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How to clean relay contacts?

Discussion in 'Electronic Design' started by Andrey, Feb 3, 2005.

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  1. Andrey

    Andrey Guest

    Hello All,

    I am dealing with a bunch of relays that have burnt out contacts: Mechanics
    works fine but contacts measure 3 - 15 Ohms between them instead of zero. I
    opened several - the contact pads turned dark gray, almost black. It is not
    coal, still looks like metal surface. The dark surface is very thin, several
    passes of sandpaper clean it and relay works fine after that.

    I wonder if there is a way to clean the contacts without opening relays (the
    relays are sealed, opening destroys them)

    Appreciate any wisdom

    Andrey

    PS: the relays are used to switch RF power at 2 - 30 MHz
     
  2. Tim Wescott

    Tim Wescott Guest

    It sounds like you either have el-cheapo relays or that they're being
    abused -- are they actually switching when the power is on, or should
    they only be switching with power off?

    If the system is designed to switch with power on then it'll take a
    special relay to not break down under that treatment; I don't even know
    if such a thing exists at all.

    If the system is _not_ designed to switch with power on then either
    something is wrong and you _are_ switching with power on or something
    (like way high SWR) is wrong and the relays are seeing RF voltages high
    enough to arc the contacts when they're open.
     
  3. Could also be oxidisation if they dont have some 'wetting' current which is
    not easy clean off without physically touching them. Are they located in
    good conditions ?
     
  4. Terry Given

    Terry Given Guest

    Two words: Ragnar Holm. Hard to find but worth the effort.

    Failing that, read the book Schrack published on relays. very
    interesting stuff. here are 2 great ways to ruin relays PDQ:

    1) use a low current relay to switch a large inductive load - that'll
    splat the hell out of the contacts, quite possibly welding them closed.

    2) use a whopping great relay (say the one that should have been used
    for the load above) to switch a thermocouple. The contacts will go
    intermittent, and perhaps stick open or closed.

    relays which are designed to switch high currents have really grunty
    contacts, carefully designed by hordes of fanatical metallurgists. Often
    a current of 200mA or so is required to blast the inevitable
    accumulation of crud off the contacts - using such relays for signal
    apps fails to clean the contacts, and problems ensue. I personally have
    overseen the replacement of a few dozen "big" (well puny really, 10A
    230V) relays used to switch signals in a low voltage tester. Rather than
    getting the requisite gazillion switching operations, they crapped out
    after a few tens of thousand. So I replaced them (OK, got a tech to
    replace them) with a suitably chosen low-current "signalling" relay

    Conversely low-current relays use totally different contact materials,
    and invariably rely on some mechanism other than a good belt of amps to
    keep the contacts clean. Try to switch an inductive load with one of
    these, and count the hugely reduced number of operations - hundreds,
    maybe thousands, but nowhere near millions.

    I once looked at RF relays, and man were they *expensive*. I suspect you
    just have the wrong relay for the job.

    Cheers
    Terry
     
  5. Chris

    Chris Guest

    Hi, Andrey. I'll leave the wisdom to others, but here's some advice.

    Sandpaper is pretty brutal. The idea is to clean the metal surface of
    the contact without decreasing the surface area by gouging it. Contact
    cleaners are available to do this job. They are a little longer than a
    matchstick, about 1/8" to 1/4" wide, and flat. They have very fine
    abraisive material, and are stiffer than sandpaper, which will tend to
    misalign the contacts. When using them, insert the flat between the
    two closed contacts and gently rub back and forth a few times, being
    careful not to twist the contacts out of alignment.

    However, if you can't open the relays, you have to find some way to
    break through the surface oxidation (the dark gray/black indicates you
    probably have silver contacts). If nothing else is wrong, it's
    possible that switching a higher voltage (AC, 50/60 Hz) at something
    below the rated current of the relay will do the trick. The heat of a
    small contact arc is sometimes enough to refresh the contacts, if
    they're not too far gone. Can't tell you exactly without looking at
    the relay contact ratings, but I'll usually use either a 24V or 120V
    light bulb. Hook up the AC, switch your bulb on and off a couple of
    times, and see if that improves your contact resistance. Note also
    that the cold resistance of the bulb is much less than when heated, so
    you're actually goosing the contacts with several times their rated
    current current for a few milliseconds. Note also that this does not
    work and should not be tried with the bifurcated contacts often used in
    signal switching, which will tend to stick when you use them in this
    manner. And that can't be fixed without opening the case.

