1. Why use optocoupler devices in the circuit?
Requirements for electrical isolation. Signal transmission is required between the A and B circuits, but the power supply level between the two circuits is too large, one is hundreds of volts, and the other is only a few volts. Two power supply systems with huge differences cannot share the power supply. Circuit A is connected with strong electricity, and there is a risk of electric shock when contacting the human body, so it must be isolated. The B circuit board is the part that the human body often touches, and dangerous high voltage should not be mixed together. Between the two, it is necessary to complete signal transmission, but also must be electrically isolated. The use of high-impedance devices such as operational amplifier circuits, and the transmission of weak analog voltage signals by the circuit, make the anti-interference processing of the circuit a more troublesome thing-the noise interference mixed in from various channels may be reverse , "Submerged" the useful signal. In addition to the safety of human contact, the safety of circuit devices must also be considered. When the input side of the optocoupler is damaged by 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 research, development and practical application of optocoupler devices. The basic function of the optocoupler is to effectively isolate the input and output side circuits; it can transmit signals in the form of light; it has a good anti-interference effect; the output side circuit can avoid the introduction of strong voltage to a certain extent And shock.
2. General properties of photoelectric coupling devices:
1. Structural features: The input side generally uses light-emitting diodes, and the output side uses phototransistors, integrated circuits and other forms to implement electrical-optical-electric conversion and transmission of signals.
2. There is light transmission between the input and output sides, but there is no direct connection with electricity. The presence and strength of the input signal controls the luminous intensity of the light-emitting diode, and the output side receives the light signal, and outputs a voltage or current signal according to the light-sensitive intensity.
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 the output side has a certain current output capability, and some can directly drive small relays. Special type optocoupler devices can linearly transmit AC and DC signals in millivolts and even microvolts.
4. Due to the structural characteristics of the optocoupler, the input and output sides need independent power supplies isolated from each other, that is, two power supplies without a "common ground" point. The input side of the following first and second types of optocouplers provides the input current path by the signal voltage, but in fact the input signal circuit also has a power supply branch; while the linear optocoupler, the input side and the output side are directly connected There are two types of isolated power supplies.
3. In the frequency converter circuit, there are three types of photoelectric coupling devices that are often used:
1. A triode photocoupler, such as PC816, PC817, 4N35, etc., is often used in the output voltage sampling and error voltage amplifier circuit of the switching power supply circuit, and is also used in 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 photosensitive triode, which is mainly used for isolation and transmission of switching signals;
2. The second type is an integrated circuit photocoupler, such as 6N137, HCPL2601, etc. The input side luminous tube adopts a new type of luminescent material with low delay effect, and the output side is composed of a gate circuit and a Schottky transistor, which greatly improves the performance improve. The frequency response speed is much higher than that of the triode photocoupler, and it is also used in the fault detection circuit of the frequency converter and the switching power supply circuit;
3. The third type is linear optocoupler, such as 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-level weak analog signals. In the frequency converter circuit, it is often used for sampling and amplifying processing of output current, and sampling and amplifying processing of main loop DC voltage.
4. Measurement and online inspection of the first type of optocoupler device:
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 it can directly drive small relays, and the output saturation voltage drop is less than 0.4 V. It can be used for the transmission of lower frequency signals and DC signals of tens of kHz. There are polarity requirements for the input voltage/current. When a forward current path is formed, the two pins on the output side present a path state. When the forward current is less than a certain value or bears a certain reverse voltage, the two pins on the output side are in an open state.
Measurement methods:
The digital meter diode gear, the forward voltage drop of the measuring input side is 1.2V, and the reverse is infinite. The positive and negative voltage drops or resistance values on the output side are all close to infinity.
The x10k resistance file of the pointer meter, measured its 1st and 2nd pins, there is an obvious difference in positive and negative resistance, the forward resistance is about tens of kΩ, the reverse resistance is infinite; the 3 and 4 pins have infinite forward and reverse resistance.
Two-meter measurement method. Use the x10k resistance file of the analog multimeter (which can provide 15V or 9V, tens of μA current output), connect pins 1 and 2 in the positive direction (black pen with 1 pin), and use the resistance file of another meter to measure x1k 3 The resistance value of pin 4, when the pin 1 and pin 2 are connected, the resistance value between pins 3 and 4 will be about 20kΩ. If the pin 1 and pin 2 are disconnected, the resistance between pins 3 and 4 will be infinite.
A DC power supply can be connected in series to limit the input current to less than 10mA. When the input circuit is turned on, the resistance of pins 3 and 4 is in the path state, and when the input circuit is open, the resistance value of pins 3 and 4 is infinite.
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