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I want to use the following relay to control a 220V AC electrical device.

USB Relay

The device may be an oven or a heating plate which I think should draw a less than 10A current (but the current should still be quite large).

I think I should cut the "live" wire of the electrical device and insert the 2 cut wire parts into the relay, while the "neutral" wire should be left unchanged. And then enclose the relay with a plastic box to protect it from touch by anyone.

As I have only some experience with small current electronics but have no experience in handling large current devices. I want to know that should I use "Heat-shrink tube" when connecting the cut "live" wires into the relay pins ? If yes, which type/model of "Heat-shrink tube" should I use for an enough protection ?

Moreover, is there any online tutorial/example for connecting large current device to a relay in a safe manner ?

Thanks for any suggestion.

EDIT :

After some googling, I find that there are two youtube videos that show the use of this kind of SONGLE relay to switch some 120V AC light blubs. They are located here and here.

Although this kind of relay seems do well on short term high voltage/current AC switching, safety concerns (e.g. overheat, etc) about its long term usage are still under question.

migrated from engineering.stackexchange.com Nov 5 '15 at 14:43

This question came from our site for professionals and students of engineering.

  • Relays typically have 5 terminals. Common (C), Normally Open (NO), Normally Closed (NC), and two Coil terminals. Power will connect to Common. The line to the device, will connect to the Normally Open (NO) terminal. And it looks like the coil is controlled by the USB connection. When the coil is energized, the relay will be pulled closed. At this point you'll have continuity between the C and NO terminal, so your device should turn on. When the coil is not energized, you'll have continuity between C and NC, so the device will be off. – Tester101 Nov 6 '15 at 1:23
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    BTW, in power electricity "low voltage" means 0-1000V, "medium voltage" is 1000-60000V and "high voltage" starts over 60000V. It's not "high" if it can't jump the air to bite you without even touching. : ) – Agent_L Nov 6 '15 at 10:40
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    OK. I have not said "high voltage" in a professional way. I have just used this term in a home user's respective. – user1129812 Nov 7 '15 at 3:07
  • Correction : I have just used this term in a home user's perspective. – user1129812 Nov 7 '15 at 3:21
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the Idea you have is good , but the components although "rated" for 10 amps really will not hold up long at that level. I usually only load these small relays at 50% and as low as 10% if it has a short cycle time. the problem is contacts in the mechanicial ones will soon stick and overheat your hot plate , The electronic ones the solid state relays internally short untill the junction is blown apart but usually dammaging the divice they are driving. as for plastic it melts, maybe a plastic liner in a metal box or standoffs in metal but use caution it will probably work for a while but when it fails thats when things get ugly

  • I've had these relays fail. They just stop working, usually in the open position. They don't blow up. – gbronner Nov 6 '15 at 13:59
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A relay has two sides, primary and secondary, coil and contacts. Each side is rated for the voltages and currents that it can handle.

The stamping on this one looks like it's saying that it can handle 10 amps at 250V on the secondary side. (I am skeptical...) But you'd have to refer to the literature / documentation to determine that for sure.

Likewise the oven / hot plate will be marked for it's current requirements, you can verify this in operation by testing with a meter. I think most will draw more than 10 amps.

The wiring would be stripped and clamped in the screw terminals. You'd want to strip it just long enough that it makes solid contact in the screw terminal, without bare wire sticking out.

The enclosure that houses the relay and wiring should be rated / listed for the purpose, there should be strain relief for the cables entering / exiting, the wiring should be properly sized and rated, the wiring and strain relief should also be properly listed / reated, etc.

Just my opinion, this looks like an accident looking for a place to happen, and 240V can kill you or burn down your house, so I'd ask for some live, in-person, qualified help with this.

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You should not use "Heat-shrink tube". The tube is used for insulating bare wires. What you have is already too much insulation, you need to strip a bit of it. You got the live wire part right. Perhaps what you wanted to mean is a crimp connector, that is a small copper or brass tube you slip over bare end of a stranded wire and then crimp with specialized pilers. This is indeed used to secure a multi-stranded wire in a connector, as the screw alone tend to push the strands aside rather than tightening them safely. This applies only if you're using flexible stranded wire, as solid wire does not present this kind of problem. Some types of screw terminals are designed for stranded wire so you don't need to treat tips of the wire.

I agree with batsplatsterson and Ed Beal - any relay smaller than a pack of cigarettes is unlikely to reliably switch a serious heating equipment. I would personally use this USB-driven relay to drive a big, fat, mains-driven contactor - one able to switch several kWs of an oven reliably.

  • It seems that camp the 2 cut "live" wire parts into 2 crimp connectors and then connect the crimp connectors into the screw terminals of the relay is a more firm and safe way for connecting large current wires through such a relay. In fact, I want a reliable way to connect large current wires into my relay. However, as I am not familiar with crimp connectors, I wonder whether there is crimp connector with such a small head that could be plugged into the small screw terminals of the usb relay. Could you suggest one if there exists. – user1129812 Nov 6 '15 at 0:33
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    Yes, it's a good suggestion to use such a usb relay to turn on another higher power relay if the current in the electrical device is large. – user1129812 Nov 6 '15 at 0:43
  • +1. Using this device to control a larger contactor, is likely the safest approach. – Tester101 Nov 6 '15 at 1:34
  • @user1129812 Sorry, I'm not a native speaker so I don't even know if the "crimp" is proper term for what we're talking about here. They surely do exist, as I noticed them on many store-bought devices (like ceiling lamp). Covering the tips with solder is a poor man's way of making them compatible with screw connectors, as it doesn't require the specialized crimp tool. BUT in your position I would simply look for a contactor with terminals that work with stranded wire. Check image at biersin.com.pl/biersin-en/oferta/… - the one before last. – Agent_L Nov 6 '15 at 9:37
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    OK. I think "the one before last" is a wire ferrule that seems a suitable candidate for my application. It could be bought from here and a youtube tutorial is found here. – user1129812 Nov 7 '15 at 2:56
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I've run these on hot plates and for electric motors.

There are three terminals:

  • live
  • closed
  • open

It is not clear from the picture which is which.

You need to hook up the plug end of hotplate 'hot' wire to the live terminal, and the other cut end to the 'open' end. Try this with a battery first if you don't have a wiring diagram.

These will work, but you'd be better off with a solid state relay, especially if you plan on switching it on and off rapidly. The clicking sounds can be quite annoying as well.

Also, please see this response on how to connect the stranded wire to the terminal block: https://electronics.stackexchange.com/questions/29861/tinning-wires-that-will-be-screwed-in-to-a-chocolate-block-terminal-strip

  • Thanks for a good link. It seems that wire ferrules are what I need for a firm and safe wire connection to a screw terminal. – user1129812 Nov 7 '15 at 2:24
  • The hot plate I use may not exceed a power of 2400W. I am planning to use a relay to switch the hot plate on/off several times a minute, not a high frequency. In your case do your relay get very hot ? Could you elaborate more on your scenario ? Do you just use the usb relay to switch a more high power AC relay ? – user1129812 Nov 7 '15 at 2:45

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