What kind of relay do I need to switch 20A 120VAC using a 12V coil? - Page 1
Forum section: electronics / projects

author: Marty_L
author_rank: Newbie
timestamp: February 22, 2019, 04:46:37 am
content: Basically I want to be able to turn on a space heater remotely. The one I have uses 13 amps but I wanted to be able to switch the maximum that one of them could pull which I figure is 15 amps through a regular wall plug, so I figure I should get a relay that can handle more than that just so its not operating at its max. So I was thinking I would get a relay that can switch about 20 amps 120V (AC from a US home) and I wanted to do it with 5-12V DC but really any low-ish DC voltage would work if 5-12V is not a reasonable ask or if it is cost prohibitive. But I'm having trouble finding a relay that meets those specs, I'm sure I'm just searching using the wrong terms but all I get are relays with 120VAC coils or else all they can switch is 12VDC (automotive relays). I'm hoping that somebody has some experience with this and can tell me what device I'm looking for or what I'm doing wrong when I'm searching. I've tried a BUNCH of different terms on a lot of different sites, some of the terms:12v coil 120vac 20a load relay12v coil 120vac 20a contacts relay12v coil 120vac 20a contact rating relay12v coil 20a switching current relay12v coil 20a relay20amp relaythose are the more complete terms, I've use lots of others and partials of those and the others. Oh and SPDT would be nice but I can kinda work with whatever on that front. Thats all the relevant information I can think of, let me know if I have left something out and thank you in advance for your help.

author: Tom45
author_rank: Frequent Contributor
timestamp: February 22, 2019, 04:59:00 am
content: A search at Digikey for in-stock 12 VDC coil, 15 amp contacts relay gives 37 choices.An example is a Potter and Brumfield relay (Digikey PB329-ND)

author: Marty_L
author_rank: Newbie
timestamp: February 22, 2019, 06:00:27 am
content: So on digikey, if I search out of all products it returned 0 results, if I changed it to only search in Relays it returns 2,700 and some results, not sure I understand how that works but I guess I don't have toDon't I want 20 amp or should I just go with 15 amps? I've read your not supposed to operate things at their max ratings or it greatly reduces its life. Plus since its going to be operating in a separate room possibly while I'm sleeping and its a large load, I figured I should stay well within the limits. I'm gonna put a fuse inline with it but still... But I'm open to new information.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 22, 2019, 07:02:12 am
content: Ratings are difficult; case-by-case. Sometimes you can run continuously at maximum rating no problem whatsoever, with built-in safety margins on specs. Sometimes the manufacturer is outright lying (not limited to Aliexpress sellers; reputable large Western companies do this as well); or there is a strong enough tradition of lying so that people don't even consider it lying but "just the way it is" (like the voltage ratings on tantalum capacitors).For relays? There is no generic rule. But, when switching mains voltage,definitelypick a relay with third party safety approval markings. Check that the stardard they are tested against is relevant; theseshouldbe safe at maximum load, it's the intended use for such relays.For me, Digikey shows 162 products:https://www.digikey.com/products/en/relays/power-relays-over-2-amps/188?k=&pkeyword=&sv=0&pv1411=80&pv1411=184&pv1411=88&pv1411=47&pv1411=48&pv1411=58&pv1411=177&pv1411=66&pv1411=165&pv1411=94&pv1411=52&pv1411=63&pv1411=57&pv1411=65&pv1411=39&pv1411=59&pv1411=60&pv1411=152&pv1411=148&pv1411=147&sf=1&FV=ffe000bc%2C120007c%2C1200002%2C120001b%2Cmu100A%7C1410%2Cmu120A%7C1410%2Cmu150A%7C1410%2Cmu200A%7C1410%2Cmu20A%7C1410%2Cmu21A%7C1410%2Cmu25A%7C1410%2Cmu26A%7C1410%2Cmu30A%7C1410%2Cmu31A%7C1410%2Cmu32A%7C1410%2Cmu33A%7C1410%2Cmu35A%7C1410%2Cmu400A%7C1410%2Cmu40A%7C1410%2Cmu48A%7C1410%2Cmu50A%7C1410%2Cmu60A%7C1410%2Cmu90A%7C1410&quantity=&ColumnSort=0&page=1&stock=1&pageSize=25But do note that relays are sometimes difficult - even dangerous - to shop because multiple different products with different ratings tend to be "multiplexed" in a single datasheet, with confusing and long part numbers, sometimes without explicit tables about what is available and what the actual specs are; hence, the workers at distributors like Digikey often get the specs wrong! Be extra careful with the part numbering so youknowyou are looking at the correct numbers.One lucky thing is that your heater is a nice resistive load, so inrush current, or reactive voltage spikes and sparking are not going to be problems.

author: IanB
author_rank: Super Contributor
timestamp: February 22, 2019, 07:17:20 am
content: Possibly you don't want a relay, but a "contactor"? Try searching for contactors instead of relays and see what turns up.

author: Marty_L
author_rank: Newbie
timestamp: February 22, 2019, 07:23:19 am
content: Siwastaja - that is so much good information, thank you! What do you mean about the voltage ratings on tantalum capacitors? How much are they off by?

author: Marty_L
author_rank: Newbie
timestamp: February 22, 2019, 07:27:52 am
content: IanB - thanks, I'll give that a look. I actually found a few relays that will work but I'll give the datasheets a second look after Siwastaja's information. I did notice the datasheets were confusing and was just going off of the ratings digikey used but I'll double check that now.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 22, 2019, 07:50:31 am
content: Quote from: Marty_L on February 22, 2019, 07:23:19 amBy about 2x. Manufacturers publish separate documents that tell you that you always need to derate the voltage down to 50 to 60% of the rated voltage, and sometimes even succeed to mention this in the datasheets. But you know, if the whole industry has always been lying, it's difficult to start telling the truth as it would be difficult to compare products then, and yours would appear much worse than it is. So, this is something considered to be "practical engineering skill"; engineers are supposed to know this, almost no one even calls this lying. And frankly, you are supposed to learn about the parts you are going to useanyway(there are other relevant limitations as well), so if you miss such widely known, widely documented part of knowledge, it's your fault as a designer. Yet, I find this practice hilariously stupid. Products should be specified truthfully; with a voltage rating accompanied with a lifetime spec with that voltage, and that spec should be at some relevant lifetime, not a few hours.

author: jackthomson41
author_rank: Regular Contributor
timestamp: February 22, 2019, 02:17:23 pm
content: Buy simplerelaywith single switch and 5 Terminals.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 22, 2019, 03:05:25 pm
content: 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

author: Zero999
author_rank: Super Contributor
timestamp: February 22, 2019, 08:12:29 pm
content: Quote from: Conrad Hoffman on February 22, 2019, 03:05:25 pmThere'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.

author: KL27x
author_rank: Super Contributor
timestamp: February 23, 2019, 12:50:56 am
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: February 23, 2019, 01:04:45 am
content: 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.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 23, 2019, 07:18:55 am
content: 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.