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Using an arduino relay brick to switch 120 ac volts / 5 amps safely

Discussion in 'Microcontrollers, Programming and IoT' started by mcm, Mar 1, 2013.

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  1. mcm

    mcm

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    Mar 1, 2013
    I'm building a circuit that switches ~5 amps at120 ac volts (from a wall) to power / unpower a heating coil. I'm currently using the following relay to do so:

    http://www.seeedstudio.com/depot/electronic-brick-5v-relay-module-digital-p-479.html

    The powered leads are connected to the green screw down terminals from that photo. Although the relay itself is rated for 120 v / 5 A, I am worried that using a screw down terminal to connect to a wire is unsafe. After several hundred cycles in a very humid environment, the wires blacken near their screw-in point, and in at least one case have failed poorly as in the attached photo:

    Any suggestions on how to do this safely are appreciated!
     

    Attached Files:

  2. GreenGiant

    GreenGiant

    842
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    Feb 9, 2012
    screw down terminals are the standard, but if you are concerned with that you can get a ferrule for the end of the wire to help increase contact, and if you arent planning on removing the connection you can coat is in an electric grease, epoxy, or sealant.
    Grease will not be super clean but itll allow you to remove the connections when you need to while it insulated, epoxy will hold it all together while it seals and make it very difficult to change, and sealant will allow you to change it though not super easily, but it will seal nicely (RTV Silicon or something similar)
     
  3. duke37

    duke37

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    Jan 9, 2011
    It looks to me that the connections have got hot. This will happen if the wires are not screwed down tight.

    Can you take the circuit out of the humid environment?
     
  4. GreenGiant

    GreenGiant

    842
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    Feb 9, 2012
    I didnt even notice the picture there... its too early on a friday for minute details.

    Those wires do not look big enough for 120V 5A you need 14awg at least, and if there is risk of spikes you should probably go for 12awg
    That black looks like it got too hot from passing too much current through the thin wire
     
  5. mcm

    mcm

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    Mar 1, 2013
    GG,

    I'm pretty sure those are 14 gauge but I'll double check tonight. Is the point of epoxy / grease to reduce the resistance at that connection? I've measured at < 1 ohm but I don't know how small is small enough.

    duke,

    Probably not, due to my application. I'll see what I can do to minimize this.

    Another thing I forgot to mention - if you look close you'll see there are 2 red wires going into the same screw down terminal (on the right). Could this be causing an issue? Should I slather that connection in conductive grease?

    Thanks for both of your suggestions!
     
  6. Electrobrains

    Electrobrains

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    Jan 2, 2012
    That photo does not look healthy!
    You definitely have to do something about it.

    As duke and GreenGiant already mentioned (sorry, I started to wright this earlier today and had to make a pause, thus repeating some things), the problem is obviously over temperature at the terminals.
    Assuming the module is ok and correctly dimensioned, I think you may have 2 main reasons for that problem:

    1) Your own wires might be too thin for the high current. Together with the (normal) temperature rise at the terminals, the temperature will be too high.

    2) Any kind of moisture is murderous to electronic circuits. High power and high voltage will rapidly cause electrolysis and higher contact resistance, that will heat up that spot and may cause fire. You can also get the same problem inside the relay, if it's not sealed.
    I looked up the data sheet of the relay and it's obviously not sealed!
    The relay spec says: Ambient humidity:40-85% RH

    I basically question if it's legal to sell that relay module like they do. There is no proper data sheet offered. The relay itself has only 1.5kV Dielectric Strength between Contacts and Coil, which is too little for 230VAC rails.
    Moisture will also decrease the isolation distance of any circuit and if you have electrolysis, it can produce a conducting path on the PCB, that might be lethal!

    I suggest:
    -Look that the wire cross section is enough (as GreenGiant suggests)
    -Assure that the terminal connection is made properly (ferrules). You could also solder the cables to the back side of the PCB to get a good connection.
    -Forget the grease! The whole circuit must not be exposed to a "very humid environment". Either move it outside the humid room or use a sealed box for that circuit. All other options are life dangerous!
     
  7. mcm

    mcm

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    Mar 1, 2013
    All,

    Double checked, and wow, I was way off. The wires are actually 22 gauge. But according to this site:

    http://www.powerstream.com/Wire_Size.htm

    22 gauge should be able to handle 7 amps for chassis wiring. I thought this was chassis wiring - am I wrong?

    electrobrains,

    Thanks for your suggestions - I'll get ferrules and I have some ideas on how to get this out of its humid environment. Hoping I won't have to seal the electronics box.

    How did you calculate the required dielectric strength for a given AC voltage? Note that I never plan to run this over 120 volts.
     
  8. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    Dielectric strength is a property of the material.
    Condensation will conduct easily especially if there are salts about. Some fluxes contain salts but these should not be used in electronics.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    Look at the "maximum amps for power transmission" column. It's less than 1 amp.
     
  10. mcm

    mcm

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    Mar 1, 2013
    steve,

    I saw that column, but I thought this application was chassis wiring, not power transmission. My understanding was that "power transmission" is for the case when you have a bundle of wires.
     
  11. mcm

    mcm

    5
    0
    Mar 1, 2013
    Updated my electronics based on all the suggestions - see the attached picture. Top of the picture you can see my new relay plugged into a solderless breadboard. I'm using insulated quick connect wire terminals to run a 16 gauge wire through the relay. The bottom / middle of the picture shows the barrier block screw down terminal I am now using to distribute power to this high current region (you don't want to know what I was using before!). Thanks very much to everyone for all the helpful suggestions!
     

    Attached Files:

  12. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    Great! That starts looking like "heavy duty"...
     
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