| 摘要 |
PURPOSE:To considerably reduce the number of component by providing a control circuit for applying a detection voltage, obtained by dividing the output voltage of a power supply circuit, onto the reference terminal of a shunt regulator and biasing a switching transistor based on a current flowing through a cathode terminal. CONSTITUTION:When the output voltage Vo of a power supply circuit 2 is lowered to the lowest level in order to dim a lighting panel 11, a detection voltage Vd lower than a reference voltage Vs is applied on a reference terminal 7r. Consequently, current Ic does not flow into a cathode terminal 7c, and the bias voltage Vp of a switching transistor 5 lowers to bring about non-biased state thus turning the switching transistor 5 OFF. As a result, power supply to a circuit 3 is interrupted. |
