How does an optocoupler work?

What is an introduction to optocoupler devices?

An optocoupler device (optocoupler for short) is a combination of light-emitting devices (such as light-emitting diodes) ) and photosensitive devices (such as photosensitive transistors) are assembled together to achieve coupling through light to form electric-to-optical and photo-to-electrical conversion devices. Optocouplers are divided into many types. Figure 1 shows the schematic diagram of a commonly used triode photocoupler. ?

When an electrical signal is sent to the input end of the optocoupler, the light-emitting diode emits light through current. The photosensitive element generates current after being illuminated, and CE is turned on; when there is no signal at the input end, the light-emitting diode emits light. The pole body does not light up, the phototransistor is cut off, and CE is blocked. For digital quantities, when the input is a low level "0", the phototransistor is turned off and the output is a high level "1"; when the input is a high level "1", the phototransistor is saturated and turned on, and the output is a low level. "?0". If the base has a lead wire, it can meet the temperature compensation and detection modulation requirements. This kind of optical coupler has good performance and low price, so it is widely used. ? Figure 1? The internal structure diagram of the most commonly used optocoupler? Transistor receiving type? 4-pin package

Figure 2? The internal structure diagram of the optocoupler? Transistor receiving type? 6-pin package Figure 3? Internal structure diagram of the optocoupler? Dual LED input? Transistor receiving type? 4-pin package

Figure 4? Internal structure diagram of the optocoupler? SCR receiving type? 6-pin package

Figure 5? Internal structure diagram of the optocoupler? Dual diode receiving type? 6-pin package

The reason why the optocoupler can effectively suppress spikes and various impurities while transmitting signals Signal interference greatly increases the signal-to-noise ratio on the channel, mainly for the following reasons:?

(1) The input impedance of the optocoupler is very small, only a few hundred ohms, and the interference The impedance of the source is relatively large, usually 105~106Ω. According to the principle of voltage division, even if the amplitude of the interference voltage is large, the noise voltage fed to the input end of the optocoupler will be very small, and only a very weak current can be formed. Because there is not enough energy, the diode cannot emit light. thus being suppressed. ?

(2) There is no electrical connection between the input loop and the output loop of the optocoupler, and there is no grounding; the distributed capacitance between them is extremely small, and the insulation resistance is very large, so the loop It is difficult for various interference noises on one side to be fed to the other side through the optocoupler, thus avoiding the generation of interference signals caused by high impedance coupling. ?

(3) The optocoupler can play a very good role in safety protection. Even when the external device fails or even the input signal line is short-circuited, the instrument will not be damaged. Because the input loop and output loop of the optical coupling device can withstand a high voltage of several thousand volts. ?

(4) The response speed of the optocoupler is extremely fast, and its response delay time is only about 10μs, which is suitable for occasions that require high response speed. ?

Application of photoelectric isolation technology?

Optoelectric isolation in microcomputer interface circuits?

Microcomputers have multiple input ports to receive transmissions from remote field devices. After receiving the status signals, the microcomputer processes these signals and outputs various control signals to perform corresponding operations. When the on-site environment is harsh, there will be large noise interference. If these interferences enter the microcomputer system along with the input signal, the control accuracy will be reduced and malfunctions will occur. Therefore, optocouplers can be used as interfaces at the input and output ends of the microcomputer to isolate signals and noise. A typical optocoupler circuit is shown in Figure 6. This circuit is mainly used at the interface between the digital signal output of the "A/D converter" and the control signal for the forward channel sent by the CPU and the analog circuit, thereby realizing the connection of signal paths between different systems. The electrical paths are isolated from each other, and on this basis, the analog circuit and the digital circuit are isolated from each other, which plays a role in suppressing crosstalk.

Figure 6: Optocoupler wiring principle

For linear analog circuit channels, the optocoupler must have the characteristics of linear transformation and transmission, or choose to use complementary tubes. Circuit to improve linearity, or use V/F conversion and then use digital optocoupler for isolation. ?

Optoelectric isolation in power drive circuit?

In microcomputer control systems, a large number of switching quantities are used for control. These switching quantities are generally output through the I/O of the microcomputer. The driving capability of I/O is limited and is generally not enough to drive some point magnetic actuators. A driver interface circuit must be added. In order to avoid interference to the microcomputer, isolation measures must be taken. For example, the main circuit where the thyristor is located is generally an AC strong current circuit with high voltage and large current, which is not easy to be directly connected to the microcomputer. An optocoupler can be used to isolate the microcomputer control signal from the thyristor trigger circuit. An example circuit is shown in Figure 7.

Figure 7: Triac (thyristor)

In the motor control circuit, an optocoupler can also be used to isolate the control circuit from the motor high-voltage circuit. The motor relies on MOSFET or IGBT power tubes to provide drive current. An isolation amplifier stage is required between the switching control signal of the power tube and the high-power tube. In the connection form of optocoupler isolation stage-amplifier stage-high power tube, the optocoupler is required to have high output voltage, high speed and high mode suppression.

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Long-distance isolated transmission?

In computer application systems, due to the inevitable long-distance transmission between the measurement and control system and the equipment being measured and controlled, the signal is transmitted It is easy to be interfered during the process, causing the transmission signal to be distorted or distorted; in addition, between devices that are far apart and connected through long cables, the ground potential difference between the devices often leads to ground loop currents, which affects the circuit. Differential mode interference voltage is formed. In order to ensure the reliability of long-term transmission, photoelectric coupling isolation measures can be used to separate the electrical connections of the two circuits, cut off possible loops, make them independent of each other, and improve the anti-interference performance of the circuit system. If the transmission line is long and the on-site interference is serious, the long line can be completely "floated" through a two-stage optocoupler, as shown in Figure 8.

Figure 8: Optocoupler floating treatment for long transmission lines

The "floating" of the long line removes the common ground wire between the two ends of the long line, which not only effectively eliminates the The noise voltage generated when the current of the circuit passes through the public ground wire causes mutual interference, and also effectively solves the problem of long-line drive and impedance matching; at the same time, it can also protect the system from damage when the controlled equipment is short-circuited. ?

Photoelectric isolation in zero-crossing detection circuit?

Zero-crossing, that is, zero-crossing detection, means that the zero-crossing point of the AC voltage is automatically detected and a driving signal is generated, so that the electronic switch is Open at any time. Modern zero-crossing technology has been combined with optoelectronic coupling technology. Figure 9 shows a zero-crossing detection circuit that can be used in a single-chip computer numerically controlled AC voltage regulator. ? Figure 9? Zero-crossing detection

The 220V AC voltage is directly applied to the input terminals of two anti-parallel photocouplers GD1 and GD2 after being limited by resistor R1. During the positive and negative half cycles of the AC power supply, GD1 and GD2 are turned on respectively, and U0 outputs a low level. At the moment when the sine wave of the AC power supply crosses zero, neither GD1 nor GD2 is turned on, and U0 outputs a high level. The pulse signal is used as the interrupt request signal of the microcontroller and the zero-crossing synchronization signal of the controllable silicon after being inversely gated and shaped. ?

Notes?

(1) The input part and output part of the photoelectric coupler must use independent power supplies respectively. If one power supply is used at both ends, the photoelectric coupler The isolation function of the coupler will be meaningless. ?

(2) When using a photoelectric coupler to isolate the input and output channels, all signals (including digital signals, control signals, and status signals) must be isolated so that there are no Any electrical connection, otherwise this isolation is meaningless.