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Considerations when using photorelays instead of mechanical

With years of experience working with mechanical relays collected, it is easy to forget all the factors that require consideration when designing them into an application. First there is the driver circuit that can be implemented by a simple transistor or, when supporting multiple channels, a Darlington transistor array. Then there is the back electromotive force (back-EMF) that is often handled with a flywheel diode or metal oxide varistor. While the move to a photorelay resolves many of the disadvantages of a mechanical relay, there are still some design challenges that must be considered in order to ensure optimal operation over the full lifetime of the application.

Photorelays utilize an integrated Infrared LED as light source and a Photo Diode Array (PDA). When the LED is powered on, the PDA serves to drive the gate of the photorelay’s MOSFET. Just like a conventional MOSFET, once switched on current flows between the source and drain. Obviously, to replicate the bi-directional current flow of a mechanical relay, the device uses two MOSFETs to implement this feature. Just like when controlling an LED, it is critical to consider the forward current IF of the LED. Over the lifetime of the device the light emission of the LED can drop, so it is essential that the drive current selected is high enough that it always generates enough light.

Furthermore, the light emission can also be impacted by factors such as temperature and fluctuations in power supply. The speed at which the output MOSFETs turn on after the LED is enabled also depends on its light emission and therefore its drive current IF. To simplify the design engineer’s move to photorelays and highlight the unique but important points of consideration when designing them into applications, Toshiba has prepared a video that introduces some of these unique aspects and how to approach them.

To find out more about these aspects and approaches, take a look at our video on design considerations when replacing mechanical relays available here:

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