Brief introduction of photoelectric coupling devices?

Optoelectronic coupling device (optocoupler for short) is an electro-optical and photoelectric conversion device formed by assembling light-emitting devices (such as light-emitting diodes) and photosensitive devices (such as phototransistors) together through optical coupling. There are many kinds of optical couplers. The schematic diagram of commonly used triode optocoupler is shown in figure 1. ?

When the electrical signal is sent to the input end of the photoelectric coupler, the light-emitting diode emits light through current, and the photosensitive element is lit to generate current, and CE is turned on; When there is no signal at the input end, the light-emitting diode is not lit, the phototransistor is turned off, and the CE is turned off. For digital quantity, when the input is low level "0", the phototransistor is turned off and the output is high level "1"; When the input is at high level "1", the phototransistor is saturated and turned on, and the output is at low level "? 0"。 If the base has leads, it can meet the requirements of temperature compensation and detection modulation. This optical coupler has good performance, low price and wide application. ?

Figure 1? Internal structure diagram of the most commonly used photoelectric coupler? Triode receiving type? 4-pin package

Figure 2? Internal structure diagram of photoelectric coupler? Triode receiving type? 6-pin package

Figure 3? Internal structure diagram of photoelectric coupler? Dual LED input? Triode receiving type? 4-pin package

Figure 4? Internal structure diagram of photoelectric coupler? Silicon controlled rectifier receiving type? 6-pin package

Figure 5? Internal structure diagram of photoelectric coupler? Double diode receiving type? 6-pin package

The photoelectric coupler can effectively suppress sharp pulse and various noise interference when transmitting signals, and greatly improve the signal-to-noise ratio on the channel. The main reasons are as follows:

The input impedance of (1) photoelectric coupler is very small, only a few hundred ohms, while the impedance of interference source is very large, usually105 ~106 Ω. According to the principle of voltage division, even if the amplitude of interference voltage is very large, the noise voltage fed into the input end of photoelectric coupler will be very small, and only a very weak current can be formed. Because there is not enough energy, the diode cannot emit light, so it is suppressed. ?

(2) There is no electrical connection and grounding between the input loop and the output loop of the photoelectric coupler; The distributed capacitance is very small and the insulation resistance is very large, so it is difficult to feed all kinds of interference noise from one side of the loop to the other through the photoelectric coupler, thus avoiding the generation of interference signals with impedance coupling. ?

(3) The photoelectric coupler can play a very good safety role, even if the external equipment fails, or even the input signal line is short-circuited, it will not damage the instrument. Because the input circuit and output circuit of the optical coupling device can withstand high voltage of several thousand volts. ?

(4) The response speed of photoelectric coupler is extremely fast, and its response delay time is only about 10μs, so it is suitable for occasions with high response speed. ?

Application of photoelectric isolation technology?

Photoelectric isolation in microcomputer interface circuit?

The microcomputer has multiple input ports and receives status signals from remote field devices. After processing these signals, the microcomputer outputs various control signals and performs corresponding operations. When the site environment is bad, there will be great noise interference. If these disturbances enter the microcomputer system with the input signal, the control accuracy will be reduced and misoperation will occur. Therefore, the optical coupler can be used as the interface of the input and output of the microcomputer to isolate the signal and noise. A typical photoelectric coupling circuit is shown in Figure 6. This circuit is mainly used for the digital signal output of "A/D converter" and the interface between the control signal of the forward channel sent by CPU and the analog circuit, so that the signal paths between different systems are connected and isolated on the electrical path, 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? Wiring principle of photoelectric coupler

For the linear analog circuit channel, photoelectric coupler must have the characteristics of linear transformation and transmission, or choose the opposite tube, adopt complementary circuit to improve linearity, or adopt V/F conversion and then use digital optical coupler for isolation. ?

Photoelectric isolation in power drive circuit?

In the microcomputer control system, a large number of switches are used for control. These switches are usually output through I/O of microcomputer, but the driving ability of I/O is limited, which is generally not enough to drive some point magnetic actuators. A driver interface circuit needs to be added. In order to avoid the interference of microcomputer, isolation measures must be taken. For example, the main circuit where the thyristor is located is generally an AC high-voltage circuit with large voltage and current, so it is not easy to directly connect with the microcomputer. Optocouplers can be used to isolate microcomputer control signals from SCR trigger circuits. Figure 7 shows an example of a circuit.

Figure 7? Bidirectional thyristor (thyristor)

In the motor control circuit, optocoupler can also be used to isolate the control circuit from the motor high voltage circuit. The motor relies on MOSFET or IGBT power tube to provide driving current, and the switching control signals of power tube and high power tube need isolation amplifier stage. In the connection form of optocoupler isolation stage, amplifier stage and high power tube, optocoupler is required to have high output voltage, high speed and high * * * mode suppression. ?

Long distance isolated transmission

In the computer application system, long-distance transmission is inevitable between the measurement and control system and the tested and controlled equipment, and the signal is easily disturbed in the transmission process, resulting in distortion or distortion of the transmission signal; In addition, between long-distance devices connected by long cables, the ground loop current is often generated due to the ground potential difference between devices, and the differential mode interference voltage is formed on the circuit. In order to ensure the reliability of long-distance transmission, photoelectric coupling isolation measures can be adopted to separate the electrical connections of the two circuits, cut off the possible circuits, make them independent of each other, and improve the anti-interference performance of the circuit system. If the transmission line is long and the field interference is serious, then a two-stage photoelectric coupler can completely "float" the long line, as shown in Figure 8.

Figure 8? Optical coupler floating treatment of transmission long line

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 mutual interference of noise and voltage generated by various currents passing through the common ground wire, but also effectively solves the problems of long line driving and impedance matching; At the same time, when the controlled equipment is short-circuited, it can also protect the system from damage. ?

Photoelectric isolation in zero-crossing detection circuit?

Zero-crossing, that is, zero-crossing detection, refers to automatically detecting the zero-crossing of AC voltage and generating a driving signal, so that the electronic switch starts to conduct at this moment. Modern zero-crossing technology is combined with photoelectric coupling technology. Fig. 9 is a zero-crossing detection circuit which can be used for a single-chip microcomputer numerical control AC voltage regulator. ?

Figure 9? Zero-crossing detector

The 220V AC voltage is limited by the resistor R 1 and directly applied to the input terminals of two anti-parallel photoelectric couplers GD 1 and GD2. In the positive and negative half cycles of AC power supply, GD 1 and GD2 are turned on respectively, and U0 outputs a low level. At the moment when the sine wave of AC power supply crosses zero, GD 1 and GD2 are not conductive, and U0 outputs a high level. After inverted gate shaping, the pulse signal is used as the interrupt request signal of single chip microcomputer and the zero-crossing synchronization signal of SCR. ?

Precautions?

(1) The input and output parts of the photoelectric coupler must use independent power supplies. If both ends use a power supply, the isolation function of photoelectric coupler is meaningless. ?

(2) When the input and output channels are isolated by photoelectric coupler, all signals (including digital signals, control signals and status signals) must be isolated, so that there is no electrical connection between the isolated two sides, otherwise this isolation is meaningless.