Some integrated circuits that offer galvanic isolation between two sides of a system can provide high voltage isolation through optical coupling. The best option is to use an optocoupler with DC offset on the AC signal, then filter the DC on the output with a capacitor.Feeding an optotriac with an AC signal modulates conduction in the phototriac, creating an AM signal on the output.If you have an AC signal and you want to pass it across an isolation barrier, you should apply a DC offset and use an optocoupler. The verdict at this point should be clear.

The signal could also be rectified, but this would require a zero cross detecting inverter on the output side to fully reproduce the input AC signalĭon’t Use an Optotriac to Pass an AC Signal.In the latter case, the DC offset should be larger than the AC amplitude The frequency can’t be too high, otherwise conduction could be hampered by the phototransistor’s internal capacitances.There are a few requirements on the use of a basic optocoupler for an AC signal: The short answer is “yes,” an optocoupler can be used to pass a time-varying signal across an isolation gap. Can An Optocoupler Be Used to Pass AC Signals? A simple example is shown below.ĭC voltage or a signal being used to turn on a phototriac in an optotriac IC. As we can see, the optotriac is not directly passing AC across the isolation gap. This allows the AC source on the output side to deliver power to its load. When the source is turned on, the phototriac on the output side starts conducting. Instead, it is used to act as an optically triggered switch that turns an existing AC signal ON and OFF.Įssentially, the LED on the input side will be connected to a DC source, or to a GPIO on an MCU. Use of an OptotriacĪn optotriac is not used to directly pass an AC signal to the output side. These components are the optical analog of a traditional triac and function in a similar way as a phototransistor the base (or gate) is illuminated with an input signal, and this modulates current flow on the output side from an existing power source. The photosensing component used in an optoisolator is a phototriac. One reason you will often see an optocoupler preferred in digital systems is because these components have simpler circuit construction and reasonably fast response time, yet they still provide comparable isolation as an optotriac.Ĭontrol one side of an isolated system from the other side From the data in the table, we can see that the two types of components are very similar. The table below provides a comparison of these two types of components for optical isolation. This does mean that a system will need to have power on both sides of the isolation barrier. The idea is to use an input signal to then trigger current flow on the output side. The isolation barrier normally prevents current from flowing between two sides of the component. Optocouplers and optoisolators are both used to pass a signal from one side of a system to another side when there is galvanic isolation. So which should you use in your design? It depends on the type of signal being passed between two isolated regions in a PCB. These components are less often used for optical isolation, typically because of the simplicity of using an optocoupler. There is another type of optoisolator component that can be used, known as an optotriac. The most common method is with an optocoupler, which uses an LED with a phototransistor to couple a signal across a gap. The simplest way to do this with data and control signals is to use optical coupling. The goal in all of this is to control signal flow and ensure safety.Įven when a PCB uses isolated sections to confine power and data, sometimes signals need to pass between the two regions. The isolated section of the PCB could be further isolated from another board in a larger assembly. Most often, this is used to prevent ESD from passing between one region of a PCB and into another region, where one of the isolated regions might be exposed to the external environment. Isolation in an electronics design involves separating two sides of an electronic component, or groups of components, to prevent signals from passing between two regions of a PCB.

Optocoupler vs digital isolator