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Looking for 4-20ma circuits

Discussion in 'Electronic Design' started by hamilton, Mar 4, 2012.

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  1. hamilton

    hamilton Guest

    I have a request for a 4-20ma interface. (input and output)

    googleing has found TI / Analog parts.

    These parts seem a bit pricey for the function.

    Does anyone have any links for in-expensive 4-20ma circuits.


  2. I refer you to my (other) thread of last year. Well I would if I could
    find it.

    John Larkin posted one

  3. hamilton

    hamilton Guest

    Thanks, I take it the input is PWM ?
  4. No, that's something else.

    This is an analog instrumentation standard.. 4mA = "Zero", 20mA =
    "Full Scale", similar to the old pneumatic 3-15 PSI standard.

    HART stacks a digital protocol on top using a 1200 baud modem.

    Best regards,
    Spehro Pefhany
  5. I missed the request for an input - but can't you just use a 250 ohm
    resistor say?
    Hi, No, it's an analog voltage.
  6. Jamie

    Jamie Guest

    But, It has a cap in there for a integrator process..

    so, it may still work as a PWM input..

  7. It's in the wrong place to integrate the +input.
    No, the caps just stabilise the local voltage:current loops. An input
    voltage change notionally results in an instantaneous output current
    change. Of course a real circuit will have bandwidth and slew rate
    limits, but it is not intended to convert PWM.

  8. hamilton

    hamilton Guest

    Thank you all for your insights into 4-20ma circuits.

    The application is for machine motor control, not dangerous. (I hope)

  9. I always assumed they were related somehow though?
  10. Jasen Betts

    Jasen Betts Guest

    Digital was 20mA, (or 60mA), 4-20 is analogue.
  11. Perhaps arbitrarily picked as the same 20mA as the current loop, and
    with the same zero supression as the pneumatic standard. It sort of
    feels in the right order of magnitude for long runs, immunity from EMI
    and immunity to corrosion in connections. There were other 'standards'
    too, 10-50mA, for example. I doubt anything could run off the 4mA
    (better to count on 3~3.6mA) in the olden days, but nowadays it's
    quite common to have computers and displays as well as sensor
    excitation and signal conditioning that work off of '2-wires'.

    Best regards,
    Spehro Pefhany
  12. I had an ASR33 teletype once, I think that used the 20mA loop.

    Hey, does anyone know... We have an application where the customers want
    "4-20mA" but there are several variables to communicate. A competitor
    uses a "time multiplexed" arrangement where the values are output in a
    repeating sequence like <a for 300ms> <b for
    300ms> <c for 300ms> etc. There is some kind of sync value using an
    out-of-range current to indicate the start of the sequence.

    Do you know if any of this is standardized or directly supported in
    PLCs? Or is it all ad-hoc?


  13. josephkk

    josephkk Guest

    So, are you looking for transmitter or receiver?

  14. josephkk

    josephkk Guest

    I suspect you may have misunderstood the analogy. Nor was it presented
    well. Digital 20 mA and 60 mA were bit serial "teletype" standards were
    king of the hill long before what is now TIA-232. Like the control system
    4-20 mA it was a current loop.

  15. josephkk

    josephkk Guest

    Nonsense. For those familiar with the art 4-20 mA is clearly the analog
    loop. Right in the subject line.

  16. josephkk

    josephkk Guest

    The standard did not exclude locally powered (transmitter) devices. They
    did have to be isolated though. Ground loop thing. Worked way back in
    the tube days.

  17. josephkk

    josephkk Guest

    I think it is ad hoc, but i will bet that it uses 24 mA (error value) as
    the sync.

  18. WoolyBully

    WoolyBully Guest

  19. Now tell us the science as to why it begins at 4mA.
  20. Current is current. It is VERY precise, and properly linearized, very
    accurate at any point in the loop.
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