Looking for the correct relay for my project.

Hello everyone!

My project (or part of) is to have my baseboard heaters controlled by an Arduino.

Basically, I'm looking to find the right relay switch. I do not want to buy an incorrect one as that would probably be a disaster.

I pulled off the cover to the existing thermostat to find the rating. This is exactly what it states;

Rating -
120 vac - 22 amps 1/6 hp
208-240 vac - 22 amps 1/6 hp
277 vac - 18 amps 1/6 hp

I only have basic understanding here but I assume all that means is the amps are slightly different depending on how many volts are coming from the source. Though I have no clue what "1/6 hp" means.

Anyways. I hope you guys can help, even though this is a wee bit out of the scope of the Arduino itself.

What kind or relay switch should I be looking for? Or is there a better solution to what I'm aiming for.

Thanks guys!

To be perfectly honest, if you can't tell what size relay that you want, going by those specs, then you shouldn't be playing around with mains wiring. It's far too dangerous. And if your laws are anything like our's, it's also illegal.

I'm sure that someone will help you regardless.

power is in watts.

watts are volts time amps.

so, with 120 volts and 10 amps, you get 1,200 watts.
with 220 volts and 5 amps, you get ... well, 1,200 watts

if you think of voltage as speed and amps as the size, then lots of fast ones will fill the bucket at the same rate as large slow ones.

now the 'thermostat' is really just a switch or a relay. however you want to call it.
the part that makes it a thermostat is that it has a bi-metal spring. one metal heat at one rate, and will bend the bi-metal part. this bending will push on the parts that close or open the switch part.

a relay is the same thing, only the activator is an electromagnetic coil.
a wall switch is the same thing, only you are the activator.

in each case the parts that allow power to pass are isolated from the part that does the actual actuation.

about those parts that pass the power. well, first off, you have the contacts. these are special bits of metal that are designed to undergo massive electrical sparking. every open and every close has sparking that occurs. with copper, it would heat the copper and then the copper would liquefy and cool, making a perfect welded joint.

those contacts are connected to larger bits of copper to give you a place to connect your wires.

more about those contact.....
if you turn an AC line off and on, like a light, a hair dryer or such, the load causes some electrical effec.
the light is a simple resistance heater. you close the circuit, the power passed, the heating element gets hot and glows.
your hair dryer has a motor. a motor has coils, the coils have to create a magnetic field. what happens is the coils could be thought of as a vacuum. the suck the ever living daylights out of the wire and cause a HUGE surge in power. so much that lights on the same circuit will dim. this is called in-rush current and if easily 5 times the current of the running motor. but, this is expected and is called a transient load. we only need to know it exists.

and lastly there is a DC load, car headlights or such. DC acts differently than AC on the contacts. it welds them more efficiently

why did I tell you this ? because ALL .relays are rated for a resistive load, and an inductive (coil) load and a DC load.
in your case, the H.P. stands for motor horsepower. this is the inductive load.

but, all that is moot and have no bearing on your ACTUAL question.

your un-asked question should have been :
" I have a baseboard heater in which the nameplate of that exact heater says, 1,200 watts, 120vac, 240vac"
"how to I select a relay"

more information that should be listed is the part number and a VERY simple wiring diagram.
how many of these do you have connected ?

we would then ignore everything I told you about MECHANICAL relays and...
....tell you to get a 30 amp SSR, or sold state relay. make sure it has a signal line that can use 5 volts. most have a rating something like 3-32VDC and can take pretty much any form of DC signal to activate..

the activation is an opto-isolator. it uses a simple LED to light and a light detector to know the light is on. this is the isolation as you cannot pass power through light.

the other side, the isolated power side, will be rated in EITHER AC or DC, NEVER BOTH. you MUST make sure you get the correct type of voltage. SSR's are not interchangeable.with voltage.AC or DC.

