| 摘要 |
PURPOSE:To control the ripple of a lamp current as well as to prevent any beat noise from occurring by converting a pulse width modulation dimming signal into a D.C. signal of the level conformed to pulse width, and feeding a lamp with high frequency output whose frequency is varied according to this level. CONSTITUTION:An external dimming signal is received by a photocouple 65 of a D.C. converter circuit of a dimming control circuit 6, a low frequency PWM waveform voltage similar to the external dimming signal is generated in a collector of a transistor 66. This voltage is smoothed by a resistor 67, a condenser 68 or the like, while a D.C. voltage in proportion to an ON-OFF duty ratio of pulse width of a PWM waveform is generated and a voltage- frequency converter 7, generating a high frequency signal for driving a lamp 9 via an error amplifier 64, is feedback-controlled. A beat noise is prevented from occurring by a lamp current whose ripple varying the frequency according to dimming is controlled via a D.C. signal, not directly by this A.C. dimming signal, thus an adverse effect to be exerted on constituent electronic parts is sharply abated. |
