the drive control unit so as to drive the load by constant current control when the failure detecting unit detects the failure.

2. The load controller according to claim 1 , wherein the constant control signal supply unit includes:

a second input circuit which detects that the drive instruction signal by the operation of the drive instructing unit is less or equal to a second input threshold value set to be lower than the first input threshold value;

a second constant current source that is activated in accordance with a detection by the second input circuit; and

a constant control signal generating unit that generates the constant control signal in accordance with the activation of the second constant current source when the failure detecting unit detects the failure.

3. A load controller, comprising:

a first input circuit which detects that a drive instruction signal by the operation of a drive instructing unit is less or equal to a first input threshold value;

a first constant current source activated in accordance with a detection by the first input circuit;

a first constant control signal supply unit that is activated by the first constant current source and supplies a first constant control signal;

a second constant control signal supply unit that includes a failure detecting unit that detects failure of the first input circuit or the first constant current source and that supplies a second constant control signal in accordance with the activation of a second constant current source during the failure of the first input circuit or the first constant current source which the failure detecting unit detects;

a drive control unit that generates a constant drive control signal in accordance with the first constant control signal supplied from the first constant control signal supply unit when the failure detecting unit does not detect the failure and the second constant control signal supplied from the second constant control signal supply unit when the failure detecting unit detects the failure; and

a load driving element that is controlled by the constant drive control signal supplied from the drive control unit so as to drive a load in a constant driving operation.

4. The load controller according to claim 3 , wherein the second constant control signal supply unit includes:

a second input circuit which detects that the drive instruction signal by the operation of the drive instructing unit is less or equal to a second input threshold value set to be lower than the first input threshold value;

a second constant current source that is activated in accordance with a detection by the second input circuit; and

a second constant control signal generating unit that generates the second constant control signal in accordance with the activation of the second constant current source when the failure detecting unit detects the failure.

5. The load controller according to claim 1 , wherein the first constant current source outputs a constant current to the PWM signal supply unit and the failure detection unit.

6. The load controller according to claim 3 , wherein the first constant current source outputs a constant current to the first constant control signal supply unit and the failure detection unit.