Patent ID: 8222839

Claim:
A dimming control system for a vehicular lamp comprising: a plurality of light source units each having a semiconductor light source and a current control portion to control a drive current for driving the semiconductor light source; and a control unit connected to the plurality of light source units via power supply lines, wherein the control unit includes: a plurality of switch portions respectively connected in series to the light source units and controlling dimming of the semiconductor light sources by repeating an ON/OFF operation in a predetermined cycle; a plurality of abnormality detection portions that detect a current or a voltage supplied via the power supply lines; and an abnormality determination portion that determines there is an abnormality in the power supply lines or the light source units when a value of the detected current or voltage exceeds a predetermined range while the switch portions are ON, wherein the predetermined cycle includes a first cycle and a second cycle that is longer than the first cycle, and the ON/OFF operation is repeated in both the first cycle and the second cycle; the plurality of switch portions are controlled such that an ON period of the second cycle is longer than an ON period of the first cycle; and the abnormality determination portion performs an abnormality determination during the ON period of the second cycle.