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1.光电耦合器常闭电路光电耦合器常闭电路如图1所示。在无输入信号时,虽然晶体管VT处于截止状态,但发光二极管仍有足够的电流通过,所以光敏元件即光敏电阻器的阻值很小,相当于开关接通。当有输入信号时,晶体管VT完全导通,此时VT的集电极与发射极的压降在0.3V左右,远小于发光二极管的正向导通电压(1.3～2V),所以发光二极管不发光,光敏电阻器的暗阻很大,相当于开关断开。通常将此时的电子开关称为常闭式。
1. Photocoupler normally closed circuit Photocoupler normally closed circuit shown in Figure 1. In the absence of input signal, although the transistor VT is off, but the light-emitting diode is still sufficient current through, so photosensitive elements that photosensitive resistor resistance is very small, equivalent to the switch is turned on. When there is input signal, the transistor VT is completely turned on. At this time, the voltage drop between the collector and the emitter of VT is about 0.3V which is much smaller than the forward conducting voltage of the light emitting diode (1.3-2V), so the light emitting diode does not emit light, The dark resistance of the photoresistor is large, equivalent to the switch off. Usually this time of the electronic switch called normally closed.