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back emf of relay from regulated supply

Discussion in 'Electronic Design' started by [email protected], Dec 3, 2012.

  1. Guest

    If a relay coil is powered by a regulated power supply, will the back emf generated by the coil cause any damage to the regulator as cathode of protection diode is at o/p of the regulator? From what I read about driving relays using trnasistors the protection diode protects the transistor from the back emf genereated by the relay by diverting the back emf away from the trnsistor. Now it goes to the o/p of the regulator powering the relay coil.

    thanks
    bhav
     
  2. Guest

    On Sunday, December 2, 2012 11:53:48 PM UTC-5, wrote:

    The clamp diode allows the relay current to circulate in a loop from one relay terminal to the other until it decays to zero. It does not put current back into the regulated supply.
     
  3. Phil Allison

    Phil Allison Guest

    If a relay coil is powered by a regulated power supply, will the back emf
    generated by the coil cause any damage to the regulator as cathode of
    protection diode is at o/p of the regulator?


    ** Long as there is an electro cap at the output of the PSU, it will absorb
    back emf energy from a relay - so there is no switch off voltage spike.

    With a transistor and no diode or cap, such spikes will overvoltage the
    collector.


    ..... Phil
     
  4. On Sun, 2 Dec 2012 20:53:48 -0800 (PST), the renowned
    Are you having problems?

    You might want to have at least a small bypass capacitor on the output
    of the regulator.. the distributed capacitance in the relay coil could
    otherwise cause the regulator voltage to go up a bit.. I doubt it
    would damage the regulator but it's good practice.. there are other
    ways that stuff can get coupled in with a mechanical relay.



    Best regards,
    Spehro Pefhany
     
  5. Forget about back-EMF, for a moment. The first thing to consider
    is that the relay coil is an inductor and will 'try' to keep
    its current constant. When you switch off your relay, can this
    current go somewhere without damaging anything? If so, you're
    safe.

    So, if you switch the relay with an emitter-follower, you don't
    need a catch diode. Now, what's the power supply like and how
    do you switch the relay?

    Jeroen Belleman
     
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