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thermocouple input, 0..20mA output

Discussion in 'Electronic Design' started by Eur van Andel, Sep 13, 2004.

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  1. Hi

    I needed a circuit that converted a thermocouple (40 uV/K) to a 0..20mA input
    of a PLC. Normal opamps can't touch the rails and I didn't like creating a
    negative voltage. All I had was 24V DC. Here is my circuit:
    (fixed pitch)

    +24V DC +24V DC
    | |
    | |
    | | | |
    | |\| 1k | |
    +-|-\ ___ |/ |
    | >--|___|--| |
    /---------|+/ |> | 0.8 Ohm current output: 0..20mA
    * |/| | | ___
    \---------------+ +---+--|___|+->|-->|-->|--o--+
    thermocouple | | | |
    0..16mV | +--------------------+ .-. inside PLC:
    0..400C | | | 250 Ohm to GND
    | | |
    | '-'
    | |
    | |
    === ===
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    The thermocouple generates about 16mv @ 400C (400*40uV), there is no
    cold-junction compensation (with a working point of 300C, I don't need one).
    The opamp tries to match the 16mv difference and raises its output. The NPN
    boosts the current (if your opamp can't source 20mA) until 20mA*0.8Ohm = 16mV
    over the resistor, which is fed back into the negative opamp input.
    Most opamp outputs and inputs can't come near the rails and since it is current
    output only, I added some diodes between the current measuring ressitor and the
    PLC input. That way the thermocouple and the opamp in- & outputs are raised far
    enough from the bottom rail.

    The 1k is a currrent limiting resistor to protect the NPN transistor.
    Measure the polarity of the thermocouple while holding it in a flame.
  2. James Meyer

    James Meyer Guest

    Wouldn't a single resistor work just as well as a string of diodes?

  3. Eur van Andel wrote...
    [ see edited version below ]
    A pretty nice circuit. I've taken the liberty to make a few changes.

    All single-supply opamps, and most low-cost chopper opamps, etc., work
    down to the negative rail, so the diodes aren't necessary. I replaced
    the 0.8 ohms with a more common 1.0 ohms, and made up for the change by
    adding a little gain with the opamp. This also provides a possibility
    for different temperature ranges, or gain calibration for various types
    of thermocouples.

    .. ,-----------+----------- +12 to +28V DC
    .. | |
    .. ,-------2k49---------,
    .. | |\| select | |
    .. +--|-\ |/ |
    .. | | >-- 1k --| |
    .. o--+-2k----|+/ |\> | current output:
    .. / | | |/| | | 0..20mA
    .. * 470k 10k0 | '---+-- 1R00 --+----o
    .. \ | | | LT1013 | |
    .. o--+----+-------------------------------' .-. inside PLC:
    .. thermocouple | | | 250 ohms
    .. 0..16mV | | | to GND
    .. 0..400C | '-'
    .. | |
    .. === ===
    .. GND GND

    A resistor on the opamp's + input cancels voltage offset errors from
    the input bias currents and also protects the opamp in case of static
    discharge into the thermocouple wiring. Adding a pair of back-to-back
    diodes from the opamp's + input to the other thermocouple lead would
    provide further protection. The 470k input resistor prevents a high
    output current in the event of a disconnected thermocouple (but watch
    out, because the unit will falsely indicate +175C or so).
  4. Usually we want an instrument such as this (for temperatures above
    room temperature) to fail at somewhat about the 20mA range when the
    T/C opens so that heat source will turn *off*. For a general purpose
    instrument, it's thus necessary to add another bit of circuitry to
    limit the current to 21-25mA or so, as well as putting a bit of bias
    current through the sensor (perhaps the op-amp Ib if it's the right
    polarity and about the right value, but I usually like to control it).
    If it's a one-off hack for a specific application, some series
    resistance can be added on the output that limits the current,
    assuming the loop supply voltage is known and regulated, and assuming
    the load resistance is known. Obviously without cold-junction
    compensation, RF filtering, and output polarity protection, this
    circuit is very limited in application anyhow.

    Also, note that all these circuits will behave rather badly with
    grounded-junction thermcoouples (which are otherwise generally
    preferred). Adding isolation while maintaining loop-power is

    Best regards,
    Spehro Pefhany
  5. Rich Grise

    Rich Grise Guest

    If you're going into a PLC, you probably want 4-20 mA where 4 is the lowest
    possible value. Typically, in 4-20 mA control loops, 0 means a fault of some

    Have Fun!
  6. Spehro Pefhany wrote...
    "behave rather badly" = fail completely! It seems clear this
    circuit is entirely too simple for most serious applications.

    Didn't we have a thread on isolated-sensor 4-20mA loop circuits
    a few years ago?
  7. I liked (aka made money with) the flying capacitor
    with thermocouples. Auto-zero the amplifier on the
    cold junction whilst the cap is over on the t/c.
    Solid state switching if the CMV is known to be low
    (eg, just to avoid any earth loop), or use a relay
    for high CMV and/or extreme CMRR.
  8. Not specifically, that I recall, though we've talked about some
    elements of the problem.

    Best regards,
    Spehro Pefhany
  9. Fred Bloggs

    Fred Bloggs Guest

    The OP mentioned that the junction is in a flame- so probably not grounded.
  10. That's not necessarily true. It's common to TIG weld the junction into
    the end of a metal protection tube or to TIG weld a mineral-insulated
    swaged T/C junction together with the outer shell, which is in turn
    grounded by the mounting bracket or compression fitting. Insulating
    the junction electrically from the protection tube (if present) slows
    down the response, costs more, and tends to be less reliable.

    Not so much of a factor in a relatively low temperature system such as
    this one, but I've gotten burned by DC leakage in high temperature
    insulation affecting accuracy. Best to keep relatively high DC
    voltages off the T/C if possible- AC can be filtered out easily.

    Best regards,
    Spehro Pefhany
  11. You're welcome.
    You are very right. The circuit (probably the opamp) has died already in the
    industrial environment where it was used. The 2 kW heater is switched by
    turning on a toroid transformer, which generates huge EMF pulses. The
    optocouple leads are isolated and coaxial at the sensor end, but after 20" is
    is a simple twin wire, non-twisted, about 3' long.

    I'll post the next version here too.
  12. This PLC can do 0..20mA as well, which makes it easier for me.

    If I had to do it, I'd use a PIC. I don't like PLCs.
  13. That was the testing method.

    The thermocouple is in a heated Aluminum block. It is a coaxial type K juction,
    isolated in a stainless (316) sleeve.

    Farnell 424-8326 exactly this one
    Conrad 120757 similar, not the same
  14. That mineral insulated one is isolated-junction. Made by Labfacility
    in the UK. I wonder if the same guy still runs the place- anyone
    happen to know? A big jolly fellow, can't recall his name atm.

    Best regards,
    Spehro Pefhany
  15. It works, too.
    SCH and BRD file in Eagle format. ( For these board sizes, Eagle
    is Freeware. Also PNG's of board and schematic and the circuit on my bench,
    with ugly 10k pot mod for the 2k2, which doesn't work?

    2k2 gives a zero current of about 2.7 mA.

    I forgot the 2k as well :-|
  16. Could you have picked a worse op-amp for use with a thermocouple?

    Best regards,
    Spehro Pefhany
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