Opto-isolators are a fairly common electronic device, serving to relay information between diodes without electrical current being passed. With this ability, opto-isolators are able to be used to establish electrical isolation between two regions of a printed circuit board (PCB). As a protection mechanism, opto-isolators can deter harmful electrical currents from flowing across a device, making it useful to have a general understanding of their design and functionality.
With a light-emitting diode that is operating in the near-infrared spectrum, an opto-isolator can transform an electrical input signal into a light signal. Once the conversion is complete and the signal moves across a space, a photodiode, phototransistor, or photodarlington transistor will convert the light signal into an electrical signal once again. As such, data can be transferred between diodes while maintaining the form of an electrical signal at the input and output side. By transforming the signal into a light signal as it passes through a diode, a barrier against voltage transients or overvoltage levels is established and upheld. To further prevent any issues or interference, the components will all be placed within an opaque package that mitigates any external light from entering the assembly.
While opto-isolator devices find implementation in a wide variety of applications, their most common uses include being placed within communications, control, and monitoring systems. If an application features long data cables where induced voltage transients or ground plane surges are possible, an opto-isolator can be very beneficial. Typically, this is mostly a concern if there are sensitive semiconductor components present.
When searching for opto-isolator options, it is important to not confuse such parts with optocouplers. While the two terms are regularly used interchangeably, there are some small differences that can be important to take note of. For example, while opto-isolators regularly have the ability to isolate voltages exceeding 5000V, optocouplers are more suitable for those under 5000V. Additionally, opto-isolators may perform with different speeds based on whether or not they feature solid-state components. Regardless of what device you choose, the most important parameters to consider are isolation voltage, bandwidth, linearity, current transfer ratio, and power requirements.
For the isolation voltage, one is referring to the maximum rated voltage that the opto-isolator can handle before breaking down. While distance and humidity greatly affect such conditions, it is best to refer to manufacturer specifications. Bandwidth will often affect data rates, and basic options will have a bandwidth of around 10 MHz. For linearity, it is essential that one realizes any non-linearity will cause distortion, and the right qualities will often depend on what signals are being transferred while avoiding cutoff and saturation regions. The current transfer ratio of a device will also affect efficiency, and the chosen component should meet required ratios while meeting the maximum rated temperature and operating life of the device that the isolator will be placed in. Lastly, power requirements will need to be met for efficiency, and thermal management should be upheld to protect sensitive components.
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