Patent ID: 8653740

Claim:
A load control circuit comprising: a switch device configured to control a voltage applied to a load element according to an electric power supply from a power source; a driver circuit configured to control activation and deactivation of the switch device; and a circuit portion configured to set a duty ratio and output the duty ratio to the driver circuit for causing the driver circuit to activate the switch device, wherein the driver circuit is configured to activate the switch device according to the duty ratio to control the voltage applied to the load element, and the circuit portion is further configured to set the duty ratio according to a duty ratio characteristic curve in which the duty ratio decreases gradually with increase in a voltage of the power source, and decrease in the duty ratio with increase in the voltage of the power source becomes small gradually, wherein the circuit portion is a lamp control circuit including: a periodic waveform generator configured to generate a periodic voltage waveform; a threshold circuit configured to generate a threshold voltage to be compared with the periodic voltage waveform; and a comparator configured to: compare the periodic voltage waveform with the threshold voltage to obtain a comparison result; set a duty ratio to cause the driver circuit to activate the switch device according to the comparison result; and output the duty ratio to the driver circuit, wherein the comparator is configured to cause the driver circuit to activate the switch device according to the duty ratio outputted to the driver circuit to control a voltage applied to the lamp having a nonlinear voltage-illumination characteristic, and the threshold circuit is configured to output the threshold voltage, such that the threshold voltage satisfies a relationship obtained by approximating a characteristic curve of the duty ratio, which renders an illumination of the lamp substantially constant with increase in a voltage of the power source, by a linear-function straight line using a formula (V 2 ×DUTY) 1/2 of an effective value of a voltage applied to the lamp, the DUTY being the duty ratio, and the V being the applied voltage to the lamp.