All of the IoT switches (AC) I´ve seen so far are relay based and I´ve been doing some custom ones for my home. For small loads circuits (lightbulbs max 100W, 110VAC) would it be safe to use a triac instead of a relay enclosed in a light switch box?
I´d like to have four circuits with a total of no more than 500W, 110VAC and in this case the size of 4 relays would make it difficult to fit. I was planing on going with triacs / opctocoupler making the front plate of the control made of aluminum and place all triac there to use it as heat dissipation element...
I´ve seen some dimmers going on this route but I´ve never seen an IoT switch using it - is it only related to cost? Any advices on this? No need for schematics or details just advices related to the risks this kind of usage.
I know there are lots of items related to relays vs triacs in general but I´d like some advices here lated to enclose it on a switch box.


Triac should work just fine, you need to take care of the heat dissipation though.

The reason is probably related to cost / simplicity, although a Triac (properly designed) will be more reliant over time than a relay.


Triac vs relay: You need to consider these parameters to take a call:

  1. Cost
  2. Space requirements
  3. Thermals
  4. Switching cycles
  5. Contact sticking
  6. Immunity to line noise/spikes

Relays are metal contacts that get activated when you pass a current through its coil. This gives a very low resistance path and is suitable for higher currents. Advantage primarily comes from the fact that you do not need a lot of cooling components. A compact relay can easily handle 10-15 amps without heating up a lot. Depending on the exact application, the benefits of using a relay are low cost, smaller size, better thermals, and immunity to line noise.

With relays, the major issues are limited operation cycles and contact sticking if you are driving capacitive loads.

Triacs can be used without heat sinks or active cooling for low currents (<0.5 amp). They will have a typical voltage drop of 1 volt across them. This means a larger heat dissipation as your current draw increases. You can do away with passive cooling for higher currents as well but the heat sink size will increase for higher currents. Triacs also allow you to do phase control for dimming certain loads (typically used for motor speed control, incandescent bulb dimming, compatible LED dimming etc). You have practically unlimited switching cycles with a triac and there is no concern of contact sticking. However one problem might be line noise that can false trigger your triac. This can be avoided if you use the right spec triac or snubber to absorb those spikes.

There is no general rule that one solution will be compact or the other. Same thing applies to cost and reliability. Solution selection is quite subjective and depends on the application.

High current resistive load where dimming is not required - use a relay

Low current loads where dimming is required - use a triac

Low continuous current with high inrush current (capacitive load) - use a triac

Very high switching count required (over 100,000 during lifetime) - use a triac


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