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First, why should we use optocoupler devices in the circuit?
Electrical isolation requirements. Between the A and B circuits, the signal transmission is required, but the power supply level is too wide between the two circuits. One way is hundreds of volts, the other is only a few volts; two different power supply systems cannot share the power supply. ;
The A circuit is related to the strong electricity, and the human body is in danger of electric shock and needs to be isolated. The B circuit board is the part that the human body often touches, and dangerous high voltages should not be mixed together. Between the two, it is necessary to complete the signal transmission and must be electrically isolated;
The use of high-impedance devices such as op amp circuits, and the transmission of weak voltage signals by analog circuits, make anti-jamming processing of circuits become a cumbersome task - noise interference from various channels may be anti-customer , "submerging" the useful signal;
In addition to considering the safety of human contact, the safety of the circuit device must be taken into account. When the input side of the optocoupler device is damaged by a strong voltage (field) impact, the output side circuit can be safe due to the isolation of the optocoupler.
The above four reasons have contributed to the development, development and practical application of optocoupler devices. The basic function of the optocoupler is to effectively isolate the input and output side circuits; to transmit signals in the form of light; to have better anti-interference effects; the output side circuit can avoid the introduction of strong voltage to a certain extent. And shock.
Second, the general properties of optoelectronic coupling devices:
1. Structural features: The input side generally adopts light-emitting diodes, and the output side adopts various forms such as phototransistors and integrated circuits to perform electro-optical-electrical conversion and transmission of signals.
2. There is light transmission between the input and output sides, and there is no direct connection with electricity. The presence or absence and strength of the input signal controls the luminous intensity of the LED, while the output side receives the optical signal, depending on the intensity of the light, the output voltage or current signal.
3. The input and output sides have high electrical isolation, and the isolation voltage is generally above 2000V. It can transmit AC and DC signals, and has a certain current output capability on the output side. Some can directly drag small relays. Special optocouplers are capable of linearly transmitting millivolt, even microvolt, AC and DC signals.
4. Due to the structural characteristics of the optocoupler, the input and output sides need separate power supply sources that are isolated from each other, that is, two power supply sources that do not have a "common ground" point. The input side of the first and second types of optocouplers provides the input current path by the signal voltage, but the input signal loop also has a power supply branch; while the linear optocoupler, the input side is the same as the output side. There are two isolated power supplies.
Third, in the inverter circuit, the commonly used optocoupler devices, there are three types:
1. A triode type photocoupler, such as PC816, PC817, 4N35, etc., is commonly used in the output voltage sampling and error voltage amplifying circuit of the switching power supply circuit, and is also applied to the digital signal input circuit of the inverter control terminal. The structure is the simplest, the input side is composed of a light-emitting diode, and the output side is composed of a photo-transistor, which is mainly used for isolating and transmitting the switching signal;
2. The second type is an integrated circuit type optocoupler, such as 6N137, HCPL2601, etc., the input side light-emitting tube adopts a novel luminescent material with low retardation effect, and the output side is composed of a gate circuit and a Schottky transistor, so that the working performance is greatly improved. improve. The frequency response speed is greatly improved compared with the triode type photocoupler, and is also applied in the fault detection circuit and the switching power supply circuit of the frequency converter;
3. The third type is a linear optocoupler such as the A7840. The structure and performance are quite different from the first two optocoupler devices. In the circuit, it is mainly used for linear transmission of mV-class weak analog signals. In the inverter circuit, it is often used for sampling and amplification processing of output current, sampling and amplification processing of DC voltage of main circuit.
The following figure shows the pin and function schematic diagrams of three types of optocoupler devices:
Three optocoupler circuit diagrams
Fourth, the first type of optocoupler device measurement and online detection:
The first type of optocoupler has a working voltage drop of about 1.2V at the input, a maximum input current of 50mA, and a typical application value of 10 mA. The maximum output current is about 1A, so that the small relay can be directly driven, and the output saturation voltage drop is less than 0.4. V. It can be used for transmission of tens of kHz lower frequency signals and DC signals. There is a polarity requirement for the input voltage/current. When the forward current path is formed, the two pins on the output side exhibit a path state. When the forward current is less than a certain value or a certain reverse voltage is applied, the two pins on the output side are open.
Measurement methods:
The digital meter diode file measures the forward voltage drop of the input side to 1.2V and the inverse infinity. The positive and negative voltage drops or resistance values ​​on the output side are close to infinity;
The x10k resistance file of the pointer table measures 1 and 2 feet, and there are obvious positive and negative resistance differences. The forward resistance is about tens of kΩ, the reverse resistance is infinite; the positive and negative resistances of 3 and 4 feet are infinite;
Two meter measurement method. With the x10k resistor of the pointer multimeter (can provide 15V or 9V, tens of μA current output), positively turn on 1, 2 feet (black pen 1 pin), use another table's resistance file to measure with x1k 3 The resistance value of 4 feet is 1. When the 1 and 2 foot pens are connected, the resistance value of about 20kΩ is displayed between the 3rd and 4th feet, and the test leads of 1 and 2 feet are disconnected, and the resistance between the 3rd and 4th feet is infinite.
A DC power supply can be connected to the resistor to limit the input current to less than 10mA. When the input circuit is turned on, the resistors of pins 3 and 4 are in the path state, and when the input circuit is open, the resistance values ​​of pins 3 and 4 are infinite.
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