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4-20ma controls

Discussion in 'Electronic Basics' started by [email protected], Apr 23, 2005.

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  1. Guest

    Ive seen a lot of industrial controllers such as plcs and they all seem
    to use a signal of 4-20 ma in proportion to relay sensor information to
    the controller.

    My question is .....why 4-20 ma.....why not 1-10 ma or 1-50 ma... what
    is the reasoning between using 4-20 milliamps

    thanks
     
  2. Lord Garth

    Lord Garth Guest

    It probably comes from the old standard for a Teletype serial current loop.
    I would guess that alarm systems also maintain a loop current in this range
    in order to detect cut sensor lines.
     
  3. There have been lots of competing signal standards over the years, but
    this one has enough advantages to have become very common. First of
    all, using current instead of a voltage eliminates errors from wire
    resistance. This also allows several devices to be wired in series on
    the same current loop, as long as the total supply voltage can account
    for the sum of all their drops at maximum current.

    Having an elevated (live zero) distinguishes between a zero signal and
    a broken circuit, and allows off scale below zero readings, also. The
    minimum power delivered to the field device at 4 milliamps minimum
    current and a few volts drop is enough to power lots of micro power
    devices from sensors to microprocessors to local LCD displays. And a
    short circuit across the signal lines causes no particular damage.

    There is a similar 10 to 50 ma standard, but the extra power it
    consumes does not help with anything, since micro power instruments
    have become available.
     
  4. Bob Eldred

    Bob Eldred Guest

    Several reasons: One, The current to the device powers the device and it
    cannot to go below some minimum or there would be insufficient power for the
    device. That's why it's not zero to 20mA. Four mA was chosen as a
    convenient minimum still allowing enough to run the device. Having the
    current power the device allows the device to operate on only two wires. It
    would take a minimum of three wires for a voltage output device and
    sometimes four wires. You need power, ground and signal and maybe signal
    ground. A current 4 to 20mA device has +V usually 24 volts and signal and
    that's it, two wires.

    Second: The current in a line is always the same, barring leakage, Therefore
    there are no losses regardless of the length of the line. The resistance of
    the line does not matter.

    Third: The signal is loaded at the receiving end of the line with a sense
    resistor, often 100 ohms or 250 ohms. These are low impedance loads and help
    keep down noise and interference. Voltage lines, by contrast, are usually
    high impedance loads and are more susceptable to noise.

    Fourth: One could go to higher current but that would be wasteful especially
    in a system with hundreds of channels. Twenty milliamps is sufficient for
    most purposes. Four to twenty milliamps gives one to five volts on a 250 ohm
    load, a good range for data acquisition hardware. That still leaves 19 volts
    to run the device on the transmitting end of the line with a 24 volt supply.
    Bob
     
  5. PeteS

    PeteS Guest

    As noted - 4mA is a good 'zero' that is also a valid signal. I use
    4-20mA loop devices and zero current implies (quite reasonably) that
    the sender is either broken or not connected.

    Shorting the leads causes no damage whatever - the current will still
    be whatever it was, regardless of where the short occurs, provided it
    is beyond the sender, of course. For an industrial situation, this is
    perfect.

    Another good reason for using this vs. a higher current (besides the
    fact that this is easily sufficient) is it gives greater range for a
    given output driver. I have used (and seen used) 4-20mA loops for over
    1km, where the compliance requirements can exceed 100V, but as this is
    simply I*R(loop), a higher current would have a higher compliance (Vout
    range) for a given loop.

    I will note that many pieces of industrial equipment can actually be
    switched between 4-20mA and 0-20mA, which can actually cause a great
    deal of grief for remote monitoring if one is not aware of it, and the
    switch is on the wrong position.

    Cheers

    PeteS
     
  6. Jamie

    Jamie Guest

    4..40 ma is actually the common range for many industrial units, that is
    why when you get a little current source hand held, you will noticed
    that most of them will at least 40 ma's
    the one i use will do up to 100 ma, but i have never used it that high.
    its a nice tool if you need to calibrate a device to work with in a
    range of (ma) window far from the actual controller that drives it.
    steam/plumber fitters and the like must make sure that the valves
    for example actuate properly for the described range.
    --
    currents sourcing is nice because you can assure that you are getting
    above some levels of current ambient current caused by adjacent wires in
    raceways and the like but, this only applies to small levels of course.
    current loops are also used in close loops type of communication
    devices so that all devices on the line can monitor current activity so
    that it can talk to the other units on the same wire or not collide with
    current traffic.
    these kind of devices normally need to have one unit as the current
    source, the source current device normally tries to maintain the set
    current point was devices get added on the line.
    most devices use an optical type coupling or a LED or transistor,
    depending on the direction of flow. each device creates a lost of
    voltage that the current source supplies. at some given point, the
    current source will not be able to maintain it as devices are added in
    the loop of course, this is where repeaters come into play!
    depending on the manf and device, the number of devices can really
    get up there, for example RedLion type of serial display of a type i
    have in mind usings 20 ma current loop supplied from an 18 volt source,
    these units can address up to 100 0..99, but since each device and lose
    and average of 0.7 volts in the TX state and 1.7 in the RX state, it
    kind of limits the number of devices you can actually have on the loop.

    enough of that.
    ...:)
     
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