An indicator system for a vehicle is shown in FIG. 11. This system includes an indicator unit having an open-circuit detecting function. A partial voltage of a voltage VB of a battery 100 divided by resistors R60, R70 and a zener diode ZD1 is determined as a threshold voltage Vth for the open-circuit detection. A shunt resistor R80 is connected between the battery 100 and incandescent lamps 101, 102 that are arranged at the front end and the rear end of the vehicle.
A comparator 103 is provided for comparing a voltage drop VR80 at the shunt resistor R80 with the threshold voltage Vth. When an open-circuit is present, that is, a disconnection is present, in at least one of the incandescent lamps 101, 102, a current flowing through the shunt resistor R80 is reduced. As a result, the voltage drop VR80 at the shunt resistor R80, namely, a voltage at a point between the shunt resistor R80 and the incandescent lamps 101, 102, becomes smaller. An output level of the comparator 103 varies when the incandescent lamp 101, 102 becomes open. Therefore, an open circuit in the incandescent lamp 101, 102 is detected based on the variation in the output level of the comparator 103.
Light emitting diodes (LEDs) have better power saving performance than incandescent lamps. Therefore, application of LEDs to indicator systems for vehicles has been examined in the recent years. One of such systems is proposed in JP-A-2002-76439. A single LED cannot provide sufficient brightness for direction indication. Thus, multiple LEDs are arranged in lines and used for each indicator to provide desired brightness. Open-circuit detection can be performed in this system in the same manner as the indicator system shown in FIG. 11.
However, a voltage drop at each LED varies from LED to LED and the voltage drop VR80 at the resistor R80 varies due to a variation in voltage drops at the LEDs. A relationship between the battery voltage VB and the voltage drop VR80 under normal conditions is shown in FIG. 10. The variation in the voltage drop VR80 due to the variation in the LED is also shown in FIG. 10.
A middle of a range of the variation in the voltage drop VR80 is determined based on an average voltage drop of LEDs and indicated with line L10. When the voltage drop at the LED 110 is smaller than the average, a battery voltage-voltage drop characteristic curve shifts from line L10 to the left side of the graph. The battery voltage-voltage drop characteristic curve shifts from line L10 to the right side of the graph when the voltage drop at the LED 110 is larger than the average. The characteristic curve shifts between the maximum line and the minimum line. The maximum line and the minimum line indicate the battery voltage-voltage drop characteristic in conditions that the voltage drop at the LED 110 is the largest and the smallest, respectively.
A relationship between the battery voltage VB and the voltage drop VR80 under abnormal conditions is also shown in FIG. 12. A middle of a range of the variation in the voltage drop VR80 is determined based on an average voltage drop of LEDs and indicated with line L20. A battery voltage-voltage drop characteristic curve shifts in the same manner as the normal conditions. The maximum line and the minimum line are also provided for this case.
If the voltage drop at the LED 110 is larger than the average and the battery voltage VB is low, the characteristic curve shifts more to the right than line L10. As a result, an open circuit is improperly determined even when it does not actually exist. If the voltage drop is smaller than the average and the battery voltage VB is low, the characteristic curve shifts more to the left than line L10. As a result, an open circuit is not determined even when is actually exist. Namely, improper open-circuit determination occurs in an area indicated with shade in the graph.