The commonly used isolation devices in electrical applications include optocouplers, relays, and transformers.
Optocouplers belong to flow controlled components that transmit signals through light as a medium: electricity → light → electricity. The input end is a light-emitting diode, and the output end is a photosensitive semiconductor. The core application of optocouplers is isolation, commonly used in systems where there is no common ground between input and output. So the voltage resistance between input and output can reach thousands of volts.
Many communication modules are also isolated by optocouplers, making it easier to connect between different systems without considering whether they are grounded or not.
When using a linear optocoupler to transmit switch signals, it is necessary to match the input resistance reasonably. In Figure 1, the input resistance is 360 Ω, the forward voltage drop of the optocoupler input is about 1V, the input current IC is (5-1)/360 ≈ 11mA, the optocoupler output CE impedance is more than 200 Ω, and the HFD2 coil impedance of the relay is 2880 Ω. At this time, the relay can be driven normally. If the IC current decreases, the CE impedance will increase, which will cause the relay to not be driven normally.
Linear optocouplers are mainly used for simulating signal transmission, with an output equivalent to a variable resistor. It is common in switch mode power supplies to use an optocoupler as feedback to isolate high and low voltage. The commonly used ones are EL817. As mentioned in the previous example, it is also commonly used for switch signals.
The output voltage of the switching power supply is isolated by a linear optocoupler and fed back to the control chip to achieve real-time adjustment of the output voltage.
Nonlinear optocouplers are mainly used for transmitting switch signals (or digital signals), and commonly used 4N series include 4N25 and 4N26; In addition, there are high-speed optocouplers such as 6N136, 6N137, etc. It is commonly used for communication isolation and PWM wave control (which can effectively reduce electromagnetic interference), to determine whether it is a high-speed optocoupler, and to check if the data manual indicates High speed (1Mbps, 10Mbps).
main points
① The core application of optocouplers is isolation function;
② At the same voltage, when the input resistance and output resistance of the linear optocoupler are the same, the output current IC is basically consistent with the input current IF; Even if the input and output voltages are different, matching the resistance between the output and input can be achieved;
③ Both linear and nonlinear optocouplers can be used for switching signals, while nonlinear optocouplers cannot be used as substitutes in linear optocoupler circuits.
④ Nonlinear optocouplers have a faster response speed than linear optocouplers, similar to how comparators have a faster response speed than operational amplifiers.