Measurement and online detection of triode optocoupler in frequency converter

The function of the optocoupler in the circuit is mainly to isolate the light energy from the electrical energy, so as to avoid interference while facilitating the design. Generally speaking, optocouplers are divided into three categories, and each category has different measurement and online detection methods. This article will introduce the measurement and online detection modes of the major types of optocouplers. If you are interested, please take a look.

In the classification of inverter circuits, the broad category of optocouplers is a triode-type photocoupler, which is often used in output voltage sampling and error voltage amplification circuits of switching power supply circuits, and also used in the digital signal input circuit of inverter control terminals. The structure is simple, 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.

such as PC816, PC817, 4N35 and so on.

Measurement and online detection

type photoelectric coupler, the input end working voltage drop is about 1.2V, the input current is 50mA, the typical application value is 10mA; the output current is about 1A, so it can directly drive small relays, and the output saturation voltage drop is less than 0.4V. 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

Digital meter diode gear, measuring input side forward voltage drop of 1.2V, reverse infinity. The positive and negative voltage drop or resistance value on the output side is close to infinity;

The x10k resistance file of the pointer meter, measure its 1, 2 feet, there is a significant difference in positive and negative resistance, the forward resistance is about tens of kΩ, and the reverse resistance is infinite; the 3 and 4 pins are infinite and the reverse resistance is infinite;

Two-meter measurement method. Use the x10k resistance gear of the analog multimeter (which can provide 15V or 9V, tens of μA current output), turn on 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.

The above measurement is a necessary process before the new device is installed. When it is inconvenient to measure on-line, the device can be removed from the circuit when necessary, and the device can be measured off-line to further judge the quality of the device.

In actual maintenance, offline resistance measurement is not very convenient, while power-on detection is more convenient and accurate. Measures should be taken to change the input side circuit, and measure the quality of the device according to the corresponding change (or no change) produced on the output side. That is to break the "balanced state" in the faulty circuit, make it appear "transient imbalance", thereby exposing the cause of the fault. The input and output sides of the optocoupler device are connected with current-limiting resistors in the circuit. In power-on detection, methods such as reducing (parallel) resistance and increasing resistance (opening it) can be used to cooperate with the voltage detection on the output side. Judge the quality of the optocoupler device. In some circuits, you can even directly short-circuit or open the input side and output side to detect and observe the dynamic changes of the circuit, which is helpful to judge the fault area and carry out the maintenance work.

Precautions during measurement: One side of the optocoupler device may be directly connected to the "strong current", and there is a danger of electric shock if touched. It is recommended to provide an isolated power supply for the machine during maintenance.

is the application circuit diagram of common triode optocoupler devices.

It is the digital signal input circuit of the inverter control terminal circuit. When the forward rotation terminal FWD and the common terminal COM are shorted, the voltage between pins 1 and 2 of the PC817 changes from 0V to 1.2V, and the voltage on pin 4 changes from 5V to 0V. . Similarly, when the control terminal is open, the voltage between pins 1 and 2 of the PC817 is 0V, and the voltage between pins 3 and 4 is 5V. The circuit of Figure 1 (1) can see the voltage value of each pin of the optocoupler device, and it can be judged by measuring the voltage of the input and output pins in a fault or normal state.

   The measurement between 1 and 2 is 0.7V (the average value of the AC signal), and the between 3 and 4 pins is 3V, indicating that the photocoupler has an input signal, but is the photocoupler itself normal? Use metal tweezers to short the 1 of the PC817 , 2 feet, measure the voltage of 4 feet to rise from the original 3V to 5V (or there is a significant rise), indicating that the optocoupler device is good. If the voltage does not change, the optocoupler is damaged.

There are three types of optocoupler devices in the inverter. This article only introduces the detection and online measurement methods among them. In the following article, the editor will introduce the detection and online measurement methods of the remaining two types of optocoupler devices. I hope you can gain something after reading this article.