Some terminology of industrial technology often blurs the distinction between similar concepts. Today, we find that the terms optocoupler and optical isolator are used interchangeably to refer to the same function.
The optocoupler and optical isolators allow the transmission of signals and data from one system to another in an electronic device without direct electrical connection. This is accomplished optically by placing transmitters and detectors separated by transmitters and detectors in a single package and then sending beams to the optical receiver. This allows the circuit to maintain complete electrical isolation, while information can be transmitted from one potential to another. In all optocoupler and optical isolators, input signals are converted into light pulses emitted by LEDs. This optical pulse is transmitted to a silicon-based optical sensor.
Optical sensors can be analog or digital according to the type of input signal to be transmitted within the device. If an application requires analog signals, such as 4 to 20 mA, the optical light sensor can be a photodiode or a phototransistor. Both of these devices provide analog output signals, which can be used in many analog applications.
In those applications where semaphores are very important to the operation of the system, analog response is necessary. The current output of the device referring to the input light intensity of the LED is called current transfer ratio (CTR), which is equal to the output current divided by the input current. According to the different system gains, CTR values may vary from 10% to 5000% or even higher. Usually, the smaller the CTR, the faster the rise and fall time.
Analog devices usually require additional circuits to be used by other parts of the system. Digital output devices are most suitable when pulse or bit output is required in applications.
The optocoupler is specially designed as small packages similar to miniature package for the dual in-line package (DIP) or surface mounted device (SMD) so that they only occupy the smallest space while transmitting data optically. Because the definition of isolation electric voltage and electric potential may be given by AC, RMS, or DC values, designers should pay special attention to it.
Optical isolators have many packaging types, such as rectangular, cylindrical, and some special shapes. These packaging types are designed to provide higher isolation voltage than DIP and SMD packages.
When using optical isolators, designers may need to take environmental conditions into account. For example, if the system needs isolation of the DC 50000 volts, the humidity percentage of the environment will be an important consideration.
If the humidity of the air is too high, the surface of the optical isolator or PCB may generate arcs, and then form conductive paths and form short circuits around the device. Distance, temperature, atmospheric pressure, type and degree of air pollution, and humidity together determine the breakdown voltage of the optical isolator of the surface.
The optocoupler and optical isolator have two structures: single LED or back-to-back LED (with cathode and anode connected). The structure of the optical sensor can be changed between a single optical sensor and a complete integrated receiver.