This will be my first attempt to control a 110v device (high voltage, by my standards!). Initial tests will be on a table lamp; eventual target may be a small 500W heater.

  • How should I wire the 110V device to the screw terminals?
  • Other than putting the unit in a plastic project box, what safety precautions should I take?
  • If the project box has a metal lid, should I attach the 110V ground wire to the lid?
  • what are the abbreviations NC, COM, NO?

This is the device I will be using:

enter image description here

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    Don't forget to check the datasheet of the relays to see where the line and neutral should be connected (and if that matters for this relay). More on that here – AndrejaKo Jan 5 '12 at 0:45
  • Thanks, good note! I checked for this particular relay and there's no mention. Based strictly on the aesthetics I'll connect neutral together directly and run the hot line through the relay. :-) – Mark Harrison Jan 5 '12 at 2:13
up vote 10 down vote accepted

As others have said, this should not be sitting on your table or on the carpet. Exposed 110V metal carrying 110V is dangerous, and the pins on the back side of the PCB, the exposed screws, and any stripped wires could also cause shorts or electrocution. On the note of

A decent way to mains power to your device would be to add a panel-mount power connector like the IEC 60320-C14 receptacles commonly found on PCs, monitors, and TVs for power input:

enter image description here

You (and your potential customers) probably have a few cables for these receptacles lying around.

You can get these with integrated fuses and on/of switches; check the Connectors, Interconnects / Power Entry - Inlets, Outlets, Modules section of Digikey and filter for IEC 320-C14, receptacle - male pins, panel mounted, and the features you want.

The best output would probably be a standard electrical outlet. This would allow you to connect any appliances directly to your device. You can also get panel-mount receptacles for these plugs, for the US they're NEMA 5-15 receptacles:

enter image description here.

For both the input and output, the wires can be soldered or screw-mounted to the receptacle, and connect (entirely within the enclosure) to the relay module. The project box can then completely protect the user.

And yes, if the box is metal, you must ground it in case one of the hot wires becomes disconnected and touches the box. With this ground wire, the circuit breaker or fuse will protect anyone (from electrocution) or anything (from fire) which comes in contact with the box. Without this wire, the size of the box amplifies the danger from a few small screws to an entire enclosure. Ground it, or use an insulating (plastic) enclosure.

As Brian already, said, the abbreviations stand for:

  • COM: the common terminal. Connect your power input to this terminal.
  • NC: Normally Closed, the spring inside the relay pulls the contact to this terminal in the absence of a magnetic field. Unless you have something that should be powered if your unit is in a failure mode, or need to alternate between two devices with each relay, connect this to nothing.
  • NO: Normally Open, connect this to your switched device. It will connect to COM when the relay is activated.
  • In terms of safety, should you connect the hot or neutral wire to the COM terminal, or does it matter? – MBraedley Jan 10 '12 at 20:40
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    Hmmm...I don't have my books available to check, but I think you should connect the hot wire to the relay. In theory, this will allow the entire system (which would otherwise be at mains voltage) to return to ground. This is how an ordinary lightswitch is connected (to the 'hot' wire), it seems sensible to do the same with a relay. – Kevin Vermeer Jan 10 '12 at 21:03

Screw terminals like you show are a very bad idea for dangerous voltages unless the whole assembly is normally covered with something not easily removed, like a electrical box. The screws at the top of the screw terminals will be connected to their line. It would be too easy to accidentally brush up against them.

Your ultimate 500W load will draw nearly 5A, so make sure all paths of that circuit can handle that. I see the relays are rated for it, but what about the PCB traces between the relays and the screw terminals? If you are buying this board as a unit, check its ratings. Don't assume the whole board is rated for 10A just because the relays are.

  • I thought of using two relays in parallel to handle high currents. However, if one of the relays fails to switch the remaining relay will get overloaded (without any indication beside the smoke). – 0x6d64 Jan 5 '12 at 9:40
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    @ox6d64: You can't count on relays sharing current load equally. Generally, paralleling relays is a bad idea. Get a bigger relay rated for the total current instead. – Olin Lathrop Jan 5 '12 at 14:07
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    @ox6d64: Not only will relays possibly not share loads equally, but they won't necessarily switch at the same time, meaning the full load will be exclusively on one relay for a short time--which is long enough to damage it. – JYelton Aug 12 '14 at 18:25

Let's not forget fuses or circuit-breakers as handy safety devices... Something around the 7A range might be appropriate.

Also, if you're concerned about the 110v getting to the microcontroller that I assume you're using to control these, you might consider using an opto-isolator between the board and the controller.

Olin's concerns are more than well-founded, the screws are recessed slightly into the blocks, so the danger is somewhat reduced. 110v can give you a nasty surprise when you're not paying attention.

Brian's response is correct for the Normally-open, Normally-closed and common abbreviations.

I have an assumption about the chip that's under your thumb in the photo - that's a relay control device? If it's not, there are other parts that you'll need to handle voltage spikes when you engage and release the relays electronically.

Here in the UK we have 230v mains (that's RMS, so it's around 325v peak that everything has to withstand), so we have to be even more careful. My answer here is a bit more general than the question asks), but it applies in this case too. I always do a first test on any mains equipment by connecting it to the outlet in series with an ordinary (mains voltage) 40W filament lamp. If the circuit is good, then the lamp will not light, but it will safely show up any problem like a short circuit, when the lamp takes the load and glows brightly without anything getting damaged.

I have a specially made up board with a mains socket and a bulb in series with a switch, a neon that shows when the mains is connected, and another that glows when the output is switched on. It is especially useful for a first switch on for a "smoke test". If there are electrolytic capacitors then the bulb glows brightly for a moment as they charge up, and it helps to polarise them gently if they are old or have not been used for a time. It also helps when connecting up a transformer, as again the current is limited if there is a short or wrong connection.

This is not the same as your intention to test your circuit with a table lamp, which I presume will be the load, my lamp is in the input lead, and it is a safe general procedure to apply to all mains equipment.

I treat the mains with enormous respect, and make it a rule when constructing to ensure that no mains voltage connectors can be accidentally touched when the outer cover is removed.

COM is the common terminal. It is connected to NC (Normally Closed) when the relay isn't activated and NO (Normally Open) when activated.

To hook up your load, just connect one wire of the load straight to one of your mains wires. Connect the other load wire to either COM or one of the NO or NC, and connect your second mains wire to COM if you connected the load wire to NO or NC, or vice versa.

For electrical safety, I agree with Olin, make sure you cover the assembly up before you plug in the mains. Also make sure you don't have any stray copper strands poking out from the wires when you stick them in the screw terminals...

You can attach the ground wire to a metal lid using a lug and a screw.

  • 1
    I strongly recommend to use wire end ferrules/sleeves to protect stranded wires. This also solves the problem with single strands that stick out. – 0x6d64 Jan 5 '12 at 9:33
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    Another recommendation for the ferrules. Don't try to tin the tips of your wires with solder and then use the wires in a screw terminal. Ordinary older will crack under too much stress, and deform under lower stress, so you'll never get a permanently tight connection with a tinned wire in a screw terminal. – Kevin Vermeer Jan 5 '12 at 16:22

Switching large AC loads from small DC control using relays is not really the best idea. I dislike relays: they click audibly, waste power and wear out. Large relays can't be driven directly from computer gpio 30 mA pins and all relays require diodes as Paul mentioned.

In this case I would both opto-isolate and use a triac instead of a relay. Start solid-state and stay solid-state.

  • Start solid-state stay solid-state. Awesome statement. Welcome to electronics SE :) – qdot Sep 13 '12 at 0:14

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