When designing optocoupler circuits, the model and parameters of optocouplers must be correctly selected, and the general selection principles are as follows:
1. Since the photocoupler is a one-way signal transmission device, and the transmission of data in the circuit is bidirectional, the size of the circuit board is required to be certain, combined with the actual requirements of circuit design, it is necessary to choose a device with a single chip integrated multi-channel optocoupler.
2. The allowable range of current transfer ratio (CTR) of optocoupler is not less than 500%. Because when the CTR is < 500%, the LEDs in the optocoupler need a large operating current (>5.0 mA) to ensure that the signal does not occur errors in long-term transmission, which will increase the power consumption of the optocoupler.
3. The transmission speed of the optocoupler is also one of the principles that must be followed in the selection of optocoupler, the optocoupler switching speed is too slow, and it cannot respond correctly to the input level, which will affect the normal operation of the circuit.
4. It is recommended to use linear optocoupler, which is characterized by the fact that the CTR value can be linearly adjusted within a certain range. In the design, because the input and output of the circuit are both high- and low-level signals, the circuit works in a nonlinear state. In linear applications, because the signal is not distorted in transmission, a suitable static working point should be set according to the requirements of dynamic work, so that the circuit works in a linear state.