Optical Coupler (opticalcoupler, and English abbreviation is OC) is also known as optical isolator or optocoupler, and it is shortly called the optical coupler. It is a device that transmits electrical signals through light. Usually, the luminescent device (infrared light emitting diode LED) and the photoreceptor (photosensitive semiconductor tube) are encapsulated in the same shell. When the input end adds electric signal, the light emitting diode emits light. After receiving light, the phototransistor generates photocurrent, which flows out from the output end, thus realizing the "electro-opto-electric" conversion. The optoelectronic coupler, which couples the input signal to the output signal through the medium of light, is widely used in digital circuits because of its small size, long life, no contacting point, strong anti-interference ability, insulation between output and input, and unidirectional transmission of signals.
The main advantages of optocoupler are: one-way transmission of signal, complete electrical isolation between input and output, no influence of output signal on the input, strong anti-interference ability, stable operation, no contact, long service life and high transmission efficiency. The optocoupler is a new device developed in the 1970s. It has been widely used in electrical insulation, level conversion, stage coupling, driving circuit, switch circuit, chopper, multi-resonator, signal isolation, stage separation, a pulse amplification circuit, digital instrument, long-distance signal transmission, pulse amplification, solid-state relay (SSR), instrumentation, communication equipment, and computer interface. In the single-chip switching power supply, the linear optocoupler can be used to form the optocoupler feedback circuit, and the duty cycle can be changed by adjusting the current of the control terminal to achieve the purpose of precise voltage stabilization.
In the design of optocoupler circuit, two parameters are often neglected and need special attention.
One is reverse voltage (Vr), which is the maximum reverse voltage that the original LED can withstand. If the value exceeds this reverse voltage, it may damage the LED. For general optical coupler, this parameter is only about 5V. When using in the presence of back voltage or oscillation, special attention should be paid to not exceeding the reverse voltage. For example, when using AC pulse to drive LED, an additional protection circuit is needed.
Another parameter is the current transfer ratio (CTR) of the optocoupler, which refers to the ratio between the output current and the input current of the optocoupler under DC operating conditions. The CTR of the optocoupler is similar to the current amplification factor of the transistor. It is a very important parameter of the optocoupler. It depends on the input and outputs the current values of the optocoupler and the voltage values of the power supply of the optocoupler. These parameters together determine whether the optocoupler works in the state of amplification or switching. The calculation method is similar to that of the transistor working state. If the design of input current, output current, and current transfer ratio is unreasonable, the circuit may not work in the desired state.