    This trick may work for relays and switches which are being used for
    dry contact (low voltage and low current) service, too.

    Good luck
    Chris
     
  6. Guy Macon

    Guy Macon Guest

  7. John Smith

    John Smith Guest

    Hi, Andrey -

    You already have good suggestions for cleaning.

    You didn't ask, but here is my suggestion to prevent the problem. Ideally,
    the relay should be energized before RF power is applied and de-energized
    following the removal of RF power. It may be arcing between the slightly
    open contacts causing your problem.

    Good luck.

    John
     
  8. Michael

    Michael Guest

    Tim Wescott wrote:
    (snip)

    Oh sure; ever hear of "contactor"? That's a 25-cent word for "relay
    with whacking great contacts".
     
  9. Some of us distinguish between the two based on the construction.


    Best regards,
    Spehro Pefhany
     
  10. I read in sci.electronics.design that Spehro Pefhany <[email protected]
    What are the constructional differences? I have relays that look like
    small contactors. Or....
     
  11. A "contactor" is a "relay" that has contacts that you won't want to get
    body part between. ;-)
     
  12. They are contactors iff they can truthfully be so categorized under US
    Harmonized Tariff Schedule Item 8536.41.45 or 8536.41.65


    Best regards,
    Spehro Pefhany
     
  13. Rich Grise

    Rich Grise Guest

    I've seen contactors with a separate "start" contact, that then use one of
    their own contacts to hold them closed - that way if there's a short power
    outage, you have to restart all of the motors in the factory by hand.

    Presumably, it prevents equipment damaging itself, like if the controller
    is resetting or it's some process that if it's interrupted, has to be
    reinitialized and restarted, that sort of thing.

    Thanks,
    Rcih
     
  14. AFAIUI, contactors have bridging contacts (two pairs per circuit).
    Relays have one pair of contacts per circuit and a current-carrying
    wire, braid or flexure of some kind. Admittedly, this distinction gets
    stretched pretty thin with liquid state relays/contactors.


    Best regards,
    Spehro Pefhany
     
  15. You're just making that up right now eh? ;)
     
  16. No.


    Best regards,
    Spehro Pefhany
     
  17. Well, if I were asked about the difference between relays and contactors,
    *I* would make something up about contactors for heavy stuff like large
    motors and what have you, and relays for the smaller and milder tasks.
    Talking about double contacts, I think about vibration issues, different
    lenghts of blades, each contact looks like a fork, where each contact has a
    different resonance frequency. For use in nasty industrial environments.
    This 'trick' Ive only seen on the smaller relays up to a few amps rating.

    Copper blade fork, with different lenghts of contacts, top view:
    ######################.###
    #####
    ###################.####
    Two of the above needed to make a switch.

    Some 'Relay' contacts are a sandwich of contact blades and insulation
    spacers, hold together with popnails or tiny screws. The larger contacters
    have those typical thick braid wires. +25Amps.

    One of us has it totally reversed, relays and contacters. Not that it
    is terribly important... what's in a name. Use what fits the requirement.
     
  18. Yes, the pusher type of relay. From what I've seen, that's a European
    & telecom thing. Relatively inefficient electrically, and contact life
    is reduced for a given coil wattage, but superior coil-contact
    isolation.
    If you feel like it, Google on "definite purpose contactor" and see
    what you find.

    But it could be a Pondian thing.

    Best regards,
    Spehro Pefhany
     
  19. Terry Given

    Terry Given Guest

    Contactors also have arc quenching features built in. And seriously
    large contact-contact spacing when open (the 2 pairs of contacts helps
    too) unlike most relays. It was always fun designing relays into
    circuits with a given creepage & clearance - open up the relay, and look
    at the tiny gap between contacts - often 0.5mm to 1mm.

    Cheers
    Terry
     
  20. I *always* open them up, and always life-test them under load before
    using them in a new application. Even some very cheap ones if you look
    carefully you can see signs that they were manually tweaked in
    production. Not what you want for high reliability.


    Best regards,
    Spehro Pefhany
     
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