I don't understand relays

I bought a Seed Studio relay shield at Radio Shack for 20.00.

The relay says 3A 120VAC 3A 24VDC. The board is powered by 9-12 volts DC.

First of all can you control lower voltage than specified? Say 12VAC or 12VDC?

Also, and more importantly, where do I get the power from? The board is only getting 12VDC so if I want to power something that takes 12VAC how would I do that?

TIA

You're not supposed to do line voltage with that board at all.
They didn't design it for that sort of high voltage (it's totally not UL.)
It has no agency approvals for anything of the kind.
[Especially since the traces for the terminals and relay traces are so close to the Arduino USB connector shell.]

Yes, you can use voltages lower than the rated voltages on the relay labels.

You bring the voltage that you want to switch into one of the relay terminals (COM) and look for it at that relay's NC (normally closed) or NO (normally open) contact.

I always seem to get a bit of a clue after posting on here.

Is it correct that the AC or DC voltage comes from elsewhere and that the relay only controls the ground?

I'm just trying to figure out if I can interface the shield with 12VAC landscape lighting.

(Just read the reply and didn't totally get it.) Especially the very first line if your reply.

With regard to the "first line" of my previous reply: Do NOT use this for 120 vac !

http://forum.arduino.cc/index.php?topic=165923.msg1239161#msg1239161

You can use it for 12 V AC.
But as stated, be real sure it doesn't touch any parts ofthe Arduino.
You could consider to extend the headers of the Arduino to be sure.
I've seen some shields that get too close to the USB connector shell to my taste, but haven't acutally seen these shields.
Runaway Pancake has as you can see in his link.

Interesting, because on their website they say you can.

So let's say I'm working with AC voltage, how would I wire that up, because the lines are reversible.

I’m just trying to figure out if I can interface the shield with 12VAC landscape lighting.

How much current is flowing in your 12VAC lighting circuit?

I think the power supply is more than it's capable of.

If it does 3A @ 24VAC, how would I know what amperage it could handle at 12VAC?

If I had to guess I'd say in the neighborhood of 6A, but I have a feeling that's wrong.

The output is 12VAC 3.75 amps.

Cut 1 line and connect it through the relay.
PSU side to COM and lamp side to NO.
3A is your maximal current.
The difference between the AC and DC voltages is due to AC is alternating and will go through 0 volts 100 - 120 times per second.
A switch off spark will be stopped at that moment, and this will not occur when using DC current.
So a DC voltage will generate sparks for a longer time, damaging the contacts more.

I'm not sure what you just said.

However, if I can't do more than 3AMPS @ 12VAC, can I knock it down somehow? I'd imagine the lights will still work, they just wouldn't be as bright.

Some reading on relays:

The output is 12VAC 3.75 amps

If this is the power supply rating, you probably are running less than 3.75 amps in the actual lighting circuit.
Best measure the actual current with a meter.

So I could just knock out a couple of lights from the set which would knock down the draw if necessary.

I do have a multimeter to check with.

Also, I was planning on separating the lights into four sets of two, so the load to each relay would be less right, less than an amp draw on each.

But how would I wire it?

I know how to do DC from the link Runaway Pancake posted, but AC?

PM5K:
But how would I wire it?
I know how to do DC from the link Runaway Pancake posted, but AC?

It’s the same concept, the relay contacts (being a switch, an SPDT) completes / interrupts the circuit.

DWG attached.

nodiff.JPG

Basically, a relay is simply a "switch"

The switch gets opened or closed depending on if there's a built up magnetic field (an electromagnetic) you power the electromagnetic
with your arduino (or transistor in this case), once the magnetic field occurs, it will repel off a perm magnetic which pushes the metal
contacts together...

When the contacts form, electricity will flow...

I kind of understood the relay, I just wasn't understanding where the power came from, and once I did I wasn't sure if the relay connected the power or ground.

I think I get it all now, I'll let you guys know how it goes.

There is a diagram and a video on the above page that may help explain how some relays work. It’s a DC example but you wire the AC circuit the same way.

Here is another http://arduino-info.wikispaces.com/ArduinoPower#4-8

PM5K:
I think the power supply is more than it's capable of.

If it does 3A @ 24VAC, how would I know what amperage it could handle at 12VAC?

If I had to guess I'd say in the neighborhood of 6A, but I have a feeling that's wrong.

3A is the maximum current it can handle. Period. There is a certain amount of contact resistance, so there is a certain amount of heat generated. That rating has little to nothing to do with the maximum voltage the relay can switch.

When it says 120Vac and 24Vdc max, that is the maximum voltage that the contacts can interrupt at 3A. When you open switch or relay contacts, an arc forms. How long it can get depends on the amount of current flowing through it, and the voltage driving it. More current means a hotter arc and therefore lower resistance, so it takes less voltage to sustain it. A higher voltage means the arc can get longer before the resistance rises high enough that the current through it can no longer keep it hot enough.

As someone else pointed out, AC is interrupted 100 or 120 times a second (depending on if your power is 50Hz or 60Hz), so a higher voltage can be interrupted by the same switch. DC, however, makes a continuous arc, so a lower voltage can sustain an arc at the same current.