Where an opto-isolator circuit is used to decouple ground planes or voltage sensing inputs, the rate of change of the isolated signal is relatively unimportant. Typical off the shelf opto-isolators can withstand input-to-output voltages differences of up to 10 kV and voltage transients of around 25 kV/μs. Distance and humidity are the most significant factors. The voltage at which arcing will occur will depend on the temperature, humidity, separation distance, pressure, and airborne contaminants’ presence. This is affected by the PCB design, that is how the traces for the inputs and outputs are routed and separated and the environmental conditions around the device. Similarly, an external breakdown will occur when the voltage at the input pin of the device arcs across to an output pin. An internal breakdown will occur when the voltage at the light source element of the device arcs across to the light sensor element. This isolation voltage is governed by the construction of the opto-isolator device itself and factors outside the device. The isolation voltage is the maximum rated voltage difference that can be present between the LED and the light sensor. If in doubt, study the data sheet. The image below shows the typical circuit diagram for an opto-isolator or opto-coupler with inputs on the left and outputs on the right.įor the moment, we want to focus on the different types of opto-isolators. When choosing an opto-isolator, the key parameters to consider are the isolation voltage, bandwidth, linearity, current transfer ratio, and power requirements. There are also different opto-isolator speeds, and fast opto-isolators like a solid state relay will outperform something like a diode opto-isolator in terms of data transfer. Don’t be surprised if you see exceptions to this convention. It’s worth mentioning that the terms optocouplers and opto-isolators are often used interchangeably however, the usual convention is that optocouplers are devices that can isolate voltages of up to about 5000V while the opto-isolators are devices that can isolate voltages of over 5000V. Important Specifications When Choosing Opto-isolators They are particularly useful where long data cables that could be susceptible to induced voltage transients or ground plane surges enter an electronic device containing sensitive semiconductor components. There are many different types of opto-isolator circuits that are widely used in communications, control, and monitoring systems where data signals could provide a point of ingress for harmful voltages to damage a device. The components are sealed in an opaque package to prevent interference from external light. This provides a barrier to any voltage transients or overvoltage levels that appear at the input from affecting the electrical circuit at the output of the opto-isolator. Then, within the same device, a light-sensitive device such as a photodiode, phototransistor, or photodarlington transistor converts the light signal back into an electrical signal. In simplest terms, an opto-isolator works by taking an input electrical signal and converting it into a light signal using a light-emitting diode, generally operating in the near-infrared spectrum. Opto-isolators act as a protection mechanism, ensuring that harmful electrical currents cannot flow across the device. Because there is no need to directly pass voltage or current between the inputs and outputs in an opto-isolator circuit, these components can be used to provide electrical isolation two regions in a PCB. An opto-isolator is an electronic device that can be used to pass information between a diode without passing an electrical current.
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