| 摘要 |
PURPOSE:To hold the load electric power of the lamp constant even if an input source voltage varies by comparing an effective value calculated by detecting a lamp current with a reference value and controlling the lamp current on the basis of the comparison output. CONSTITUTION:An effective value calculating means 11 detects the lamp current i(t) of the lamp 22 to calculate the effective value I. A lamp resistance varies in value greatly with filament temperature, but considered to be nearly constant when the lamp is on almost at the rated electric power point, so the lamp electric power is expressed as the product of the square value of the lamp current and resistance value. For the purpose, the lamp current is controlled to a constant value as long as the resistance value. Then, a comparator 112 compares the effective value I with the reference value I0 and outputs their difference, and a phase control means 113 controls the phase of the current i(t) on the basis of the difference output to hold the load electric power of the lamp constant. |
