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Relay switch - Live or Neutral to connect?

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I'm trying to control a small table lamp with AC power, using a relay switch board with a Songle srd-05vdc-sl-c. There are 3 screw ports for each of these relays (the board came with two). After a few experiements with an LP (the 3.3V works! i can hear the click sound from the relay) I figured how to control the on/off of the relay box.


If i understand the relay circuit correctly, either neutral or live from the AC part is to be cut and connect to the screw ports, so that the relay switch can complete the circuit and control it.


Now my question is, on the AC part, there are 3 wires - ground, neutral, and live. Should I cut neutral or live for connecting to the relay?


Thanks in advance :)

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In the UK it's brown for live, blue for neutral and green/yellow for earth.


If you're switching fluorescent lighting you may need an appropriate varistor across the relay contacts to stop them fusing. It works a bit like a flyback diode on the relay input, but for AC.

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But NEVER trust that the wire that's supposed to be neutral really is. Especially in a home environment, it's likely that the wires to the plug could be reversed. Same goes for ground. If there is an appliance leaking to ground and the outside connection isn't great it could have potential above ground in the house.


I've got an All-American 5 radio I use every weekend for late-night coast to coast and there are just some plugs in the house you can't trust the metal chassis on because it's connected to neutral directly and it's live on some plugs.



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Thank you everyone for your help and advice. Just took a deep breath and cut the live and connected to the relay switch. Took every caution I am aware of and double checked everything (even keep my left hand in pocket while turning on the switch with right hand ;) ) from the power bar to the LaunchPad and the relay.




It works!



On the right is the power bar, and the left is an extension socket which is connected with the relay. Tried with a lamp later and everything looks good, except one thing: the LP is powered from a powered USB hub, and that hub is powering a pair of loudspeakers for the PC. Occasionally when the relay is on, there is some noice from the loud speaker :eh: Did i made a bad connection and caused instability?

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What is that red board you have the relay mounted on? I am working on the same type of project except doing it wirelessly.... using a Parallax relay board kit. Just wondering if the one you are using is less expensive, because I am going to build a few of them.


Regarding your speakers buzzing, it could be that your USB hub doesn't supply enough power. I read that many USB hubs are cheaply made so the power supply to connected devices isn't very good.

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It is from a local store selling arduino stuff, for around US$4.5 (like the pic below, the 4 metallic standoffs are not included). There is another model with 8 relays for US$10. A similar one is on ebay http://www.ebay.com/itm/New-5V-2-Channe ... 3378bf38ec





Regarding your speakers buzzing, it could be that your USB hub doesn't supply enough power. I read that many USB hubs are cheaply made so the power supply to connected devices isn't very good.


Thanks, good to know it's not something bad happened.. i'll try another hub :)

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  • 7 years later...
16 hours ago, Softwaremaker said:

Sorry for this late jump in here. Can you show me how this is connected from an AC source to the Relay module ? ie . Which wire from the mains are connected to the COM and NO/NC of the relay ?
Thanks !

Presuming you are trying to switch the power to an AC-powered device, such as a lamp or small appliance:


You generally want the COM and NO (normally open) contacts. When the relay coil is not energized, these are OPEN (disconnected) so the load will be off. If you want the load to be ON when the relay coil is not energized, use COM and NC. If you want to switch between two loads, the loads go on NO and NC, and the source goes to COM.

In general, the HOT current carrying wire goes to the relay COM, and the NEUTRAL (grounded) is left unswitched. The EARTH (safety) ground is not involved and should be passed straight through to the EARTH ground lead of the load, if it has one.


The color conventions vary in different parts of the world, and not all commercial cords follow the conventions-- they are usually enforced in fixed wiring such as in a home of commercial building, but cords for plug-connected appliances are often not compliant. Several of the conventions are in messages above-- for example, in the US and Canada, the NEUTRAL conductr is WHITE insulated, and the screw or terminal on many devices like outlets is silver, the HOT conductor is BLACK (by preference. RED, BLUE, or any other distinct color than green may be used) and screw or terminal is often brass/copper color, and the EARTH (safety ground) wire is GREEN.  The BLACK (or equivalent) should be switched.



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