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4-20ma current loop question — Parallax Forums

4-20ma current loop question

xanatosxanatos Posts: 1,120
edited 2012-12-01 11:19 in General Discussion
OK, so I have a PLC designed to receive a 4-20ma current loop signal. 20ma = highest point, 4ma = lowest point. The specs say it's a 24volt system. But when I put 24 v across the 250 ohm PLC load, of course I would get a lot more than 20ma.

For example, if I was to put 24v into 250 ohms, I would get 96 ma (I=V/R = 24/250 = 0.096A)

To get 20ma into a 250 ohm load, I would really need 5V plus the V drop of the loop, correct?

Why do they spec it as a 24v system?

Thanks for disconfuzzlating me,

Dave

Comments

  • kwinnkwinn Posts: 8,697
    edited 2012-11-29 18:58
    The system is powered from 24V, which is common for industrial and building automation. The 4-20 mA loop is usually an analog signal from a sensor of some kind and when it is connected to a 250 ohm resistor it produces a 1-5V signal for input to an adc.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-11-29 19:01
    24V also provides plenty of headroom in systems with long lead runs or where multiple receivers are connected in series. The main thing to keep in mind is that a 4-20 mA transmitter is a current source or sink, whch has to be "compliant" with a range of voltage supplies.

    -Phil
  • xanatosxanatos Posts: 1,120
    edited 2012-11-30 08:32
    I've built a small 4-20ma "receiver simulator", basically just a 250 ohm resistor and some jacks to plug my meter leads into so I can simultaneously monitor voltage and current. I am guessing, in playing around with driving this item, that there must be some good ways in the TX side if the loop of guaranteeing that a current in excess of 20ma is never delivered to the PLC (such as in the case of a component failure in the TX side)... anyone have any good links to some schematics I could peruse on that subject?

    Thanks!
  • Tracy AllenTracy Allen Posts: 6,664
    edited 2012-11-30 20:13
    Some devices have a second feedback loop that hard limits at 20mA. However, for simple, reliable loop protection I've used a soft limit using the classic two-transistor circuit:
    limiting.png

    It starts to enter limiting at less than 20mA, as Q2 starts to turn on and robs the base of Q1, and is fully limiting at about 25mA. The transistors are inside the feedback loop that regulates the loop current.
    limiting.png
    584 x 202 - 17K
  • kwinnkwinn Posts: 8,697
    edited 2012-12-01 11:19
    A 3 terminal adjustable regulator with a 60 ohm resistor between the output and adj pin would provide a 20mA constant current source to the output circuitry of the sensor.
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