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Thermocouple surge protection

Discussion in 'Electronic Basics' started by JTM, Jun 4, 2004.

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  1. JTM

    JTM Guest

    We have some long runs of Type-T thermocouple and would like to protect them
    from nearby lightning strikes with a TVS or similar.

    If I simply connect a bidirectional TVS between each lead of the TC and
    ground will that affect the readings? Is there a better, but still compact
    and inexpensive, solution for TCs? Modifying the reading equipment itself
    is not an option.
     
  2. Soeren

    Soeren Guest

    Hi,


    I would guess "no", since it will be high impedance while not
    "activated", and the TC is very low impedance, but I fail to see the
    purpose ?
    Lightning goes to earth by the lowest impedance path and unless you
    actually earth one of the TC-leads, the TVS will do nothing and the
    lightning will possibly go to earth through your measuring device,
    killing it very very very dead in the process.

    I would try an earthed wire mesh around it, like the shield in a coax
    cable (isolated from the TC leads of course).

    You could test it with an ESD-generator (or an empty lighter with piezo
    ignition).


    --
    Regards,
    Soeren

    * If it puzzles you dear... Reverse engineer *
    New forum: <URL:http://www.ElektronikTeknolog.dk/cgi-bin/SPEED/>
     
  3. JTM

    JTM Guest

    Nearby strikes are taking us down so there must be an 'easy' route through
    the equipment. I need to route the surge somewhere besides the equipment so
    am considering a TVS or such.
    The unshielded cable is in place and, like the reading equipment, cannot be
    modified.
    thanks
     
  4. Soeren

    Soeren Guest

    Hi,



    Bummer !

    Well, it might be worth trying a TVS from each lead to earth (a Y-
    coupling) like this.
    ____________
    _____________SS______________| |
    / | | |
    * TC | | instrument |
    \_____________SS______________| |
    . | |____________|
    | |
    [TVS] [TVS]
    | |
    +--+--+
    _|_
    /// Earth

    (View in notepad or similar)


    --
    Regards,
    Soeren

    * If it puzzles you dear... Reverse engineer *
    New forum: <URL:http://www.ElektronikTeknolog.dk/cgi-bin/SPEED/>
     
  5. Bob Masta

    Bob Masta Guest

    Here's an untested idea: Add a resistor in series with each
    TC lead at the equipment, followed by reverse-biased
    diodes to the positive and negative rails of the equipment
    circuit. That will clamp the incoming spikes to the rail
    voltages, same as the ESD protection circuits built into
    CMOS logic ICs.

    If you don't want to open the equipment to tap into
    the rails, then instead follow the series resistor with
    parallel back-to-back diodes to ground. The TC
    output is so small the diodes will have no effect in
    normal operation since you'll be so far from forward
    conduction.

    You might need to fool around with the input resistors.
    Also, that might screw up some cold-junction compensators
    that use resistive networks right on each TC input.





    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  6. Bill Schuh

    Bill Schuh Guest

    Would optical isolation with a transmitter would be too expensive? I
    guess that would just put the problem into losing the transmitters
    rather than losing your current instruments. That may not be any more
    acceptable to what happens now.

    Are the TCs sheathed and ungrounded?

    I don't think a high impedance TVS as you describe would affect the
    reading but a quick test would tell.

    Bill Schuh
     
  7. Soeren

    Soeren Guest

    Hi Bob,

    And the resistors (acting with the reverse leakage of the diodes) would
    throw the measurements so far off, that guessing a random number would
    be more reliable ;)


    --
    Regards,
    Soeren

    * If it puzzles you dear... Reverse engineer *
    New forum: <URL:http://www.ElektronikTeknolog.dk/cgi-bin/SPEED/>
     
  8. Bob Masta

    Bob Masta Guest

    TCs are normally read with high-Z input measuring circuits, so there
    should be no appreciable current flow, hence no drop across the
    resistors. TC outputs are typically 50 mV or less, so the
    reverse-parallel shunt diodes wouldn't really be into conduction.
    You could use reverse-series zeners in shunt as well. (Or probably
    red LEDs, which have a super-sharp knee at 1.4 V forward bias.)


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
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