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I am developing one consumer product in which I am going to operate Resistive and Inductive loads. In this project there is also a need of dimming of the loads.

I have completed programming part. But doing study of switching through programming I got confused at what to use for switching. Relay or Triac?

I have studied that every Automation companies are using Relay for switching of loads. And for dimming purpose they are using Analog Dimming. While Triac can do both Switching and Dimming through phase cutting.

So my confusion is, why everybody is going with Relays not Triacs? What can be the possible points for not using Triacs?

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  • \$\begingroup\$ "Which way I should go", ah, we've got ourself a Yoda. \$\endgroup\$ – Harry Svensson Nov 15 '17 at 11:09
  • \$\begingroup\$ Read this answer and have an idea you should : electronics.stackexchange.com/questions/339747/… \$\endgroup\$ – Solar Mike Nov 15 '17 at 11:18
  • \$\begingroup\$ @HarrySvensson my other comment is your fault ... :) :) \$\endgroup\$ – Solar Mike Nov 15 '17 at 11:19
  • \$\begingroup\$ What voltage? What current? AC/DC? \$\endgroup\$ – HandyHowie Nov 15 '17 at 11:36
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Relay

  1. Has low voltage drop when closed. Other than not exceeding the max current rating, you don't worry about dissipation.

  2. Inherently isolated.

  3. Can switch off at any time.

  4. Is relatively slow, ms to 10s of ms to respond either direction.

  5. Is a mechanical system, so eventually wears out from use. Consider each relay as capable of a finite number of switch cycles. Good relays come with specs for this. Note also that the current, voltage, and inductive component of the load being switched can significantly effect the number of switch cycles a relay is capable of.

Triac

  1. Has a volt or more drop at useful currents. The little bit of reduced voltage to the load is usually not a issue when dealing with line voltages, but the power dissipation can be.

  2. Not isolated. This can be a serious issue when switching line power. Sometimes the control circuitry can float with the line voltage. Sometimes optical isolation is used right before the triac. That's so common, triacs with optical isolators come packaged in a single device called a solid state relay.

  3. Turns on pretty much instantly, certainly much faster than a relay.

  4. Turning off only happens at a current zero-crossing. This can be both good and bad. It is good in reducing emissions. It is bad if you want to turn something off quickly.

  5. As long as maximum specs are adhered to, lasts pretty much indefinitely.

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  • \$\begingroup\$ Triac #6 - Silent switching. \$\endgroup\$ – R Drast Nov 15 '17 at 12:40
  • \$\begingroup\$ Triac #7 - Subject to damage in many commercial lamp products when some types of filament bulbs burn out. \$\endgroup\$ – Michael Karas Nov 15 '17 at 14:04
  • \$\begingroup\$ +1 I'd also add Relay #6, Well understood, and easy to design with. The converse for Triacs. \$\endgroup\$ – Trevor_G Nov 15 '17 at 15:00
  • \$\begingroup\$ Also Relays #7, usually easily maintainable if not PCB mount. Much easier to unplug and plug in a new standard relay package, than replace some $300 PCB because the TRIAC fried. See.. my furnace died, and they want $500 for a part to fix it... \$\endgroup\$ – Trevor_G Nov 15 '17 at 15:01
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First, a relay can switch off both DC and AC. Triac only switches off when crossing zero, ie only AC.

Then for AC loads, what justify using a relay or a triac are usually of three kinds, not mutually exclusive:

  • need of galvanic isolation on the AC (for safety reason for example) - only a relay offers that

  • money! Often a relay will be cheaper and simpler to design (ie less expensive too); it usually comes at the price of longevity too 😞

  • need of good dynamic performances, like fast switching On or phase control; in that case the triac has clear advantage

When money is well spent, a design becomes more and more popular: the hybrid relay. Here is a link to an AN from ST on the subject.

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