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I am a software developer with little knowledge of Electrical Engineering. I have delivered some Raspberry PI projects that extensively use Relays.

I recently heard that Triacs are much cheaper and safer than Relays and can do the same job. If that is true, my question is:

Why are Triacs so under emphasised in Raspberry PI (or Arduino) projects and people prefer using Relays?

Also, are Triacs suitable for 120V AC or is there any Triac that can control 12V DC as well?

Thank you,

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  • \$\begingroup\$ And for some (important) things not said there: electronicdesign.com/components/… \$\endgroup\$ – Fizz Oct 22 '15 at 6:39
  • \$\begingroup\$ And blog.heilind.com/2012/10/… has a nice bullet-style recap. I suspect the combination of cost and know-how issues are decisive in the arduino community. \$\endgroup\$ – Fizz Oct 22 '15 at 6:46
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Relays are as dead simple as it gets. You turn them on and off, they work asynchronously, set it and forget it. They work with AC and DC.

Everything that makes a relay great makes a triac terrible. Triacs only work with AC. Triacs require support circuitry, for the drive to isolate it from the controller, a snubber if you are running inductive loads and possibly a zero crossing detector. Triacs are synchronous, meaning they (may) require constant babysitting.

That being said a triac can make up for the short comings of a relay. With a triac you can get a soft start to avoid large inrush currents. You can throttle a load which can sometimes give speed control (AC is weird that way). Also triacs are immune to physical shock where relay contacts can bounce.

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  • \$\begingroup\$ This has come up surprisingly often in recent days, but have a look at this for what's required to control a triac: electronics.stackexchange.com/a/148084/53375 I doubt that a Pi running a high-level OS can be made to do it reliably; it would have to command a slave device that can control its timing with sub-millisecond accuracy when referenced to an external signal (the zero-crossing detector). That said, there are triacs that can time themselves with built-in zero-crossing detectors, which would make the job slightly easier, but they can't be dimmed. \$\endgroup\$ – AaronD Oct 22 '15 at 17:08
  • \$\begingroup\$ Another advantage of relays is that the closed contacts are effectively a wire. The analysis is about that simple, except for some "bouncing" at the transitions. A triac has an unavoidable voltage drop across it, even when fully on. This is more-or-less constant regardless of current (unless the current is zero), so the output waveform becomes distorted around zero-crossing. Resistive loads like incandescent bulbs and heaters don't care at all, but electronic power supplies like CFL's and computers may hate it. \$\endgroup\$ – AaronD Oct 22 '15 at 17:15

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