1. About optocoupler

Optocouplers use light as a medium to transmit electrical signals. It has a good isolation effect on input and output electrical signals, so it is widely used in various circuits. At present, it has become one of the most diverse and versatile optoelectronic devices. The photocoupler generally consists of three parts: light emission, light reception and signal amplification. It has the characteristics of strong anti-interference ability, stable operation, no contact, long service life and high transmission efficiency.

2. Optocoupler selection and replacement

Sometimes due to various reasons, the optocoupler on the existing circuit board cannot be replaced by the same model, but there are optocouplers from other manufacturers or even other models on hand. At this time, you need to give a reasonable choice to see if Can replace the existing optocoupler, so what issues should be paid attention to when replacing?

1. Package type

Optocouplers are classified into coaxial type, dual in-line type, TO package type, flat package type, chip package type, and optical fiber transmission type according to the package. Which package depends on the actual circuit, which directly determines Whether this optocoupler is available;

2. Linear or non-linear

The linear relationship refers to the CTR-IF relationship. Within a certain range, the current transmission characteristic curve of a nonlinear optocoupler is nonlinear. This type of optocoupler is suitable for the transmission of switching signals and not suitable for the transmission of analog quantities. The current transmission characteristic curve of the linear optocoupler is close to a straight line, and the performance is better when the signal is small, and can be isolated and controlled with linear characteristics. The commonly used linear optocoupler is the PC817 series.

For example, the commonly used optocoupler in the switching power supply is a linear optocoupler, and a non-linear optocoupler cannot be used, which may deteriorate the oscillation waveform and cause parasitic oscillation in severe cases.

3. Current transfer ratio CTR

The CTR corresponding to different IFs is also different, and the data difference is still very large. Under normal circumstances, when IF=5ma, the CTR value should be roughly the same as the CTR value of the original optocoupler. Otherwise, the product dynamics are very poor. It is necessary to pay attention to selecting the appropriate IF current when replacing, so that the change of the input control current can be fed back to the output terminal in time to ensure the stability of the product feedback loop.

Generally, the allowable range of the current transmission ratio of the optocoupler is 50% to 200%. If the CTR is less than 50%, then the LED in the optical lotus will need a larger working current to drive, and even increase the power consumption of the circuit. Moreover, the I/O port of the single-chip microcomputer cannot provide excessive current. If the CTR value is too large, the Start the circuit or when the load has a sudden change, the system may be unstable and affect the normal output. In addition, it should be noted that optocouplers of the same model will have different grades, and the corresponding CTR will also be different.

4. Other

For example, the operating temperature, the optocoupler has a temperature within a working range, pay attention to whether the input control current IF will be greatly affected by the temperature; in addition, there are response time, etc.; when replacing the optocoupler, it is best to choose the brand, which is easier to find on the market.

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