About the Types of Optocouplers

First, about optocoupler

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

Two, optocoupler selection and replacement

Sometimes, due to various reasons, the existing optical coupler on the circuit board cannot be replaced by the original model, but there are other manufacturers, or even other models of optical coupler at hand. At this time, it is necessary to give a reasonable choice to see whether the existing optical coupler can be replaced. Then what problems should be paid attention to when replacing?

1. Encapsulation type

The optical coupler is classified according TO the package: coaxial type, dual-in-line type, TO package type, flat package type, patch package type, and optical fiber transmission type, etc. According to the actual circuit, which is the package, which directly determines whether the optical coupler is available;

2. Linear or nonlinear

Linear relation refers to the relation of CTR-IF. Within a certain range, the current transmission characteristic curve of nonlinear optical couplings is nonlinear. Such optical couplings are suitable for the transmission of switching signals, but not suitable for the transmission of analog quantities. The current transmission characteristic curve of linear optocoupler is close to a straight line, and the performance is better when the small signal, can be isolated control with linear characteristics, commonly used linear optocoupler PC817 series.

For example, linear optocoupler is commonly used in switching power supply. It is impossible to use nonlinear optocoupler, which may cause the oscillation waveform to deteriorate and parasitic oscillation to appear in serious cases.

3. Current transmission ratio CTR

In general, when IF=5ma, CTR value should be roughly consistent with the CTR value of the original region used by the photocoupling, otherwise the product dynamics is very poor. It is necessary to select appropriate IF current during replacement, so that the change of the input control current can be timely fed back to the output. Ensure the stability of product feedback loop.

Generally, the current transfer ratio of the photocoupler is within the allowable range of 50% to 200% of the CTR. If CTR<50%, the LED in the optocoupler will need a larger working current to drive, and even increase the circuit power consumption, and the single chip I/O port can not provide too much current, if the CTR value is too large, when starting the circuit or when the load changes, it may be unstable system, affecting the normal output. In addition, it should be noted that there will be different grades for the same type of optical coupler, and the corresponding CTR will also be different.

4. Others

For example, the operating temperature, the optocoupler has a working range of temperature, pay attention to whether the input control current IF will be greatly affected by the temperature; There's response time, etc.; Replace the optical coupler when the best choice of brand, the market is relatively easy to find.