Vehicle lighting device

1. A vehicle lighting device comprising:

a light source unit including a plurality of light-emitting elements connected in series; and

a power supply unit to turn on the plurality of light-emitting elements, wherein

in the light source unit, the plurality of light-emitting elements connected in series are divided into a first light-emitting element group which constitutes a high potential side group and a second light-emitting element group which constitutes a low potential side group which are connected at a first connecting point,

the light source unit further including,

a pair of comparison resistors connected in series to generate a voltage equivalent to a voltage at the first connecting point between the first light-emitting element group and the second light-emitting element group when all light-emitting elements are operating, one of the comparison resistors being connected between a high potential side of the first light-emitting element group and a second connecting point and another of the comparison resistors being connected between the second connecting point and a low potential side of the second light-emitting element group; and

a transistor having a base connected to one of the first and second connecting points and having an emitter connected to the other of said first and second connecting points,

the first light-emitting element group, the second light-emitting element group, and the pair of comparison resistors form at least one bridge circuit;

wherein, when one or more light-emitting elements included in the first light-emitting element group or the second light-emitting element group are short-circuited, the transistor operates.

2. The vehicle lighting device according to claim 1 , wherein the plurality of light-emitting elements connected in series are divided into three or more light-emitting element groups to form the at least one bridge circuit to include a plurality of bridge circuits.

3. The vehicle lighting device according to claim 2 , wherein a first light-emitting element group or a second light-emitting element group included in one of the plurality of bridge circuits is also used as a first light-emitting element group or a second light-emitting element group included in another bridge circuit among the plurality of bridge circuits.

4. The vehicle lighting device according to claim 2 , wherein collectors of a plurality of transistors respectively included in the plurality of bridge circuits are connected to each other.

5. The vehicle lighting device according to claim 1 , wherein a voltage that drops when one light-emitting element included in the first light-emitting element group or the second light-emitting element group is turned on is twice or more a base-emitter voltage generated when the transistor operates.

6. The vehicle lighting device according to claim 1 , further comprising a condition switching unit to switch a load condition of the light source unit with respect to the power supply unit when the transistor operates.

7. The vehicle lighting device according to claim 6 , wherein the condition switching unit places both ends of the light source unit in a short-circuit condition when the transistor operates, the both ends being connected to the power supply unit.

8. The vehicle lighting device according to claim 6 , wherein the condition switching unit cuts off the light source unit from the power supply unit when the transistor operates.

9. The vehicle lighting device according to claim 6 , wherein the condition switching unit repeatedly performs switching, when the transistor operates, between a condition in which both ends of the light source unit are short-circuited and a normal condition, the both ends being connected to the power supply unit.

10. The vehicle lighting device according to claim 6 , wherein the condition switching unit repeatedly performs switching, when the transistor operates, between a condition in which the light source unit is cut off from the power supply unit and a normal condition.

11. The vehicle lighting device according to claim 1 , wherein

the light source unit includes a plurality of substrates on which the plurality of light-emitting elements are divided into respective groups and mounted, and

a light-emitting element mounted on each of the plurality of substrates forms at least a part of the at least one bridge circuit.

12. The vehicle lighting device according to claim 11 , wherein at least one of the plurality of substrates on which the plurality of light-emitting elements are divided into the respective groups and mounted includes a condition switching unit to switch a load condition of the light source unit with respect to the power supply unit.

13. The vehicle lighting device according to claim 1 , wherein the light source unit is a light source of a headlight, a brake light, or a turn signal light.