Spark as Switch Makes

Discussion in 'General Electronics Chat' started by john_nohj, Jul 7, 2016.

  1. john_nohj

    Thread Starter New Member

    Dec 10, 2014
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    Hello, I am familiar with the damage caused to switch contacts and the arcing that can occur when breaking (opening) inductive loads (relays, contactors, etc), and arc suppression methods.

    However when investigating a damaged switch recently, when hooked up to the circuit it was controlling, a spark flashes across the contacts as the switch is closing (making). I have never witnessed this behaviour before and was hoping someone could shed some light on what could cause this type of arcing?

    The switch is turning on power to a 24-to-6vdc voltage regulator which is then turning on a 6v ~20w contactor. The supply to the regulator has a 2mH 2-pole inductor and a bank of 3 x 100uF capacitors for smoothing. I have measured the voltage across the capacitors when the switch is open and they do not appear to be collecting an unexpected charge.

    I hope this isn't to vague and one of you may have some advice about this type of arcing??
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you are turning the circuit on by first powering the voltage regulator you may be seeing some inrush as the capacitors charge.
    Is it possible to leave the convertor powered and switch the contactor 6v?
    Max.
     
  3. benta

    Member

    Dec 7, 2015
    101
    24
    Typical for turning on a capacitive load. The inrush current can be quite high.
    This is not high-voltage arcing, as when disconnecting an inductive load, but rather low-voltage, high current "burning" of the contacts.
    Not healthy, and you might need to think about some kind of inrush-current limiting.
    Actually, the 2 mH inductor should help here, but apparently not, or not sufficiently, probably due to saturation caused by the high current.

    Benta.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    BTW, if the load is just the 6v contactor you do not need a regulated supply, a simple 6vAC source with just a bridge should be sufficient.
    Max.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,055
    3,245
    The arcing you are seeing when the contacts are closing is due to contact bounce.
     
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  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    That would be my guess too, though the OP states the switch in question is damaged, so it may be that the plating on the contacts has gone giving a high resistance contact.
     
  7. EM Fields

    Member

    Jun 8, 2016
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    The arc is due to contact bounce on MAKE.
     
  8. EM Fields

    Member

    Jun 8, 2016
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    Relay contacts designed to switch power aren't generally plated, and are coined using the same alloy throughout the bodies of both mating contacts.

    Damage occurs when the contacts bounce, an arc occurs, and metal is removed from one contact and deposited on its mate, as noise, through the plasma of the arc.
     
  9. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Two things can be happening. The first is that the open switch voltage is sufficient to span the gap....this can occur if there are burrs or dirt or pitting that creates a sharp point for corona to form. This is not too likely a scenario. More likely, you are experiencing CONTACT BOUNCE, where, upon closing the contacts bounce apart slightly which can seriously excite inductive effects. Contact bounce detection is one reason i LOVE digital storage oscilloscopes!
     
  10. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    Make sure the switch as the right ratings. AC/DC; switched current allowed etc. Contacts are made differently for low and high level currents and AC and DC ratings.
     
  11. john_nohj

    Thread Starter New Member

    Dec 10, 2014
    2
    0
    Thanks for all the replies! Excellent info.

    Unfortunately the supply voltage or contactor voltage cannot be changed. Also, the contacts in question are part of a micro-switch pushbutton, not relay.
    I have tested the capacitor voltage with the circuit off and it is not unexpectedly high, so I think the arcing is most likely due to a high inrush and contact bounce.

    The contacts are indeed getting the typical pitting described above and when held closed for long periods are beginning to lightly weld!

    The circuit is going to require some more substantial inrush limiting I think...

    Thanks again.
     
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