4-20ma..need help

Discussion in 'The Projects Forum' started by perjan6666, Jul 20, 2013.

  1. perjan6666

    Thread Starter New Member

    Jul 7, 2013
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    hello is it possible to create a circuit by an lm324 and a 2n3904 transistor to make 4-20ma?

    i have this circuit and its working great only differense is the digipot that one is a X9C103 10kohm but i can only get around 5 to 17 ma. tryed to change values of resistors but i have a lack of know how :p

    so if anyone can help me i will be happy

    thank you
     
  2. perjan6666

    Thread Starter New Member

    Jul 7, 2013
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  3. tubeguy

    Well-Known Member

    Nov 3, 2012
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    To start, you can decrease R17 to get more current. Or decrease R14 on the input of the LM324 to increase the voltage input which will increase the current out.
    Hint: The voltage applied across R17 determines the current output.

    Do you need very accurate min and max values, or just close ?
     
    Last edited: Jul 20, 2013
  4. panic mode

    Senior Member

    Oct 10, 2011
    1,321
    304
    voltage divider formula is: Vout = Vin * Rb/(Ra+Rb)
    that's all you really need to solve this issue.

    wiper voltage of digital pot ranges 0-5V (it should go 0-Vcc and here Vcc is 5V).
    This is voltage range you should be able to observe on lower end of R13 when you select min/max current value (verify this!!!). If this range is not 0-5V then we need to know what it is to calculate resistor values.

    In current configuration:

    when digiat pot wiper output is 5V, then voltage between R12/R13 is 4.4V and this is further divided by R14/R15 to 1.98V at opamp input.
    1.98V/R17 = 1.98V/100 Ohm = 19.8mA (close to 20mA but not quite)

    when wiper of digital pot is at 0V, we get 0.9V at R12/R13 which is further divided by R14/R15 to 0.4V, then
    0.4V/R17 = 0.4V/100 Ohm = 4mA

    so the output is not 4..20mA, it is expected to be 4...19.8mA, but still this looks better than what you are observing.

    possible issues can be:
    1. 5V is not exactly 5V
    2. for whatever reason your circuit does not produce 0-5V at wiper of digital pot
    3. your resistor values are off (you should use 1% or better for R12, R13, R14, R15 and R17)
     
  5. panic mode

    Senior Member

    Oct 10, 2011
    1,321
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    if you need quick fix:

    to increase max current you can either:
    - reduce R17 (by adding 820 Ohm for example in parallel with R17).
    - reduce R14
    - increase R15

    note, all of this will increase max current (but also shift upwards the low limit of 5mA). ultimately you could replace R14/R15 with trimmer potentiometer (wiper to pin 3 of opamp) and call it "max current"


    To increase span, (get wider range) you can either:
    - increase R12
    - decrease R13

    Of course you could replace R12/R13 by another trimmer pot and call it "span".
     
  6. perjan6666

    Thread Starter New Member

    Jul 7, 2013
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    thanks for answer. yes this is to control frequensy converters for 3-phase motors and they have 5v output to the loop and sense between 4-20ma.

    i put a pot in the place of R17 and R14 and i can adjust the current but then its the same "gap" like i got 20 on max and 10 on min or 4 on min and 14 on max.

    i need to adjust both max and min values..

    thanks for answer :)
     
  7. perjan6666

    Thread Starter New Member

    Jul 7, 2013
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    thank you for explaining how the op amp works i think i understand :)
    to morrow i will measure and see, i think the voltage is not 0-5 on the digipot. so its just to calculate the new values :) is it some calculators online for this? it will make it easy for me since im no professional :p
     
  8. perjan6666

    Thread Starter New Member

    Jul 7, 2013
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    0
    now its all working. i put the pot where you told me and it all become adjustable:) the voltage was 0 on the digipot with the wiper at bottom. at max i got 4.80 volt with the loop connected to the 5v rail where i have a 5 volt voltage regulator. when i disconnected the loop i get 4.90 volt eaven the voltage regulator is rated for 1,5A anyway the loop wil be powered by the frequency converter so then it is just to adjust the pots to make it work. thank you wery much mow i need to make a pcb :)
     
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