Introduction to the introduction of optocouplers

Photocoupler

Guide: As the optocoupler has good isolation of input and output electrical signals, it is widely used in various circuits. It has become one of the most versatile and versatile optoelectronic devices. Let's take a look at some of the basics of optocouplers.

First, a correct understanding of the optocoupler

An optical coupler (OC) is also known as an opto-isolator. The photocoupler is an electric-optical-electrical conversion device that transmits electrical signals by means of light. It consists of two parts: a light source and a light receiver. The light source and the light receiver are assembled in the same sealed casing and separated from each other by a transparent insulator. The pin of the light source is the input end, and the pin of the light receiver is the output end. The common light source is a light emitting diode, and the light receiver is a photodiode, a phototransistor, and the like. It has good isolation for input and output electrical signals, so it is widely used in various circuits. It has become one of the most versatile and versatile optoelectronic devices.

Second, the anti-interference characteristics of the optocoupler

The reason why the optocoupler can effectively suppress the spike and various noise interference while transmitting the signal, so that the signal noise ratio on the channel is greatly improved, mainly for the following reasons:

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

(2) There is no electrical connection between the input loop and the output loop of the optocoupler, and there is no common ground; the distributed capacitance between the coupler is extremely small, and the insulation resistance is very large, so it is difficult to interfere with various noises on one side of the loop. Feeding to the other side through the optocoupler avoids the generation of interfering signals of the common impedance coupling.

(3) The optocoupler can play a very good security role, even when the external equipment fails, even when the input signal line is shorted, it will not damage the meter. Because the input and output loops of the optocoupler can withstand high voltages of several thousand volts.

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

Third, the simple test of the optocoupler

Since the composition of the optocoupler is not the same, different detection methods should be adopted for different structural features during the detection. For example, when detecting the input end of a common photocoupler, it is generally referred to the detection method of the infrared light emitting diode. For the photocoupler of the phototransistor output type, the detection output should be carried out with reference to the detection method of the phototransistor.

1. Multimeter detection method

Here, the MF50 type pointer multimeter and the 4-pin PC817 optocoupler are taken as an example to illustrate the specific detection method: First, the pointer multimeter is placed in the "R×100” (or “R×1k”) electric block, red and black. The test leads are respectively connected to the two pins of the light-emitting diode at the input of the optocoupler. If the needle index is infinite, but the red and black test leads have a resistance value of several thousand to ten thousand ohms, then the pin connected to the black test pen is the positive pole of the light-emitting diode, and the red test lead is connected. The pin is the negative terminal of the LED.

Then, the forward voltage is connected to the input of the optocoupler, and the pointer multimeter is still placed in the “R×100” electrical blocking, and the red and black test leads are respectively connected to the two pins of the photocoupler output. If the needle index is infinite (or the resistance value is large), but the red and black test leads are exchanged with a small resistance value (<100Ω), then the pin connected to the black test pen is the internal NPN type. The collector c of the phototransistor and the pin connected by the red test pen are the emitter e. When the forward voltage of the input terminal is cut off, the phototransistor should be cut off, and the multimeter index should be infinite. Thus, not only the pin arrangement of the 4-pin photocoupler PC817 is determined, but also its optical transmission characteristics are detected. If the multimeter pointer does not swing at all times during the test, the optocoupler is damaged.

Fourth, the principle of selection of optocouplers

When designing the optocoupler opto-isolated circuit, the model and parameters of the optocoupler must be correctly selected. The selection principles are as follows:

(1) Since the photocoupler is a signal unidirectional transmission device, and the data transmission in the circuit is bidirectional, the size of the circuit board is required. In combination with the actual requirements of the circuit design, it is necessary to select a device with a single chip integrated multi-channel optocoupler. ;

(2) The allowable range of the current transfer ratio (CTR) of the optocoupler is not less than 500%. Because the LED in the optocoupler requires a large operating current (>5.0 mA) when CTR<500%, it can be guaranteed. The signal does not generate errors during long-distance transmission, which increases the power consumption of the optocoupler;

(3) The transmission speed of the optocoupler is also one of the principles that must be followed to select the optocoupler. The optocoupler switch speed is too slow to make a correct response to the input level, which will affect the normal operation of the circuit.

(4) Linear optocouplers are recommended. It is characterized by a linear adjustment of the CTR value within a certain range. In the design, since the input and output of the circuit are both high and low level signals, the circuit operates in a nonlinear state. In linear applications, because the signal is transmitted without distortion, the appropriate static operating point should be set according to the requirements of dynamic operation to make the circuit work in a linear state.

Under normal circumstances, single-chip integrated multi-channel optocoupler devices are slower, while fast-speed devices are mostly single-channel. A large number of isolation devices require a large layout area, which also greatly increases the cost of design. In the design, due to factors such as board size, transmission speed, design cost, etc., it is impossible to select a single optocoupler with a very high speed. Here, TOSHIBA's TLP521-4 is selected.

5. Matters needing attention when using optocouplers

1) Independent power supplies must be used in the input and output sections of the optocoupler. If one power supply is shared at both ends, the isolation of the optocoupler will lose its meaning.

2) When optocouplers are used to isolate the input and output channels, all signals (including digital signals, control signals, and status signals) must be isolated so that there is no electrical connection between the isolated sides. Isolation is meaningless.