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Relay Throw Question

Discussion in 'Electronic Basics' started by BillyYoung, Jan 4, 2006.

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  1. BillyYoung

    BillyYoung Guest

    Dear All,

    I have a circuit with a 12V, SPDT relay (250V 10A) and the load is ac
    resistive, 220V, 8A.
    When I apply 12V to the coils from a dc power supply, I can hear that
    the relay gives a "click" sound, but when I measure the proper output
    legs, I found that the relay actually didn't throw (i.e. opens the NC
    contact and makes the NO contact closed).

    I tried several times and suddenly the relay "click" sound became much
    louder and then it worked properly.

    Any clues? Poor contact or anything else?

    Thank you very much!
    Billy
     
  2. tlbs

    tlbs Guest

    The contacts may have been oxidized. By operating the relay a few
    times, you may have "scrubbed" the contacts clean, by the normal wiping
    action.

    Another possibility could be that some soft object was stuck in between
    the pole and the armature and was loosened by operating the relay a few
    times.

    It still may be wise to remove the cover and clean the relay contacts.

    Tom
     
  3. BillyYoung

    BillyYoung Guest

    Thank you very much!

    Somehow the relay is a sealed one and so I cannot easily remove the
    cover.

    Is it true that if my load is a bit inductive (e.g. cos theta = 0.9),
    then without a load snubber (R-C), the relay contacts may have higher
    oxidation/weld possibility? How do I know when I should add a snubber?

    Thanks.
    Billy
     

  4. Are you sure that the 12V power supply is working properly and is able to
    supply enough current to drive the relay coil?

    Personally, I don't agree with Tom's suggestion that the contacts may have
    been oxidized: if that were the case, the NC contacts would have opened,
    even if the NO contacts didn't close. From your post I *think* you're
    saying that the NC contacts stayed closed. And anyway, I've never heard of
    oxidized contacts changing the sound of a relay.

    But Tom's suggestion of some soft obstruction (or perhaps a small piece of
    grit in the works somewhere) is very plausible. That's probably where I'd
    bet my money. And if it's a sealed relay, I wonder if that piece of
    material is still inside, waiting to cause problems again in the future.
     
  5. Tom Biasi

    Tom Biasi Guest

    Sounds like your relay was stuck open and then broke free.
    Could be stuck NC contacts or an armature cocked due to physical abuse.
    Tom
     
  6. BillyYoung

    BillyYoung Guest

    Walter Harley 寫�:
    Yes Yes.. the NC still stays NC... can other be any possibilities? The
    12V is clean and the current agrees with the datasheet.

    Somehow from around 50 relays I have, few of them really have this kind
    of "random stuck". The phenomenon sometimes occurred and somehow I
    "activated" it again then the problem went away. But maybe of course
    it will occur again in the future, I don't know.

    Any other possibilities? Thanks!
    Billy
     
  7. BillyYoung

    BillyYoung Guest

    Sorry, what does "stuck open" mean? In my case the NC stays NC and the
    NO stays NO, somehow it sounds like the relay throws but bounced back
    (but the 12V was still applied there!)

    Thanks.
    BIlly
     
  8. Rich Grise

    Rich Grise Guest

    How long had it sat without operating before this happened? There could
    be some kind of lube grease on the armature that gets gummy with time,
    and you loosened it up. Or if they're like that out of the box, it could
    be crappy manufacturing. :)

    But it definitely sounds mechanical in nature.

    Good Luck!
    Rich
     
  9. Rich Grise

    Rich Grise Guest

    It means that. :)

    Cheers!
    Rich
     
  10. Sjouke Burry

    Sjouke Burry Guest

    It means, that the contacts have been fused together.
     
  11. Jasen Betts

    Jasen Betts Guest

    if a used relay possibly the NC contacts were welded. and exercising it
    unstuck them.

    Bye.
    Jasen
     
  12. BillyYoung

    BillyYoung Guest

    If it is mechanical... then what can I do about it? Actually is it
    common to drive a 12V relay with, say, 15V to ensure proper driving?

    Thanks.
    Billy
     
  13. BillyYoung

    BillyYoung Guest

    Then, to avoid the same problem occuring again, should I add R-C
    snubber to the relay, especially when I use it with inductive loads?

    Thanks.
    Billy
     
  14. No. A 12V relay should be operated, ideally, by 12V. If you look at the
    datasheets for some relays you'll see that they are specified in terms of a
    number of voltages, including a max rated voltage (more than this will fry
    the coil), and a minimum pull-in voltage (it takes at least this much to
    make the relay turn on). A 12V relay might have, say, 9V pull-in and be
    rated for up to 18V maximum. There's also a drop-out voltage; as long as
    you maintain the coil voltage above the dropout voltage, after the relay is
    energized, it will stay energized. The dropout voltage is usually a bit
    less than the pull-in voltage.

    If you've got 50 relays that claim to be 12V relays, and a handful of those
    are not reliably activated by 12V, then what you have is a defective batch
    of relays. If you purchased these from a manufacturer or legitimate
    distributor, you should be contacting their technical support for
    guaranteed-good replacements. If you purchased them from a surplus
    supplier, or over eBay or some such... well, now you know why you got such a
    good deal on them.
     
  15. ehsjr

    ehsjr Guest

    No. Relays generally will operate below their rated voltage.
    It is not usual to overdrive them to ensure they transfer.
    What you want to do is ensure that the path from the supply
    to the relay coil does not reduce the current below what the
    relay needs.

    As far as old relays that may be sticking mechanically, you
    can rig up a 555 with a driver transistor to "exercise"
    them. Set the timer for something like 1 second on, one second
    off. Let the circuit operate the relay for a few hours.
    That may help to overcome mechanical binding.

    +12 --------------------+-----+
    | |k
    [Rly] [Diode]
    | |
    +-----+
    |
    555 /c
    Timer pin3 ---[470]---| PN2222A
    \e
    |
    Ground

    The transistor is not absolutely mandatory, as
    many relays can be driven directly by the 555,
    but it's worth adding. The diode is mandatory.

    Ed
     
  16. Rich Grise

    Rich Grise Guest

    Yes, but this is a different issue than your original question. I
    had the impression that the relay was just stuck, not fused.

    Thanks,
    Rich
     
  17. BillyYoung

    BillyYoung Guest

    Thank you... then I will keep the 12V. Let me try to find more
    information from my supplier.

    Thanks.
    Billy
     
  18. BillyYoung

    BillyYoung Guest

    How do I determine which situation I have now?

    Thanks.
    Billy
     
  19. Rich Grise

    Rich Grise Guest

    I have no idea, other than to open the relay and look at it.
    Is it really "sealed", or is it one of those ones with the
    plastic case that you need two little screwdrivers to get
    off?

    Thanks,
    Rich
     
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