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OptoCoupler

Optical coupler

  • Optocouplers, also known as optoisolators or optocouplers, are commonly referred to as optocouplers. It is a device that uses light as a medium to transmit electrical signals, usually encapsulating the light-emitting device (infrared light-emitting diode LED) and the light-receiving device (photosensitive semiconductor tube, photosensitive resistor) in the same tube shell. When an electrical signal is added to the input end, the emitter emits light, and the receiver receives the light, generating a photoelectric current that flows out from the output end, thus achieving "electrical optical electrical" control.

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The main advantages of optocouplers are: unidirectional signal transmission, complete electrical isolation between the input and output terminals, no impact of the output signal on the input terminal, strong anti-interference ability, stable operation, contactless, long service life, and high transmission efficiency. Optocouplers are a new type of device developed in the 1970s and have been widely used in electrical insulation, level conversion, inter stage coupling, drive circuits, switch circuits, choppers, multi harmonic oscillators, signal isolation, inter stage isolation, pulse amplification circuits, digital instruments, long-distance signal transmission, pulse amplification, solid-state relays (SSRs), instruments, communication equipment, and microcomputer interfaces. In a single-chip switching power supply, a linear optocoupler can be used to form an optocoupler feedback circuit. By adjusting the control terminal current to change the duty cycle, precise voltage regulation is achieved, and signal reception and transfer are achieved.

The main characteristics of optoelectronic coupling are as follows:

1. The insulation resistance between the input and output terminals is generally greater than 10000M Ω, and the withstand voltage can generally exceed 1kV, with some even reaching 10kV or above.

2. Due to the fact that the optical receiver can only receive information from the light source, and vice versa, there will be no feedback phenomenon when the signal is transmitted unidirectionally from the light source to the optical receiver, and its output signal will not affect the input end.

3. Due to the fact that light-emitting devices (gallium arsenide infrared diodes) are impedance current driven devices, and noise is a high internal resistance micro current voltage signal. Therefore, the common mode suppression ratio of optoelectronic coupling devices is very high, so optoelectronic coupling devices can effectively suppress interference and eliminate noise.

4. Easy to cooperate with logic circuits.

5. Fast response speed. The time constant of optoelectronic coupling devices is usually in the millisecond or even microsecond range.

6. No contact, long service life, small size, and impact resistanceThe main advantages of optocouplers are unidirectional signal transmission, complete electrical isolation between the input and output terminals, strong anti-interference ability, long service life, and high transmission efficiency. It is widely used in level conversion, signal isolation, inter stage isolation, switching circuits, long-distance signal transmission, pulse amplification, solid-state relays (SSRs), instruments, communication equipment, and microcomputer interfaces. Due to the smaller input impedance of the optocoupler compared to the impedance of general interference sources, the interference voltage at the input end of the optocoupler is smaller, and the current it can provide is not large, making it difficult for the semiconductor diode to emit light; Due to the sealed outer shell of the optocoupler, it is not affected by external light; The isolation resistance of the optocoupler is large (about 1012 Ω) and the isolation capacitance is small (about a few pF), so it can prevent electromagnetic interference caused by circuit coupling. A linear optocoupler operates by applying a control voltage to the input of the optocoupler, and a proportional voltage is generated at the output to further control the next stage of the circuit. A linear optocoupler consists of a light-emitting diode and a photosensitive transistor. When the light-emitting diode is turned on and the photosensitive transistor is turned on, the optocoupler is a current driven type that requires a sufficient current to make the light-emitting diode turn on. If the input signal is too small, the light-emitting diode will not turn on and its output signal will be distorted. In switching power supplies, especially digital switching power supplies.