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Choosing a relay

  1. Mar 22, 2016 #1
    I have a 5v dc circuit and a 12v dc motor. If I had to use a relay, when looking at the specifications, what do I look for ?, will the coil voltage be 5v and the switching voltage be 12v ?

    Will these relays work ?
    http://www2.mouser.com/Electromechanical/Relays/_/N-5g31?P=1z0x3tdZ1yyfbg3 [Broken]

    Also, how do I know the minimum current required to activate the switch ?
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Mar 22, 2016 #2


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    Gold Member

    By activation the current will be
    Icoil = Vcoil / Rcoil
    However you may insert a save resistance in the circuit, decreasing the coil current when the relay has been activated. Often this save resistance has already been included in a relay with a dc-coil.

    Also you should protect the NO-switch ( the righthand in the attached ) by means of a RC series connection. Otherwise you may "burn" the switch, breaking the dc-motor current several times. The values for the RC depends on the motor current and how fast the NO-switch will open.

    Attached Files:

  4. Mar 22, 2016 #3
    Ok, I want to connect an LED to show when the relay is on, but because the LED drops 1.7V I would have to connect the relay in parallel so as to recieve the full 5V supply voltage

    Here's a schematic

    Would this connection be fine ?
  5. Mar 22, 2016 #4


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    Gold Member

    Also, usually when you have a relay coil you insert a reversed diode across the coil to absorb the voltage spike when the coil shuts off.
  6. Mar 22, 2016 #5


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    Not only could you damage the transistor when left out, in this case you could damage the LED also.
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