Measuring voltage on a 4-20mA output

Discussion in 'The Projects Forum' started by russellhawley, Feb 5, 2016.

  1. russellhawley

    Thread Starter New Member

    Sep 15, 2015
    2
    0
    G'day,
    Quick question regarding taking measurements on 4-20mA circuit.

    We have a 24vdc control system on site controlled by PLCs. Most instruments feedback a 4-20mA analog signal to the PLC inputs. Now if I'm in the field taking a voltage measurement with a multimeter at the output terminals of a transmitter what reading should I expect? I would have expected to see between 1 and 5v. On some transmitters I do get a 1-5v reading but others I just see 24v.
    If I break the circuit and place the leads in series I get the correct mA reading but donot always see the corresponding 1-5v when measuring across the output terminals.

    The actual instrument in question is a Rosemount Flow tube/Transmitter. A number of these on site give me 1-5v across their outputs whereas another sees about 24v.

    Cheers
    Russ
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    You can not measure a voltage on a current source.
    You will need a load to measure a voltage.
    If you want to convert 4-20 mA to 1-5 Volts, you will need a resistor of 250 Ohms as load.

    Bertus
     
  3. Picbuster

    Member

    Dec 2, 2013
    374
    50
    Bertus is correct however; the 250Ohms should include the wire resistance and could, due to the voltage drop, put the other equipment in the loop in error. (effect in 2 wire system is different than in a 3 or 4 wire system)
    A better way is to use 50Ohm in series and measure 50* 20*10^-3 = 1V max.
     
  4. russellhawley

    Thread Starter New Member

    Sep 15, 2015
    2
    0
    There is already a 250 ohm resistor in the PLC circuit. I'm curious as to why sometimes 1-5v is measured and other times a 24v reading is seen?
     
  5. ebeowulf17

    Active Member

    Aug 12, 2014
    678
    79
    Bertus is correct. The voltage you read depends on many factors. The transmitter isn't controlling for voltage output, it's controlling for current output.

    How many receiving devices are on the circuit connected to the transmitter in question? Are the receivers on that circuit of the same type and quantity as those on circuits with other transmitters? The transmitter, if working properly, is going to limit current flowing through it to the 4-20mA range described, but the voltage at the transmitter that it takes to do that depends on how much resistance there is in the rest of the circuit.

    Is this a 2, 3, or 4 wire transmitter? Is it possible that you're reading across the wrong wires? Are you reading these voltages on a transmitter terminal relative to ground, or relative to another transmitter terminal?
     
  6. ebeowulf17

    Active Member

    Aug 12, 2014
    678
    79
    To see a voltage scaled the way you expect, you need to be reading across the 250 ohm resistor, not any where else.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,798
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    If the transmitter is inactive and drawing no current then presumably you would expect to see 24V?
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,047
    By inactive I think you mean unloaded? If it's trying to drive 4-20mA thru air, it'll put out its max voltage.
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,798
    1,103
    Yes. That's a better term.
     
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