| 摘要 |
PURPOSE:To protect a forward current of a light-emitting diode part at a permissible value or lower without lowering the current detection sensitivity of a photocoupler by a method wherein a constant-voltage diode is connected in parallel with a circuit where a resistance for current detection use has been connected in series to the photocoupler. CONSTITUTION:A constant voltage diode 16 is connected in parallel with a series circuit of a resistance 12 for current detection use and a light emitting diode part 81. When a forward voltage is applied to the diode part 81, a total electric current iL is extremely small, a voltage drop caused on both ends of the series circuit of the resistance 12 and the diode part 81 is lower than a breakdown voltage VZ of the diode 15 and VZ>iF.R12+VF (where R12 is a resistance value of the resistance 12 and VF is a drop in the forward voltage of the diode 18), an electric current IZ can be neglected; when the electric current iL is increased and exceeds iL=(VZ-VF)/R12, an excess portion flows to the diode 15 and is limited to an above-mentioned value. When a reverse voltage is applied, the total electric current flows to the diode and a photocoupler is protected. |
