The working principle of optoisolators!

　　Optocoupler relay transmits electrical signal by light as medium. It has good isolation for 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.

　　Optocoupler relay 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.

　　Optocoupler selection and replacement

　　Sometimes due to a variety of reasons so that the existing circuit board can not find the same model to replace the original optocoupler, but there are other manufacturers, or even other models of optocouplers on hand, it is necessary to give a reasonable choice to see if the existing optocouplers can be replaced, so what issues need to be noted when replacing it?

　　1、Package type

　　Optocouplers are classified according to the package coaxial type, double inline type, TO package type, flat package type, SMD package type, and fiber optic transmission type, etc. According to the actual circuit to see which package, which directly determines whether the optocoupler is available;

　　2、Linear or non-linear

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

　　For example, for switching power supply is commonly used in the linear optocoupler, can not use non-linear optocoupler, may make the oscillation waveform is bad, serious parasitic oscillation.

　　3、Current transfer ratio CTR

　　IF different different corresponding CTR is also different, and the data difference is also very large, in general, in IF = 5ma, CTR value should be roughly the same as the original optocoupler used in the region CTR value, otherwise the product dynamic is very poor, the replacement needs to pay attention to the selection of the appropriate IF current, so that changes in the input control current can be timely feedback to the output, to ensure the stability of the product feedback loop.

　　In general, the allowable range of the current transmission ratio CTR of the optocoupler is 50% to 200%. If CTR<50%, then the LED in the optical coupler needs a larger working current to drive, which will even increase the circuit power consumption, and the microcontroller I/O port cannot provide too much current. It should also be noted that for the same model of optocoupler will have different grades, which corresponds to the CTR will also be different.

　　4、Other

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