| 摘要 |
<p>PURPOSE: To prevent a rush current without deteriorating the buildup characteristic of a lamp. CONSTITUTION: A current detecting circuit 22 detects a lamp current flowing through a variable resistance R1, and a control circuit 23 controls a field-effect transistor Q1 according to the voltage values of the current detection circuit 22 and of the variable resistance R1. If a rush current is generated in the variable resistance R1, the voltage of a step-down chopper circuit 21 is lowered by the control circuit 23. Under normal conditions, a lamp current is monitored, and the field-effect transistor Q1 is controlled by the control circuit 23 with the lamp current held constant to hold the light output of a halogen lamp IL constant. When the resistance value of the variable resistance R1 is smaller than 5% of the resistance value of the halogen lamp IL, the rush current cannot be decreased sufficiently; when the resistance value of the variable resistance R1 is greater than 15%, the buildup characteristic and thus circuit efficiency decrease; therefore, the resistance value must be in the range 5 to 15%.</p> |
