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Using photorelays instead of mechanical relays in thermostat designs

Tried and tested technology tends to be the first choice when developing new products, unless the design demands a significant review of the components being used. Mechanical relays have been a staple for designers for decades, allowing large currents and voltages to be safely controlled, isolated from low voltage circuits such as microcontrollers. However, with the integration of touch screens and color displays come sleeker housing designs, leaving less volume for large electronic components. Thermostats are no longer gray or beige boxes on the wall but sleek, colorful input and feedback devices for which bulky mechanical relays are suddenly no longer suitable, leaving design engineers searching for a suitable, more compact alternative.

Mechanical relays are also still in regular use despite their known shortcomings. Their switching noise is an obvious first disadvantage, but the degree to which this is seen as an issue depends on the environment within which it is used. The coil also requires a specific voltage, typically 5 V and above, that, in today’s low-voltage electronic systems, may demand its own power regulator to act as a supply. The contacts can become less reliable over time due to arcing when switching loads, and the back-EMF generated by the collapsing electromagnetic field needs to be dealt with.

Photorelays overcome these issues, providing a compact form-factor that matches that of other devices being pick-and-placed in the manufacturing environment. They integrate easily with microcontrollers and other low-voltage control circuitry without the need for a separate supply. Additionally, their switched output is much cleaner, devoid of the contact bounce attributed to mechanical relays and is resistant to the influence of impact or vibration.

To find out more about how photorelays can be used as a replacement for mechanical relays, using a thermostat design as an example, take a look at our video available here:

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