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Isolated 4-20mA output

Discussion in 'Electronic Basics' started by selbakan, Oct 12, 2006.

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  1. selbakan

    selbakan Guest

    Dear Sirs & Ladys

    I have touchpoint4 four channel gas detector controller equipment.This
    controller is giving Isolated 4-20mA output.How can I measure with
  2. Ban

    Ban Guest

    Put resistor 100 ohm between out and gnd, measure 2V=20mA.
  3. A common method is to add a silicon diode in series with the
    current on some convenient terminal strip. Test by
    connecting a milliamp meter across the diode. If the meter
    has a resistance low enough that its total voltage drop, at
    20 mA is less than about .3 volts, essentially all the
    current will be diverted through the meter, and none will
    pass through the diode. This uses up about the minimum
    voltage possible, leaving the maximum for the other
    resistances in the loop.
  4. Jenalee K.

    Jenalee K. Guest

    John Popelish a écrit :
    Why the diode?

    Jenalee K.
  5. Chris

    Chris Guest

    A current source has a voltage limit based on its power supply. For
    instance, if you add a 1K resistance in series with your current loop,
    and the power supply of the current source is only 12V, it will only be
    able to source 12mA (12mA * 1K = 12V). Placing a diode in parallel
    with the ammeter ensures that:

    1) The added voltage burden on the current loop will never be more than
    one forward diode drop (0.7V), and

    2) In the event of an open fuse on the ammeter or inadvertent open
    circuit (errant test leads), the gas detector will still work.

    Mr. Popelish speaks from experience. Particularly if the gas detector
    performs a safety function, it's good form to ensure the test won't
    interfere with equipment operation.

  6. It carries the loop current while limiting the voltage drop
    to less than a volt, when you are not measuring the current
    with the meter.
  7. Bob Eld

    Bob Eld Guest

    A 4 to 20mA detector is a two wire device. It's output is simply the current
    that it draws from a power source. The power source is usually 24VDC. All
    you have to do is measure the current it draws. You can do this with a
    current sense resistor in series with the controller and ground and measure
    the voltage across it. Or, you can place the multimeter, in a current mode
    in series with the controller and ground. Either way, the sensed current is
    the output.
  8. Jamie

    Jamie Guest

    you can get a low level DC clamp meter but the simplest thing
    to do is use your DMM in series with the connection in AMP mode or
    get our self a stand alone current and monitor source unit supply
    designed to test and align the units.
  9. Jenalee K.

    Jenalee K. Guest

    John Popelish a écrit :
    Oh, I see. You mean that the diode is there all the time and just
    provides a test point so that you can measure the current without
    breaking the loop, right?

    Jenalee K.
  10. Yes.
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