Relay diode

Discussion in 'General Electronics Chat' started by Blackbull, Apr 25, 2010.

  1. Blackbull

    Thread Starter Well-Known Member

    Jul 26, 2008
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    I wish to switch a voltage supply from one op - amp to another via a relay, will it be necessary to place a protection diode across the relay coil? That is will the back emf damage the op – amp?
    Many thanks.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Can you post a schematic of your idea?
    That way we can see how to place the diode.

    Bertus
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Many relays have coil diodes already installed. If yours doesn't, post a schematic like bertus suggested and we can help.
    And yes, it is to suppress back-EMF.
     
  4. Blackbull

    Thread Starter Well-Known Member

    Jul 26, 2008
    70
    6
    Thanks for the replies Retched and Bertus; will try to sort a schematic, it is still in my head at the moment. I was reading that the protection diode has an adverse affect on the reliability and life of the relay, otherwise I would have put one in anyway.
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
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    consider a Solid State Relay or SSR if that is a concern.
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
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    Only in very very rare occasion (0.1%) that the speed of contact opening must be realized in an extremely short time and the current flowing via the contact is high.

    I have never encountered such a case.

    So in 99.9% of all cases one would definitely benefit from fitting a diode across the relay coil.
     
  7. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    Hi,

    Placing a diode across the relay will slow down the drop-out time and therefore lengthen the time the contacts must sustain an arc. This shortens the contact life. How much, is application dependant.

    However, if the relay is switching at low voltages and current, as in your case, then there is no arcing anyway so adding a diode will not effect the life of the relay.

    An alternate solution is to use a snubber circuit (RC in series) across the relay driver. The back EMF will still go above the supply, but so long as it is limited to less than the Vbr of the driver, then no harm is done, and relay drop-out time is vertually not effected.

    Relay coils are large inductors and should not be driven directly with an opamp unless the opamp is designed for that purpose.

    Regards,
    Ifixit
     
  8. Blackbull

    Thread Starter Well-Known Member

    Jul 26, 2008
    70
    6
    Thanks also LC and Ifixit
     
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