Go Down

Topic: Relay wiring help/confirmation? (Read 4679 times) previous topic - next topic


Hey everybody,

OK so I recently bought this relay of Ebay (http://www.ebay.co.uk/itm/160881308759?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649) which I was informed will do the job I require it to do (I require it to turn on and off a heat cable which runs on 240v at 15 watts).

Now I noticed it has 6 terminals and I am assuming that the three smaller ones are for connecting to the Arduino like so, - to Arduino negative, + to Arduino positive(not sure if it is 3.3v or 5v?) and then s to the digital pin on the Arduino. Is this correct?

The the three screw terminals I am assuming connect to the heat cable and power source but not sure how? Help would be appreciated?

Thanks in advanced! :)


The small terminals connect as you stated; the relay should be powered with 5V.

The middle screw terminal is your "hot" 240V line. For the remaining screw terminals, one is normally open when the relay is off and one is normally closed. The white printing near those terminals shows that (going by the picture) the terminal closest to the camera is normally closed ("on") and the farthest one is normally open ("off").


Nov 23, 2012, 10:06 am Last Edit: Nov 23, 2012, 10:09 am by guix Reason: 1
So you must plug only 2 wires (in fact only one, but one part is input, other one is output, like a simple interrupter) in the screw connector, like this:



Thanks for the replies and sorry about my stupidly delayed reply but been super busy lately.

I'm afraid that diagram is a little confusing as mine only has 3 screws not six, and I'm not sure what the white bits are (power or the powered devices?).

As for Chagrin's reply it makes more sense to me, but just wanted to check that you would have the live in one screw and live out the other and its just a case of having a switch in the wire as such? Also does this mean that if the terminal is open when off, it's closed when on and vice versa?

Thanks again!


a) I'd be very, very doubtful of proceeding with this... safety grounds. See...


... however, as you are likely to proceed anyway (<^_^>)... be very, very "pessimistic" about what "could happen". Enclose the relay board PROPERLY. Think hard about how you can reduce the chances of any interconnection between wires to Arduino and mains power lines. And make the whole thing physically robust.

b) Regarding interpretation of the diagram....

The three circlurlar things in squares with three wires out of them are meant to be "cross sections" of the cables for powering your heater. (Three: Live, neutral and earth (also sometimes called ground))

The top square is for the wire coming from the plug from which the electricity is coming.

The relay board is similar to the one you have... but has TWO relays... it is like two of your boards, one above the other. The diagram shows controlling two heaters. For your purposes, just "erase" the lower relay (blue rectangle), and the lower (on right) square-circle-3-wires.

Which leaves the "upper" (of the pair at the lower right) square-circle-3-wires. This is the cable TO your heater.


Yes tkbyd is right :)

Sorry if that wasn't clear, the square-round things are standard wall-sockets (of France)...the one at the top is the one in the wall, the 2 other on the right are to connect devices that will be controlled by the relays :)



Thanks again for the replies and I greatly respect everything you say about mains electricity, however it is for my University major project and I also have a great deal of common sense, plus always do full research on my devices before plugging it in.

On that note, does anybody believe I should have a diode in this type of set-up, if so what type?

Thanks for explaining the diagram as it now makes a great deal more sense.



Diodes ARE needed with relays... but on the other side. There should already be one in the little PCB you have.

Basics of relay diodes...



OK perfect, thanks very much again! Is there a way of finding out if the relay I have bought has one built in? Or should I just do it anyway lol?


OK perfect, thanks very much again! Is there a way of finding out if the relay I have bought has one built in? Or should I just do it anyway lol?

Well the picture you linked to in the first posting does show a diode on the board (two terminal device with M7 printed on it). I suspect that all those asian Ebay relay boards do include a protection diode across the relay coil winding. My two relay board certainly does.



Thanks retrolefty,

I see what you have pointed out on the picture which is great. However my apologies as I have failed to mention that because that seller sucked I have bought another from a UK seller (http://www.ebay.co.uk/itm/221005545219?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2648) which looking at it, also shows diodes built in which is also good!


Go Up