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I'd like to find out which to use: a SSR (Solid State Relay) using TRIAC and optocoupler or a relay.

My consideration are from the point of power consumed, my challenge here is to choose the less power consumer one.

The biggest problem I found with a relay is through holding its state; the power is consumed first to activate the relay, then to hold the relay activated. If that lasts for long time, it will consume so much power.

My application is using a microcontroller that controls the flow of AC electricity that requires latching the state, what ever it is, for a long time.

So, from the point of power consumption, is the SSR (using a TRIAC and a MOC) or the relay more power hungry?

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A common relay may need 350 to 400 mW, and the current will depend on the nominal voltage. You'll be able to decrease the voltage after the relay is activated, however, and often go below 100 mW. But not all relay datasheets mention the minimum hold voltage.

An SSR has an IR LED which drops less than 1.5 V, often as low as 1.1 V, and may have enough at 10 mA, then that's less than 15 mW, far less than the electromechanical relay.

But while the SSR is more economic at the driving side, it will lose much more energy on the load side: a voltage drop of 1 V (optimistic) at 1 A is 1 W, while the relay with a resistance of200 mΩ will dissipate only 200 mW at 1 A.

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    \$\begingroup\$ What about latching relays? \$\endgroup\$ – helloworld922 Sep 27 '12 at 21:53
  • \$\begingroup\$ @hello - they're certainly a viable option, and also discussed here. The main issue with these is status feedback: you may need one of the contacts the let the controller know if the relay is on or off. \$\endgroup\$ – stevenvh Sep 28 '12 at 6:41

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