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  1. #1
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    Where do I install the protection diode on a relay?

    I'm confused on where i should install this diode on this relay to prevent the spike which relays give. Once source tells me to place it between COM and +12VDC, but another tells me to place it across the coil. Where does it go and how do I know its facing the right direction?
    Last edited by blueblib; January 3rd, 2007 at 03:50 AM.

  2. #2
    Human voltmeter DanU's Avatar
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    Quote Originally Posted by blueblib View Post
    Once source tells me to place it between COM and +12VDC, but another tells me to place it across the coil.
    I believe that's saying the same thing, just in different ways. The diode is supposed to be placed across the coil in the direction that current WILL NOT normally flow when the coil is active. If you put it in the wrong direction, you will create a near short across the coil, which will render the relay inoperative and possibly burn out your power supply or the diode or both. In other words, put the "arrow" or "silver band" towards the positive terminal of the coil.

    BTW, how is your project coming along?

  3. #3
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    So is this what you mean? (forget the transistor)


    That sounds dangerous...

    So there are two terminals for the relay coil. Im guessing that the +12V goes in and out. But I'm now confused since you've mentioned the positive terminal. Someone else said that i have to connect the diode to he COM and the +12V, how is that the same (like you said)?

    So i have the red wire for +12V coming from my power supply, I connect it to one terminal for the coil, and at that same point I attach the diode and connect the other side to the other terminal of the coil. The Terminals aren't labelled positive, negative or anything, so how do I know which is the right way? So if I continue the red wire out of the second terminal, will I still get +12V going through?

    I seems impossible, so do I have to connect the red wire and the black ground wire to the two terminals?

    Still confused ... but I promise I'll start up a thread about the project when it's hopefully finished!
    Last edited by blueblib; January 4th, 2007 at 02:25 AM.

  4. #4
    Human voltmeter DanU's Avatar
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    Nevermind about the COM comment. I thought you were referring to COM as the negative terminal for the coil, and the +12V as the positive terminal for the coil.

    Ignore the diode for now. You need to determine what the positive and negative terminal of the coil is. It should be labelled on the relay. If not, it's easy enough to figure out. Connect one of the coil terminals to +12V, and the other to ground. If you hear a "click", then you got the polarity right. Double check with a ohmmeter to make sure the switch contacts are shorted. If not, then the polarity is the reverse of what you have wired.

    The diode's "arrow" points towards the positive terminal of the relay coil. In the schematic it's assumed that the positive terminal is on the top, and the negative on the bottom.

    If you were to ONLY connect +12V to one terminal of the relay coil and leave the other terminal open, you will get +12V out the other end of the relay coil. HOWEVER, in operation, the voltage will drop across the relay coil according to Ohm's law.
    Last edited by DanU; January 4th, 2007 at 05:35 PM.

  5. #5
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    Thanks DanU, all clear for me now, all the best and have a great 2007!

  6. #6
    Human voltmeter DanU's Avatar
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    Thanks. Have a great new year, and good luck with your project. I would love to see some pics when it's all done!

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