Triacs VS relays

Other Hardware General Electronics
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On ebay I see a lot of single relay boards, 8 channel relay boards, optocoupled relay boards, and so on, but I never see any triac boards. I recently had a project with solenoid valves, but switching them with relays made my LCD display go all wonkey, so I used a (bt139) triac instead. After this everything worked fine.

Why aren't triacs used more widely? (It isn't particularly expensive, for a triac + optocoupler is only about 2 dollars.)
|s there a big disadvantage I don't know of?

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They both have their advantages & disadvantages. Relays are probably more universally usable - they can be used for AC, DC, low current and high current (assuming they are designed for it), but have issues related to the magnetic coil (EMF, inductive voltage spikes, etc.) Triacs/thyristors/SSRs generally have more resistance (leading to heat, voltage drops, etc,) more current limitations (minimum & maximum.)