Does a relay driving motor need a snubber?

Discussion in 'General Electronics Chat' started by ErnieM, Aug 8, 2012.

  1. ErnieM

    Thread Starter AAC Fanatic!

    Apr 24, 2011
    I'm going to control a motor via a relay. Motor requires some 12V/10A, and only runs for a few minuits several times a week. However, I don't want it to arc the relay contacts. Should I add some sort of snubbing network across the motor such as a diode, MOV, or some other element?

    Anyone have any experience with this?
  2. Bernard


    Aug 7, 2008
    There have been long discussions on snubbers on AAC, might look in find box. I would use a 5A 200V diode.
  3. maanga

    New Member

    Aug 20, 2012
    12 volts at 10 amps is a large load for a relay.
    You can fit a free wheeling diode across the motor only if you never reverse the polarity to the motor. I suggest you connect two relay contacts in series which will reduce the arc across the contacts. A double break contact will be better than single break type, but all in series.
    PackratKing likes this.
  4. JDT

    Well-Known Member

    Feb 12, 2009
    The problem is that a DC motor, when it is first connected, is not turning. So, for a short time it will draw the stall current for the motor. This can easily be 10 - 20 times the running current. So the contacts need to be big high current.

    Or the relay contacts will weld together and not turn off.

    Snubber not really required. Just oversize contacts.

    BTW: Snubber is used to remove high voltage sparking when the contacts open. When switching an inductive load. Not really a problem in this case.
  5. #12


    Nov 30, 2010
    I just ran into a vibration problem when placing a MOV across a DC motor. Haven't figured out the mechanism (of why it vibrates) yet.

    Edit: The MOV had nothing to do with the vibration.
    Last edited: Aug 20, 2012
  6. cork_ie

    Active Member

    Oct 8, 2011
    If the power to the motor is not reversible then use diodes as previously suggested. If it is reversible then use a 0.1μF 100 V or more (non electrolytic) capacitor across each contact.

    Contacts connected to an inductive load spark when opened, due to back EMF and ionisation of the tiny air gap as they open until a certain point is reached when the gap gets too wide.

    If you provide an alternative path with a diode or capacitor the spark will be greatly reduced and vastly increase contact life
  7. ramancini8

    Active Member

    Jul 18, 2012
    You require some kind of suppressor as discussed in this thread. I recommend a 12 volt contactor, which is a much hardier type of relay, because they are made for switching motors. Regular relay contacts will arc away or weld in a short time.
  8. panic mode

    Senior Member

    Oct 10, 2011
    i do a lot of seat testing application (on assemble line, one seat is tested every minute, on each test station). power seats have several DC motors and draw anything in range 5-20A, 10A is very common. without diode you will see nice arcs eating away relay contacts.