CTR current transfer ratio is a parameter used to describe the characteristics of optical couplers

The technical parameters of the optocoupler mainly include the forward voltage drop VF of the light-emitting diode, forward current IF, current transfer ratio CTR, insulation resistance between the input and output stages, collector emitter reverse breakdown voltage V (BR) CEO, and collector emitter saturation voltage drop VCE (sat). In addition, when transmitting digital signals, it is also necessary to consider parameters such as rise time, fall time, delay time, and storage time.http://www.ic-bom.com/

The CTR-IF characteristic curve of ordinary optical couplers is nonlinear, and the nonlinear distortion is particularly severe when the IF is small, so it is not suitable for transmitting analog signals. The CTR-IF characteristic curve of linear optocouplers has good linearity, especially when transmitting small signals, its AC current transmission ratio（ Δ CTR= Δ IC/ Δ IF) is very close to the CTR value of the DC current transmission ratio. Therefore, it is suitable for transmitting analog voltage or current signals, and can maintain a linear relationship between output and input. This is a very important feature of it.

The current transfer ratio is an important parameter of optocouplers, usually represented by the DC current transfer ratio. When the output voltage remains constant, it is equal to the percentage of the DC output current IC and the DC input current IF. The range of CTR for optocouplers using a photosensitive transistor is mostly 20% to 300% (such as 4N35), while for PC817 series, it is 50% to 600%. This indicates that in order to obtain the same output current, the latter only requires a smaller input current. Therefore, there is a certain similarity between the CTR parameters and the hFE of the transistor.

The main purpose of using optocouplers is to provide isolation between input and output circuits. When designing circuits, the optocouplers selected below must comply with domestic and international standards for isolation breakdown voltage. Given that this type of optocoupler exhibits switching characteristics and poor linearity, it is suitable for transmitting digital signals (high and low levels) and can be used for output isolation of microcontrollers; The selected optocoupler must have a high coupling coefficient.

In the isolation of switching power supplies and the design of optocoupler feedback switching power supplies, it is necessary to correctly select the type of linear optocoupler.http://www.ic-bom.com/