I want to use a relay for emulating a switch with an Arduino Uno.
There are probably better ways to do this, but for now for a beginner like me this will be fine if it will work.
But the device the switch is on is 12V DC.
Does that matter for relays if the controlled power is AC or DC?
I have a typical 1 channel relay shield right now: arduino relay shield 1 for sale | eBay
ghosttrain:
But the device the switch is on is 12V DC.
Does that matter for relays if the controlled power is AC or DC?
The limitations are printed on the relay.
10A 30volt DC.
Leo..
That info is for the controlled current, huh?
I have no idea why you would control a relay with a 10A 30V 'signal', but then again I'm very new to all this that's why I'm asking.
Also, to find out what the limit is for 12V, I just use Ohms law?
Hi,
The 10A 30V is the rating of the contacts,
Not the current to operate the coil.
Tom.... 
Oh, so it can do DC too then,great
When it says "10A 30V", does that mean I can use it with a 12V powered device and the current limit will be 10*30/12 Amps?
The relay coil uses 5volt/~80mA from Arduino's 5volt rail when "on".
The relay contacts are just a switch with a 10A max current limit.
If you switch 12volt devices, keep it under ~120watt. (12V x 10A = 120W).
Leo..
OK, I just don't understand one thing:
Why 12x10? It says on it 10 Amps for 30V right?
Not that I'll need anything close to 1 Amps even for this switch, jut curious.
Oh, and there is no polarity for the contacts, right?
Oh, and there is no polarity for the contacts, right?
Nope. The contacts don't care which direction current is flowing through them. - Scotty
ghosttrain:
That info is for the controlled current, huh?
I have no idea why you would control a relay with a 10A 30V 'signal', but then again I'm very new to all this that's why I'm asking.Also, to find out what the limit is for 12V, I just use Ohms law?
No, absolutely not, the limits are 10A for DC, 30V for DC, you can't trade current for voltage or vice versa!
ghosttrain:
Why 12x10? It says on it 10 Amps for 30V right?
The ratings are Maxima.
For DC you may use it for anything up to 30 V, and anything up to 10 A.
What was being explained to you was that if you then use it for 12 V, that is well under the 30 V rating, so you can use it up to the 10 A rating, and if you then used it for the full 10 A at 12 V, then that could be a load of 120 W.
MarkT:
No, absolutely not, the limits are 10A for DC, 30V for DC, you can't trade current for voltage or vice versa!
It would be ridiculous to suggest you could use it to switch 0.1 A at 3000 V! 
So as I understand the Max Voltage and Amps rating should be taken separately. Okay.
When there are two pair of contacts like on this, http://uk.farnell.com/multicomp/hrah-s-dc12v-c/relay-pcb-spco-12vdc/dp/9480170
What does that mean?
Both can be up to 30V and 10A?
Both can be up to 30V and 10A?
Yes, that would apply to each sets of contacts.
BTW - What everybody is saying is true with regular mechanical switches too. i.e. You can use a regular light switch rated for 120/240VAC at 15Amps to switch on & off your Arduino or a little LED, etc.
The coil voltage (and current) is a strict rating... A 5V relay will probably burn-up if you run 12V through it. If the coil is designed for AC, it will usually work with DC, but a coil designed for DC may not work properly with AC. (The specs should tell you.)
With electronics (Arduino, etc.) we are almost always activating the relay with DC. In some "industrial" or "electrical" applications we may have AC through the coil. I've got 12VAC coil relay on a fan relay in my home furnace, and I used a 120VAC coil relay to turn-on the lights in my girlfriend's garage when the garage-door opener is activated.
Solid state relays are a bit different. The input-side usually has a wider voltage range (maybe between 5 and 12V) and the control/output side is more strict/limited. Occasionally, there is no current limiting on the input-LED, so you can choose an appropriate resistor and control it with any voltage of your choice. Almost all solid state relays are DC controlled.
Solid state relays my switch DC only or AC only. Most AC solid state relays use TRAICS and they will lock-on with DC and won't shut off. Many 120V or 240V AC relays won't switch low voltages.
Interesting.
What about a tiny mechanical 12V resistor like this one?
Do you think Arduino with 5V can control it? http://uk.farnell.com/multicomp/hrah-s-dc12v-c/relay-pcb-spco-12vdc/dp/9480170
"What about a tiny mechanical 12V resistor like this one?"
I suspect you meant 12v relay.
NO.
Maybe a 5v relay board such as:
http://www.ebay.com/itm/5V-1-Channel-Relay-Module-Expansion-Board-Shield-For-Arduino-ARM-PIC-AVR-DSP-MCU-/331594196176?hash=item4d348de4d0
Okay. Why 3 pins?
Hi,
Three pins should be marked somewhere as C or COM, NC and NO.
The switch when not activated, connects COM ,common to NC, normally closed.
When activated it connects COM to NO, normally open, and disconnects COM from NC.
The switch changes the COM terminal from NC to NO when activated.
Tom.....
DVDdoug:
If the coil is designed for AC, it will usually work with DC,
Or maybe not.
AC solenoids - as used in contactors - operate somewhat like transformers. Until the armature pulls in, they draw a heavy current though it is still limited by inductance. If fed with DC, it is like feeding DC into a transformer winding; the current is not limited by the inductance and is likely to burn out the winding.
TomGeorge:
Hi,
Three pins should be marked somewhere as C or COM, NC and NO.The switch when not activated, connects COM ,common to NC, normally closed.
When activated it connects COM to NO, normally open, and disconnects COM from NC.The switch changes the COM terminal from NC to NO when activated.
Tom.....
Okay. Thank you.
I hear people saying that relays cant be controlled by Arduino's pins as they cant provide enough current, then I see people do exactly that in videos. Mixed messages, what's really the case?
There is a link in your original post to a relay module.
A board with relay, diode, transistor, resistors, etc.
In this case the Arduino pin is not driving the relay directly.
The Arduino pin just has to provide a small base current for the transistor.
The transistor drives the relay coil.
Leo..