Contact Resistance of Relays

Discussion in 'General Electronics Chat' started by Friz, Nov 17, 2016.

  1. Friz

    Thread Starter New Member

    Oct 28, 2014
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    HI,

    I have been looking for relays with low contact ratings and I've been noticing a lot of them say Contact Resistance(initial) = XXmOhms. Why does it say initial? Do the resistance of the contacts become less than or more after initial contact?

    Thanks
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Exactly, it can change to less or more, but usually more.

    It will depend on how you use them and how they were built. Low current relays are designed with a better contact metal, such as gold plating to work well with low currents. A high current may burn thru this. High current relays have no such plating and actually require some minimum current to burn thru any oxide layer that forms.

    Best choose a relay rated to work with the currents you will use.

    Aside: I have a test stand that uses 10 amp relays to switch low current sense signals (not my design). Periodically we need to switch in a 1 amp power supply to clean these contacts so they work for another year or so.
     
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  3. Friz

    Thread Starter New Member

    Oct 28, 2014
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    Okay, my problem is that I need a relay to be able to withstand 16ADC but it has to be 10mOhm contact resistance. The best I have found is 30mOhm. I have been looking at mouser, digikey, and newark for relays and have had no luck. Would you know of a specific series with 10mOhm resistance by any chance?
     
  4. #12

    Expert

    Nov 30, 2010
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    This is why mosfets were invented. Is there any way to use a mosfet in your circuit?
     
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  5. Friz

    Thread Starter New Member

    Oct 28, 2014
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    I was trying to stray away from them, but I might have too....
     
  6. tcmtech

    Distinguished Member

    Nov 4, 2013
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    I think you are over analyzing the problem given the basic math is not adding up to real world working conditions.

    For example, A standard 20 amp rated switch may have a initial contact spec of say 75 milliohms which by the numbers would show that at 16 amps it would have a ~.48 volt drop which would then equate to a thermal dissipation of ~ 7.7 watts yet practical hands on experience clearly shows that a 20 amp switch carrying 16 amps has no noticeable heating from contact resistance or at least no where close to 7.7 watts. Maybe a few tenths of a watt but not 7.7 watts!

    In fact if you do some digging you will find that many 50+ amp rated switches and relays have higher than 30 milliohm intintal contact rating numbers and many large 100+ amp solenoids at full current don't come close to the 7.7 watt theoretical heat load number either in real life operation of the 20 amp switch with a 30 milliohms contact resistance numbers would imply for itself.

    I would say your best solution would be to spend less time in the books and more time with your hands on some actual circuits poking around with some multimeters to see what real life says about those by the book numbers. Good chance that in rall life operation most any new 20 amp rated relay or switch will only show a few millivolt drop at 16 amps at best.

    Also what are you powering from what type of source?o_O
     
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  7. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Get a 3P or 4P relay with 30mOhm contact resistance and parallel the contacts
     
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  8. BillB3857

    AAC Fanatic!

    Feb 28, 2009
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    Back in the dark ages, mercury wetted relays gave very, very low contact resistance.
     
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    A second for putting multiple contact pairs in parallel.
     
  10. #12

    Expert

    Nov 30, 2010
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    I second the motion of realizing the specs are the "guarantee" under worst case conditions.
    If you're calculating for a production run, you pretty much have to use the "worst case" numbers.
    If you're building one, you are allowed to pay special attention to the one you're building.;)
     
  11. tcmtech

    Distinguished Member

    Nov 4, 2013
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    Have you ever seen any standard 20 amp rated switch or relay designed for a 7+ watt thermal dissipation on its contacts or any device one was in designed around having that much in line loss? o_O

    I can't say I have. At 7 watts loss even the heavy duty 20 amp 277 VAC rated rocker switch on my drill press would melt down in minutes. :(
     
  12. #12

    Expert

    Nov 30, 2010
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    Not understanding. Do you object to me agreeing with you?
     
  13. tcmtech

    Distinguished Member

    Nov 4, 2013
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    Yes I am! I'm grouchy and defensive today. :p
     
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  14. #12

    Expert

    Nov 30, 2010
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    I had one of those two days ago. I didn't realize how bad my legs were hurting until they stopped hurting.:(
     
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