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Minimum switch current

Discussion in 'Electronic Design' started by Robert Scott, Aug 1, 2007.

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  1. Robert Scott

    Robert Scott Guest

    I noticed that some switches are specified for a minimum current as well as a
    maximum current. Someone once told me that this is because that minimum current
    is needed to "burn away" oxides where the metals contact. Well, this is a
    problem for me because I am designing a low-power device that will run on a coin
    cell battery. The switch that I want to use is specified for 1 ma. minimum
    current. But I want to use the internal microcontroller pull-up for this user
    input switch. Are there some types of switches that have no minimum current?
    How much trouble can I expect if I just ignore the spec? What about putting a
    .01 uF cap around the switch to produce a short burst of high current when the
    switch is closed?


    Robert Scott
    Ypsilanti, Michigan
     
  2. Guest

    Switches with "noble" metal contacts - gold, platinum or palladium -
    work fine at low currents. Base metal contacts are cheaper, and work
    fine if you are switching more than the minimum current.

    "Dry" reed relays were intended to switch low currents, and were built
    with noble metal contacts so that they would continue to work, even in
    the absence of a "wetting current".
     
  3. Eeyore

    Eeyore Guest

    A capacitor is probably the worst idea ever.

    What you're describing is usually called 'dry switching'. Just use switches intended
    for the job.

    Graham
     
  4. Also if you believe the "burn away" theory, you would only get an arc
    when the contacts *open*. So the capacitor is no help because it is
    discharged at that point. Now a series inductor on the other
    hand... Yeah, second worst idea ever :)
     
  5. JosephKK

    JosephKK Guest

    Robert Scott [email protected] posted to sci.electronics.design:
    Basically you need a switch with gold contacts. The anti-corrosion
    properties of gold allows switches to reliably switch at the
    picoampere level.
     
  6. Fred Bloggs

    Fred Bloggs Guest

    Search on Hall-effect switches...these are ideally suited for low power
    portable with sleep power down mode and the on/off is used to pull a
    CMOS input low to wake the circuit up.
    unreliable operation
     
  7. John Fields

    John Fields Guest

     
  8. Wiping action (preferably at a decent pressure) is a necessity.
    Othewise crap in the air can deposit on the contacts and you'll get
    unreliable operation even with precious-metal contacts.


    Best regards,
    Spehro Pefhany
     
  9. John Fields

    John Fields Guest

     
  10. Could be a bad choice since it's not only rather low, but usually very
    poorly characterized. A resistor gives you much better control, and
    unless the switch is very close you'll probably want some series R as
    well.
    Can you design it so that the switch is open most of the time, as on a
    remote control?


    Best regards,
    Spehro Pefhany
     
  11. Eeyore

    Eeyore Guest

    It's the term I'm familiar with. In a typical application you might be switching say
    50uA as a control signal. That's dry enought for my liking.

    I don't agree with your post.

    My answer does at least point out that there are switches designed for this kind of
    use. Tact switches would be a classic example. Gold plated contacts are not essential.
    http://www.google.com/search?&q=tact+switch

    Graham
     
  12. Eeyore

    Eeyore Guest

    I rather doubt your video recorder (or whatever) uses any gold in the switches.

    Graham
     
  13. Eeyore

    Eeyore Guest

    You're insane.

    Graham
     
  14. Robert Scott

    Robert Scott Guest

    Yes, that's exactly how I intend to use it. It is a pushbutton for a user
    interface.

    Robert Scott
    Ypsilanti, Michigan
     
  15. Maybe you could do something like feed the pullup from a port pin and,
    if the switch is closed for too long, lower the pullup and go to
    checking it every now and then.


    Best regards,
    Spehro Pefhany
     
  16. John Fields

    John Fields Guest

    ---
    As with many other terms you're "familiar" with, your interpretation
    of the term is flawed. By definition, if the contacts are switched
    and there's _any_ voltage present across them when they're open or
    _any_ charge flowing through them when they're closed, then that's
    called "hot switching".

