First, the structural characteristics
The main features of optoelectronic coupling are as follows:
1 Insulation between the input and output terminals, the insulation resistance is generally greater than 1010Ω, the withstand voltage can generally exceed 1kV, and some can even reach 10kV or more.
2 Due to the unidirectionality of the "light" transmission, no feedback occurs when the signal is transmitted unidirectionally from the light source to the optical receiver, and the output signal does not affect the input.
3 Because the light-emitting device (GaAs infrared diode) is an impedance current-driven device, and noise is a high internal resistance micro-current voltage signal. Therefore, the common mode rejection ratio of the optocoupler device is large, so the optocoupler device can suppress interference and eliminate noise well.
4 easy to work with logic circuits.
5 fast response. The time constant of a photocoupled device is typically at the microsecond or even nanosecond.
6 no contact, long life, small size, impact resistance.
Second, technical parameters
The technical parameters of the optocoupler mainly include LED forward voltage drop VF, forward current IF, current transfer ratio CTR, insulation resistance between input stage and output stage, and collector-emitter reverse breakdown voltage V (BR). ) CEO, collector-emitter saturation voltage drop VCE(sat). In addition, parameters such as rise time, fall time, delay time, and storage time need to be considered when transmitting digital signals.
The current transfer ratio is an important parameter of the optocoupler and is usually expressed by the DC current transfer ratio. When the output voltage remains constant, it is equal to the percentage of the DC output current IC to the DC input current IF.
The use of optocouplers is mainly to provide isolation between the input circuit and the output circuit. When designing the circuit, the following principles must be followed: The optocoupler device selected must meet the national and international standards for isolation breakdown voltage; The 4N×× series (such as 4N25, 4N26, 4N35) optical couplers produced by Isocom and Motorola of the United States are widely used in China. In view of the switching characteristics of such optocouplers, the linearity is poor, and it is suitable for transmitting digital signals (high and low levels), which can be used for output isolation of the single chip microcomputer; the selected optocoupler device must have a high coupling coefficient.
Third, the role of products
The input and output electrical signals are isolated. The optical coupler generally consists of three parts: light emission, light reception, and signal amplification. The input electrical signal drives a light-emitting diode (LED) to emit a certain wavelength of light, which is received by the photodetector to generate a photocurrent, which is further amplified and output. This completes the electro-optical-electrical conversion, thereby functioning as input, output, and isolation. Since the input and output of the optocoupler are isolated from each other, the electrical signal transmission has the characteristics of unidirectionality, and thus has good electrical insulation capability and anti-interference ability. Moreover, since the input end of the optocoupler belongs to a low-resistance component of current type operation, it has strong common mode suppression capability.
Fourth, product testing
1. Use a multimeter to judge whether it is good or bad, disconnect the power supply at the input end, measure the resistance of the 1 and 2 feet with the R×1k file, the forward resistance is several hundred ohms, the reverse resistance is several tens of kilo ohms, and the resistance between the 3 and 4 feet should be Unlimited. 1, 2 feet and any of the 3, 4 feet, the resistance is infinite, after the input is connected to the power supply, the resistance of the 3rd and 4th feet is very small. Adjusting the RP, 3, 4 foot resistance changes, indicating that the device is good. Note: R × 10k file can not be used, otherwise the launch tube will break down.
2, simple test circuit, when the power is turned on, the LED does not emit light, press SB, the LED will emit light, adjust the RP, LED luminous intensity will change, indicating that the measured optocoupler is good.
Five, the principle of use
1. The allowable range of current transfer ratio (CTR) of the optocoupler is 50% to 200%. This is because when CTR<50%, the LED in the optocoupler needs a large operating current (IF>5.0mA) to properly control the duty cycle of the monolithic Switching Power Supply IC, which will increase the work of the optocoupler. Consumption. If CTR>200%, when the circuit is started or when the load is abrupt, it is possible to falsely trigger the single-chip switching power supply, which affects the normal output.
2. If an amplifier circuit is used to drive the optocoupler, it must be carefully designed to compensate for the temperature instability and drift of the coupler.
3. It is recommended to use a linear optocoupler, which is characterized by a linear adjustment of the CTR value within a certain range. The above-mentioned optocoupler works in a linear mode, and a control voltage is applied at the input end of the photocoupler, and a voltage for further controlling the next-stage circuit is proportionally generated at the output end, which is a closed-loop adjustment control of the single-chip microcomputer. It acts as a voltage regulator for the power supply output.
In order to completely block the interference signal from entering the system, not only the signal path is isolated, but also the input or output circuit is isolated from the power supply of the system, that is, these circuits use separate isolated power supplies. For common mode interference, isolation technology is used, that is, the input ground is disconnected from the output ground by a transformer or a linear photocoupler, so that the interference is suppressed without a loop. In the switching power supply, the optocoupler is a non-essential peripheral device, the designer can fully utilize its input and output isolation to anti-interference design of the single-chip microcomputer, and adjust the closed-loop voltage regulation of the converter.
Sixth, application
Due to the wide variety of optocouplers, unique structure and outstanding advantages, it is widely used, mainly in the following applications:
(1) Application on logic circuits
The optocoupler can form various logic circuits. Since the anti-interference performance and isolation performance of the optocoupler are better than those of the transistor, the logic circuit formed by it is more reliable.
(2) as a solid switch application
In the switching circuit, it is often required to have good electrical isolation between the control circuit and the switch, which is difficult for a general electronic switch, but it is easy to implement with a photocoupler.
(3) Application on the trigger circuit
The optocoupler is used for the bistable output circuit. Since the LEDs can be respectively connected into the two-tube emitter circuits, the problem of isolation between the output and the load can be effectively solved.
(4) Application in pulse amplifier circuit
The optocoupler is applied to a digital circuit that amplifies the pulse signal.
(5) Application on linear circuits
Linear optocouplers are used in linear circuits with high linearity and excellent electrical isolation.
(6) Application for special occasions
Optocouplers can also be used in high voltage control, replacing transformers, replacing contact relays, and for A/D circuits.
Seven, learning experience and experience
Among the three components of diodes, triodes, and optocouplers that were studied this week, optocouplers are relatively unfamiliar, so it is not as easy to learn from the other two components. The degree of mastery can only be said to be 50%. First, I started with its working principle, followed by its classification. The last chapters are closely related to the work, including technical parameters, product functions, and usage principles. A deeper understanding of optocoupler devices is needed in future work.
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