| 摘要 |
<p>PROBLEM TO BE SOLVED: To prevent a light-emitting diode from being lighted on uselessly after connection to an AC power source is cut, without complicating a circuit in a power supply circuit.SOLUTION: At the side of a secondary winding 2b of a transformer 2 of a switching power source 3, a detection circuit 22 for detecting that variation of an output voltage exceeds a predetermined value and a light-off circuit 23 for forcibly reducing a drive current of a light-emitting diode 5 in the case where the detection circuit 22 detects the variation of the output voltage, are provided. The detection circuit 22 detects the variation of the output voltage in accordance with whether a potential difference between terminals of a Zener diode 26 exceeds a Zener voltage. The light-off circuit 23 includes a transistor 32 which is switched to conduction when the detection circuit 22 detects the variation of the output voltage, and a transistor 33 which switches a bypass line L1 of the light-emitting diode 5 from disconnection to connection in response to switching of the transistor 32 to conduction. When the bypass line is conducted, a current supplied to the light-emitting diode 5 is bypassed and reduced, such that the light-emitting diode 5 is lighted off.</p> |