    "Dry switching" occurs when there is no voltage across or current
    through the contacts and the action, one on the other, is strictly
    mechanical.
    ---
    ---
    Neither would you agree if I stated that black was black.
    Why am I not surprised?
    ---
    ---
    All you've given an example of is low current hot switching, not dry
    switching, and the switches you're referring to are designed to hot
    switch low currents.

    In some applications where hermetic sealing isn't an option and
    greater reliability is needed than can be provided by base metal
    contacts, gold or rhodium plated contacts may very well be
    essential.
     
  17. Guest

    Bell Labs invented the reed relay in 1932 for used in telephone
    exchanges. Please explain what switch they used to protect the dry
    reed relay switches from having to carry current when opening or
    closing even though they had to carry some current when closed. Then
    tell me what kind of switch they used to protect the protective
    switch ...

    http://en.wikipedia.org/wiki/Reed_switch

    In fact, it looks as if we are both wrong - the only reference I could
    google claims that "dry reed" relays are so called to distinguish them
    from mercury-wetted reed relays (which really don't need any wetting
    current).

    http://www.answers.com/topic/dry-reed-relay?cat=technology

    I happen to like mercury-wetted reed relays - they offer a lower and
    more stable contact resistance than you can get from any other kind of
    relay, and they don't bounce. The fact that they have to be mounted
    within 15 degrees of being vertical can be a problem. People kept on
    offering orientation-insenstive mercury-wetted reed relays, but nobody
    evers seems to have mastered the art of manufacturing them reliably.
     
  18. Guest

    I, at least, would jump at the chance to congratulate you for getting
    something right at last.

    You do seem to have caught one of my (rare) misconceptions here, but
    since your theory about the meaning of "dry" in this context is even
    less plausible than mine you haven't exactly enhanced your own
    reputation in the process.

    Not that I'm not claiming that you are wrong all the time, but one
    could probably get equally reliable advice by consulting a random
    number generator. After all, a stopped clock is still right twice a
    day.
    That is easy to explain. An expert is someone who knows how little
    they know. You don't know how little you know, and interpret
    corrections as personal attacks, rather than educational episodes.
    Thus one more correction becomes one more attack from someone whom -
    you chose to believe - hates you, rather than one more misconception
    erased from your personal data bank (which probably ought to be
    transcribed and sent to some kind of museum of primitive engineering,
    for exhibition with the stone axes and the electronic circuits built
    around the NE555).

    You never learn, mores the pity. Obviously, you used to be able to
    learn, but you seem to have started losing the capacity a few decades
    ago and the process seems to have pretty much gone to completion.
     
  19. Robert Scott

    Robert Scott Guest

    Well, the problem has been solved. It turns out the Tyco tact switch I wanted
    to use has a minimum current of 10 microamps. My micro weak pullup sources
    typically 200 microamps with a minimum of 50 microamps, so I am within the spec
    after all.


    Robert Scott
    Ypsilanti, Michigan
     
  20. John Fields

    John Fields Guest

    ---
    Well, Bill, how nice to hear from you again, even though your
    commentary is flawed, as usual.

    In the second place, manufacturing orientation-insensitive
    mercury-wetted reed relays reliably isn't the problem, having them
    _perform_ reliably in any orientation seems to be.

    In the first place, what's being discussed are contacts used in a
    dry _circuit_, not the difference between mercury-wetted and non
    mercury-wetted contacts.

    Had you had your wits about you when you decided to use Google, you
    might have searched for "dry circuit" and found, excerpted from the

    "Dry circuit loads: No current is switched. The contacts carry
    current only after they are closed or before they are opened. The
    currents may be high, as long as they are not switched. Since there
    is no arcing, contact resistance is kept low by using gold plating
    or gold alloy contacts."

    at:

    http://www.leachintl2.com/english/english2/vol6/properties/how4.htm
     
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