12v relay kickback?

Discussion in 'General Electronics Chat' started by boonxiong, Aug 31, 2013.

  1. boonxiong

    Thread Starter New Member

    Oct 17, 2011
    10
    0
    Hi everyone,

    It's my first time working with a 12v automotive relay. Do i need to concern about any kickback from the coil?

    Thanks
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,134
    268
    It depends on what you are using to drive the relay, if you are driving it with a semiconductor, then yes, you should include a diode across the coil, in reverse bias connection.
    This diode will act as a current path when the switch opens, it should be rated for the coil current X 2 at least. Without the diode, the voltage will rise until *something* provides a current path for the inductive spike, usually that *something* will be your semiconductor blowing up.
     
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  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    They will typically make a back EMF spike of 120-150v or so.

    A diode across the coil is a good idea (as Sensacell said). :)
     
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  4. cornishlad

    Member

    Jul 31, 2013
    196
    25
    Relay coil suppression is a more complex issue than is commonly supposed. A simple diode is said to be the worse solution as it slows the relay release, encouraging arcing, and can shorten it's operational life. If the relay is controlled by another relay or a simple switch don't bother. The low impedance of am automobile supply line will soak up any back emf. Of course if it is to be switched by a solid state device, in spite of the drawbacks, the reverse bias diode is a common solution.
    There are several informative articles on the net about it. Goggle "relay coil suppression" or similar. here's one: http://www.te.com/commerce/Document...v&DocNm=13C3264_AppNote&DocType=CS&DocLang=EN
     
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  5. boonxiong

    Thread Starter New Member

    Oct 17, 2011
    10
    0
    Thank you all for the insightful information. Can you guys let me know the right protection?

    1. Relay will be activated by connecting the coil directly to a 12v source.
    2. Relay will be activated by a 5v->transistor->12v.

    Thanks again.
     
  6. cornishlad

    Member

    Jul 31, 2013
    196
    25
  7. cornishlad

    Member

    Jul 31, 2013
    196
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  8. boonxiong

    Thread Starter New Member

    Oct 17, 2011
    10
    0
    So I won't be needing any kind of kickback protection if I'm just hooking the coil directly to a 12v?
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,571
    2,381
    Probably not needed if it is not being switched, i.e. Energized all the time.
    Although presumably you will remove power at some point?
    If so there will still be a spike at power down.
    Max.
     
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