here is an example from e-bay :

you can see that the power side is between 24 and 380 VAC, so would work.
the current rating is 40, or 40 amps
at about $6 it is a pretty safe bet and it will work with an Arduino

the trigger current is 7.5ma
http://www.fotek.com.hk/solid/SSR-1.htm

this is another page of data sheet.
this has application hints.

tells you to use your 5 volts, have a NPN (PN2222 or any switching transistor)
use a 1k relay on your output pin to the base of the transistor.
connect the 5v to the relay after a 560 ohm resistor (could go as low as 330 if that is what you have)
only need a 1/8 watt resistor, but 1/4 watt or larger would be fine.

note that it will turn ON when you apply power to the arduino. the Arduino will need to pull the pin high to close and on start, the pins will not be pulled high immediately.

dave-in-nj:
http://akizukidenshi.com/download/ds/fotek/fotek_ssr.pdf
this is another page of data sheet.
this has application hints.

That's not the datasheet for a real Fotek SSR. It's the datasheet for a very dangerous fake Fotek SSR.
When buying Fotek solid state relays, a great deal of care must be taken to ensure that you're getting a 'real' one. The fakes cannot handle the rated current and have been known to burst into flames.
On eBay a while ago, I actually found more fake Fotek SSRs than real ones. Specifically, the Fotek-25DA and Fotek-40DA

Here are a couple of links that 'CodingBadly' posted in another thread a couple of months ago:-
https://www.google.com/search?q=fake%20fotek%20ssr
http://canada.ul.com/safetyalerts/ul-warns-of-solid-state-relay-with-counterfeit-ul-recognition-mark-release-13pn-52/
And from that same thread:-

knut_ny:
..dont leave that (Fotek) SSR unattended. I used one for a 220V, 1500W heater - it burned after a month.

And it appears that some of the fakes are even harder to distinguish than the ones in the links. The fakes used to be labelled "Solid State Relay" whereas the real ones said "Solid State Module", but now the fakes have been corrected to say "Solid State Module" too.
Here's a photo. The one on the left is a fake:-

great, now the fake Italian made Arduino's are to be avoided,
as well as fake relays.

I wish politicians and those fakes in the media would explode into flames and not our electronical bits.

dave-in-nj:
great, now the fake Italian made Arduino's are to be avoided,
as well as fake relays.

Yeah, it's a pain isn't it? The latest fakes (SSRs) are damned hard to tell from the real ones, too, unless they're side-by-side.
I made a point of reporting one seller on eBay, but there are so many there's really no point. And new ones pop up every day.
At least the counterfeit Arduinos aren't likely to be life-threatening like the relays.

I wish politicians and those fakes in the media would explode into flames and not our electronical bits.

Me too.
I guess the best way to avoid those fakes is to only buy from large, reputable sellers. Digikey, Mouser, RS components etc.
Even Sparkfun have the datasheet for a fake Fotek SSR on their website, so I'd personally be wary of buying one from them too. But maybe they have the fake datasheet, but the real SSRs.

That's the exact relay I had in mind. Now It looks like I'd to just find a 'real' one.
I'm going to do a bit more research before I go through with that, but some very useful information here.

Thanks for the help guys.

SGAShepp:
That's the exact relay I had in mind. Now It looks like I'd to just find a 'real' one.
I'm going to do a bit more research before I go through with that, but some very useful information here.
Thanks for the help guys.

I still reckon you shouldn't really be playing with mains wiring, but I'm glad I posted that info now. I'd hate to hear in a week or two that your house had burned down. :frowning:

Just be very careful where you buy your relay. It's worth paying a little more and buying from Digikey or similar, for peace of mind. Don't buy cheap SSRs from eBay, even other brands besides Fotek. You never know, they might be fakes too. There's too much at stake. A few extra $$ is nothing in this situation.

Might be cheaper in the long run, and a whole lot safer, to replace the baseboard heater with one that has a built-in relay and a low voltage wall thermostat. Many wall thermostats that directly control the power to a baseboard heater have faulted and caused house fires.

Paul