4-20 ma to 1-5 V correctly?

Discussion in 'Embedded Systems and Microcontrollers' started by Inthewoods, Mar 12, 2013.

  1. Inthewoods

    Thread Starter New Member

    Mar 12, 2013
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    Im looking for some clarification on a setup. I am using a RJ-45 breakout connector where each conductor has a screw terminal on a circuit board.
    I have a 4-20 ma pressure sensor that is powered by a 12 volt or 24 volt PS. There are two wires from the sensor for the loop current.

    I am connecting the signal to a ADC controller that can read 1-5 volts.

    The red will connect directly to the PS.

    Will the black have the 250 ohm resistor pigtailed off the black wire to read the 1-5 volts of the pressure sensor?

    Just ensuring I have it all correct.

    Thanks.
     
  2. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Much easier is to post the actual circuit.
     
  3. Inthewoods

    Thread Starter New Member

    Mar 12, 2013
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    I attached a crude diagram. Maybe it will make sense.

    Two leads for the power and tapping off the negative wire with the resistor for the 1-5 Volt reading.
     
  4. ErnieM

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    Apr 24, 2011
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    FYP:


    [​IMG]
    The resistor needs to be in series with the sensor to sense and drop the current to produce a voltage.
     
  5. Inthewoods

    Thread Starter New Member

    Mar 12, 2013
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    so put the resistor between the positive side and the negative.... the reading will post from the negative side?
     
  6. ErnieM

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    Apr 24, 2011
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    OK, Fixed MY post:

    [​IMG]
    The 1 to 5V gets dropped across your resistor. Place it this way and the 1-5V signal is referenced to ground.

    Usually that makes handling the signel simpler.
     
  7. ErnieM

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    Also note: this assumes the sensor can work correctly off 7 volts (it no longer sees the whole 12-24 v power).
     
  8. thatoneguy

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    Feb 19, 2009
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    A "correct" way to do it is in This Maxim App Note 823

    It uses a 10Ω sense resistor, and op amps to get the voltage to the 0-5V range.

    The single resistor will work, IF:

    • The Device runs on 7V
    • no other devices are in the loop
    • no other devices are going to be added to the loop in the future
    If those are met, then you can use the single resistor for a single sensor. If there is any possibility the system may be expanded by another person, etc, later on, there may be issues.
     
  9. ErnieM

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    I'm glad you put "correct" in quotes. I'm sure it works, but it uses a lot of parts, and it has the wrong range: the OP InTheWoods asked for 1-5V, this has 0-5V.

    I don't see any real need to use a high side current monitor either. Why not just drive IC2 pin 3 from a resistor to ground? That only makes sense if you're a large chip manufacturer looking to Maximize the number of chips you sell.

    The 7V limit was stated, but as it is a crucial it can't be stated enough.

    In any current sense loop then can only be one and only one device as the device sets the current thru the loop. You need to place multiple loops in parallel so each loop current is independent of the others.

    It can't be stated enough the sensor needs to be able to work down to 7 volts.

    Personally I'd prefer the 0-5V range but it's not my project to choose that. :rolleyes:
     
  10. #12

    Expert

    Nov 30, 2010
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    I heard a rumor that it is good to use a little bit lower resistance to make sure the output never exceeds the 5 volt limit of the ADC...then compensate in the software. The real problem here is that I'm way tired from being up all night and can't seem to commit to an idea while my brain is so fuzzy. Please consider this conservative method to determine if it is right for you.
     
  11. thatoneguy

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    Feb 19, 2009
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    I should have specified "other measurement devices", such as a second tap for another ADC sensor that is unrelated. The voltage drops would become an issue, as would this resistor becoming floating (or an added one).

    I can see a reason for 1-5V, under 1V and there is a malfunction, but using an op amp to make it ½V-5V would suffice for that, and fill the ADC range as well.

    In any case, the resistor the OP uses must be a precision, 1% or better, resistor, with a stable temperature response, otherwise the ADC reading will vary with ambient temperature.
     
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