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Spark in relay contacts

  1. Aug 2, 2016 #1
    Why is there a spark in the relay contacts?

    Suppose that the coil and the contacts are connected to two different power supplies. If i suddenly disconnect the power supply to the coil, then the voltage across the coil will increase and a spark will jump across the switch that I used to disconnect the power supply from the coil. But the relay contacts are isolated from the coil circuit, and should be unaffected by the voltage from the coil. The only way for a spark to jump across the contacts is if there is an inductive load connected to it.

    I am asking this, because every time I read about this topic online, they say that it is due to the voltage in the relay coil. Thank you for your time.
     
  2. jcsd
  3. Aug 2, 2016 #2

    Averagesupernova

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    Why do you think there has to be an inductive load to have a spark?
     
  4. Aug 2, 2016 #3
    When you see that response they are probably talking about the same switch that you are, the one powering the coil. That's where most people run into trouble.

    For relay contact arcing, you are mostly correct. In addition to an inductive load, a high voltage across the contacts alone can strike an arc on contact opening. A high voltage is more usually found with an inductive load due to the "inductive kick" or back EMF that occurs when the inductor current is interrupted. At the moment of current interruption, the magnetic field rapidy collapses and induces a high voltage in the windings. Even with a resistive load an tiny arc is frequently present, but when AC is being switched the arc is quenched at the next voltage zero crossing. You may have noticed that relay contact ratings are lower for switching DC. That's because there is no zero voltage point in the supply voltage and any arc that starts will continue until the contacts separate far enough to extinguish it.

    P.S. I see Averagesupernova snuck in. Guess I was typing thinking slowly. :smile:
     
  5. Aug 2, 2016 #4
    With an inductive load, abrupt current interruption or variation induces a high voltage. I couldn't see how it can happen with a purely resistive circuit. After reading your replies, my understanding is that a spark could jump if the relay contacts are extremely close to each other, which happens just as the contacts start to open. And as Tom.G said, if there is a high voltage present at the contacts, a spark could be sustained at an even wider gap. Did I understand it correctly?
     
  6. Aug 2, 2016 #5
    For AC, one would switch ON when the voltage zero crosses, but switch OFF when the current zero crosses.
     
  7. Aug 3, 2016 #6
    Mechanical contacts can have a bounce effect when opening or closing. You won't see the movement with your eye but the it is there.
    With a resistive load, that means the current can be in a cycle of flowing and not flowing several times before the contacts fully engage or separate.

    A big problem for digital circuitry, so that is why a solid state circuit is used a buffer between an electromechanical switch and the digital circuitry.
    http://www.elexp.com/Images/Contact_bounce_and_De-Bouncing.pdf

    The cycles of current flow during the bounce sets up higher frequency oscillations, or EMI that one can notice on your AM radio.
     
  8. Aug 3, 2016 #7
    Yup. And I see there is more information being coming in too.
     
  9. Aug 3, 2016 #8
    Thank you for answering my questions. :D
     
  10. Aug 3, 2016 #9

    Averagesupernova

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