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What kind of relay do I need to switch 20A 120VAC using a 12V coil?
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Zero999:

--- Quote from: Conrad Hoffman on February 22, 2019, 03:05:25 pm ---You might also consider a solid state relay. Maybe a bit costlier, but the contacts never go bad. They come in many different styles. Here's an example, but you can probably find a cheaper one. https://www.digikey.com/product-detail/en/sensata-crydom/EL240A30R-12/CC2459-ND/6612867

--- End quote ---
There's nothing wrong with solid state relays. The main advantage is they can be turned on an off an infinite amount of times, without failing, but they do have disadvantages, such as dropping more voltage than a mechanical relay, thus dissipating more power and operating one at 20A would require a reasonable size heatsink.
KL27x:
Ratings on solid state relays are even more confusing. The current rating on an SSR is what it will handle when the relay is artificially held at 25C. This has very little bearing on the temps it will see in your actual circuit, and the actual temp of the device if you were to try to pass the rated current is almost universally going to eventually reach much higher. This is one reason why people say to always "derate" an SSR by 30% or 50% or whatever. It's because they don't get into the details of the datasheet and just read the current rating... then they decide the part is "inferior" or "falsely advertised." Actually, around here, the most common conclusion is that the relay is "cheap chinese crap." But this is in fact how all transistors are rated.

For something that isn't going to turn on/off but a few times a day, I think the mechanical relay is better, anyway.

As others have mentioned, at really high current levels, you start getting into contactor territory. But for what you are asking, a mechanical relay is probably the best thing for you, anyway, so you don't have to learn about these other things, yet.
james_s:
If you do get a SSR, watch out for all the counterfeit ones from Chinese sellers, most do not even come close to claimed specs.

SSRs are great for applications that cycle on and off frequently, stuff like PID temperature controllers. As already stated they have disadvantages, they produce more heat so they are less efficient than a mechanical relay and when they fail they usually fail shorted which is not what you want for a heating appliance.

There are lots of suitable relays that will work. If you can find a junk microwave oven most of those will contain a suitable relay that you can salvage. The main power relay usually has spade terminals on the top for the load and PCB pins for the coil.
Siwastaja:
The typical failure mode for a mechanical relay is the same: shorted. So for both SSRs and mechanical relays, it comes to using quality parts and understanding each and every specification on the datasheet, and never exceeding any of them - then neither will fail.

If having the load stuck on is catastrophic, a relay type called "contactor" gives a lot of extra safety margin, by having two separate contacts in series, typically much larger physical gaps between the open contacts, and more spring force to separate the contacts.

A resistive 15A heater on a 120VAC supply is one of the easiest cases you can think of. 120VAC arc dies very easily, causing negligible sparking; a relay rated to switch 250VAC, 20A is already well overrated and won't fail, if proper quality, and driven properly. You still need to know how to drive it. One way to significantly increase the contact stress by accident is adding a freewheeling diode to the coil "because it's needed" when the coil is driven by a transistor. Don't do that. Read on the subject and use a TVS or resistor clamp instead. Also, make sure your software controlling the relay doesn't accidentally switch it more frequently than necessary.
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