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    2021-10-21 17:38:13
    Photocoupler Basis: Description, Applications and Working Principle

    Ⅰ. What is Photocoupler?


    PC817 Photocoupler

    Photocoupler is also known as optocoupler. To communicate electrical impulses, optocouplers employ light as a channel. It is frequently utilized in many circuits because it provides an excellent isolation effect on input and output electrical signals. It has evolved into one of the most diversified and adaptable optoelectronic devices available today. Light emission, light reception, and signal amplification are the three main components of an optical coupler. The input electrical signal causes the light-emitting diode (LED) to emit light of a specific wavelength, which is detected by the photodetector and converted into a photocurrent, which is then output after amplification. This completes the electrical-optical-electrical conversion, acting as input, output, and isolation at the same time. The optocoupler offers good electrical insulation and anti-interference characteristics since its input and output are isolated from each other and the electrical signal transmission is unidirectional.


    Ⅱ. The classification of Photocoupler

    Because there are so many different types and varieties of photoelectric couplers, the optoelectronics DATA handbook has over a thousand models, which can usually be categorized using the following methods:

    (1) It can be classified as an external optical path photoelectric coupler (also known as a photoelectric interruption detector) or an interior optical path photoelectric coupler, depending on the optical path. The transmission type and reflection type photoelectric couplers are the two types of external optical path photoelectric couplers.

    (2) It may be classified into two types based on the output form: a. Photosensitive device output type, which includes photodiode output type, phototransistor output type, photocell output type, and light thyristor output type, among others.

    b. Type of NPN triode output, such as AC input, DC input, complementary output, and so on.

    c. Darlington transistor output type, which includes both AC and DC inputs.

    d. The type of logic gate circuit output, such as gate circuit output, Schmitt trigger output, tri-state gate circuit output, and so on.

    e. Output type with low conduction (output low-level millivolts).

    f. Type of optical switch output (on resistance less than ten).

    g. Type of power output (IGBT/MOSFET, etc.)

    (3) Coaxial type, dual in-line type, TO package type, flat package type, chip package type, and optical fiber transmission type are the different package types.

    (4) Digital photocouplers (OC gate output type, totem pole output type, tri-state gate circuit output type, etc.) and linear photocouplers can be classified based on the transmission signal (can be divided into low drift type, high linearity Type, broadband type, single power supply type, dual power supply type, etc.).

    (5) Photoelectric couplers can be classified as low-speed (phototransistor, photocell, etc. output type) or high-speed (phototransistor, photocell, etc. output type) (photodiode with signal processing circuit or photosensitive integrated circuit output type).

    (6) It can be classified as a single channel, dual channel, or multi-channel photocoupler based on the channel.

    (7) It may be separated into regular isolation optocouplers (usually, optical glue potting is less than 5000V, and the empty seal is less than 2000V) and high-voltage isolation optocouplers based on isolation properties (can be divided into 10kV, 20kV, 30kV, etc.).

    (8) It can be classified as a low power supply voltage type photoelectric coupler (usually 515V) or a high power supply voltage type photoelectric coupler based on the working voltage (generally greater than 30V).


    Ⅲ.What’s the advantage of Photocoupler?

    (1) It effectively suppresses ground loop noise, eliminates ground interference, and electrically isolates the signal field from the main control terminal, preventing unintentional main control system damage.

    (2) It has the ability to transport electrical signals between different potentials and impedances, as well as functionalities like as signal amplification and shaping, substantially simplifying the real circuit construction.

    (3) The switching speed is quick, and the high-speed photoelectric coupler's response time is on the order of ns, substantially expanding the photoelectric coupler's applicability in digital signal processing.

    (4) Compact size; most devices use dual-in-line packaging, which comes in single-channel, dual-channel, and up to eight-channel configurations and are very easy to operate.

    (5) It can be used in place of transformer isolation, produces no peak noise owing to contact bounce, and is anti-vibration and impact resistant.

    (6) In addition to power monitoring, high-linearity photocouplers are utilized in medical devices that can efficiently protect patients' lives.

    Ⅳ.What’s the working principle of Photocoupler?


    Working Principle of Photocoupler

    The photocoupler's principle is to supply an electrical signal to the photocoupler's input to cause the light-emitting source to emit light. The magnitude of the excitation current determines the light intensity. The photoelectric effect generates a photocurrent once this light is irradiated on the packed light receiver. Is led out of the light receiver's output end, completing the electric-optical-electric conversion. The optocoupler's idea is that an electrical signal drives a light-emitting diode (LED) to emit a specific wavelength of light, which is detected by a photodetector, which generates a photocurrent, which is then amplified and output. This completes the electrical-optical-electrical conversion, acting as input, output, and isolation at the same time. The optocoupler offers good electrical insulation and anti-interference characteristics since its input and output are isolated from each other and the electrical signal transmission is unidirectional. Furthermore, because the optocoupler's input end is a low-resistance element that operates in current mode, it has a high common-mode rejection capability. As a result, as a terminal isolation element in long-line information transmission, it can considerably increase the signal-to-noise ratio. It may considerably improve the dependability of computer operations as a signal isolation interface device in computer digital communication and real-time control.


    Ⅴ.Where can Photocoupler be used?

    Optocouplers have a wide range of applications due to their distinctive architectures, unique advantages, and wide range of uses, primarily in the following situations:

    (1) Logic circuit application

    Optocouplers can be used to create a variety of logic circuits. Because optocouplers have stronger anti-interference and isolation properties than transistors, the logic circuits they generate are more trustworthy.

    (2) For use as a solid switch

    In the switch circuit, good electrical isolation between the control circuit and the switch is frequently required, which is difficult to achieve with standard electronic switches but relatively simple with a photocoupler.

    (3) Use in the trigger circuit

    Because the light-emitting diodes can be linked in series to the two emitter loops, the photoelectric coupler is employed in the bistable output circuit to effectively handle the problem of output and load isolation.

    (4) Pulse amplifier circuit application

    In digital circuits, photocouplers are used to amplify pulse signals.

    (5) Linear circuit application

    Linear photocouplers are employed in linear circuits because of their strong linearity and electrical isolation.

    (6) Use in specific circumstances

    Photocouplers can also be utilized for high-voltage control, transformer replacement, contact relay replacement, A/D circuits, and other applications.

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