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Contact resistance

Discussion in 'Electronic Design' started by Yzordderrex, Jul 3, 2008.

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  1. Yzordderrex

    Yzordderrex Guest

    Greetings,

    I am using a gold pogo pin to make contact in a 2amp application. The
    metal it is in contact with is stainless steel. I am measuring 0.12
    ohms across the interface. Does this seem correct? I would like to
    know if anyone has a link to a paper which would discuss this. I
    might have to take a look at changing the stainless to something else.

    regards,
    Bob
    N9NEO
     
  2. Guest

    What method are you using to measure the resistance? Hopefully not an
    ohm meter alone. I can't say 0.12 ohm is correct but at 2-amps that's
    a 0.24v drop. How about getting 4-clip leads, a constant current
    source and a voltmeter and make a 4-point measurement. Tie the current
    source to the pogo pin and the steel plate, connect the voltmeter to
    the pogo and steel plate near the point of pogo contact then touch
    the pogo to the steel plate and measure the voltage. Apply ohms law
    and determine the resistance. If the measurement point is away from
    the point of contact you may be measuring the resistance of the steel
    plate. Perhaps you could apply multiple pogo's and/or a sensing scheme
    to minimize the losses.

    good luck!
    al
     
  3. Guest

    Stainless steel is stainless because it is covered with a coherent
    layer of not-all-that-conductive chromium oxide - 0.12 ohms sounds
    very like the sort of resistance levels that caused me to dump
    stainless steel electrodes in a conductivity meter and go for
    platinum.

    For you application gold would be traditional. Printed circuit
    manufacturers used to routinely selectively plate a micron or so of
    hard gold onto the contact fingers on plug in printed circuit boards.
    Even that couldn't take enough pressure to force metal-to-metal
    contact through the gold sulphide that builds up - someone had to
    polish the connector pins every six months or so to keep the contacts
    reliable - and two part connectors eventually took over.

    Pogo pins do provide enough force but they will probably wreck the
    target surface if you make contact more than a few hundred times,
    which is plenty for production testing (where they are normally used).
     
  4. Stainless gets abrasively polished. Any coating placed on the raw stock
    before polishing gets abraded off. Everything that stainless is, is
    throughout the alloy matrix that makes up the medium.

    He should go by a set of lawn mower engine points. Easy to find and
    nice-n-cheap. Both contacts are platinum plated, which NEVER oxidizes,
    and is the proper point contact medium if longevity is a concern. All
    that would be needed then is an actuation mechanism.
     

  5. Another solution is to place a rheostat style variable resistor inline
    with the contact being used, and dial up the resistance during placement
    and removal of the contact pin.

    Hot swapping heavy amperages always leads to degradation of the "points"
    between which the circuit is "make" or "break". If there is any inductive
    loading, there may be a reversion spike on "make" and "break" as well. So
    accounting for that additional amperage or "pulsed flow" is wise as point
    degradation is fast and furious in most hot switched applications.
     
  6. Al

    Al Guest

    Contacts should be like metal to like metal. The gold to stainless will
    be a problem. There are many papers out there on the subject.

    Al
     
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