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reed switch fails........open or closed?

Discussion in 'Electronic Design' started by Lee Carkenord, Jun 28, 2004.

  1. When a reed switch fails, does it usually go into an "open" condition
    at time of failure, or does most often show a "closed" condition at
    time of failure, and there-after?

    Thank you..... Lee Carkenord
     
  2. Lee Carkenord wrote...
    Q. Reed switch fails........open or closed? A. Yes, correct.

    Thanks,
    - Win

    (email: use hill_at_rowland-dot-org for now)
     
  3. Ken Smith

    Ken Smith Guest

    It depends a lot on the circuit it is used in.

    It is very rare for the coil in a reed relay to go bad if it is not
    abused but it can happen. This results in the always open case.

    If the contacts are used for small signals only, they can fail as a higher
    than specified resistance. This is a neither open nor closed failure.

    At higher currents, the contacts can stick in the closed condition.
     
  4. Ken Smith

    Ken Smith Guest

    No, actually in my experience they can also fail betwix the two. The
    contact resistance can exceed the spec.
     
  5. John Fields

    John Fields Guest

    ---
    Since the attraction between the contacts in a normally open
    non-biased reed switch is magnetic and they want to close the gap
    between them when in the presence of a magnetic field, it's unlikely
    that they'll fail open because the only thing that's keeping them
    apart is the springiness of the moving contact. OTOH, If the contacts
    weld because they're improperly hot-switched, then when the magnetic
    field is removed the springiness of the moving contact won't be great
    enough to break the weld and the contacts will stay made.

    However, if it's a reed _relay_, the coil could burn open, in which
    case the failure would be and open, unmakeable relay.

    So, as usual, it depends...
     
  6. I've used quite a few reed relays. The most frequent
    initial sign of failure is that it goes 'sticky', ie,
    slow to open when the power is removed, progressing
    to staying closed permanently.

    A good way to extend the life of reed contacts is
    to use a series resistor (at least a few kohms) wired
    as close to the capsule as possible. The resistor
    limits the current the contacts have to handle and
    the close mounting minimises the capacitive current
    at each switch closure.
     
  7. See also my recent thread:
    Newsgroups: sci.electronics.design
    Subject: Reed contacts sticking
    Date: Tue, 08 Jun 2004 20:42:04 +0100
    Message-ID: <>
     
  8. Ian Stirling

    Ian Stirling Guest

    And at even higher currents, you can melt off the ends.
    But you'r not supposed to use reed relays like that...
     
  9. Terry Given

    Terry Given Guest

    I just did a design incorporating a little motion sensor. I likewise used a
    resistor to limit peak current to about 1/2 the rated value, and got
    harangued for doing so. When I pointed out the desired lifetime, along with
    the 100nF cap across the switch, the lone haranguer was silenced (I have
    been waiting years to use that phrase :)

    Cheers
    Terry
     
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