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Schematic to build a 4ma-20ma supply

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by rperch, Jan 17, 2009.

  1. rperch

    rperch New Member

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    I would like to build a circuit using a 24VDC power supply that would give me an output of 4-20ma or wider range to supply the loop power to manually control a valve while a new controler is installed. My initial idea is to use a
    24VDC Omron power supply with a 5K pot and a 1.2K resistor. I think this should work but is there something that would be better. This project would also provide us with a piece of equipment for bench testing. Any ideas or suggestions would be appreciated.

    R. Perch
     
  2. Analog

    Analog New Member

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    The DC supply is unlikely to work in the manner you mentioned, because you did not take into consideration the resistance of the valve actuator (unless I missed something). Why not build a current source with a variable output range? This can be done with an op-amp and transistor in the feedback loop. Typically they are considered voltage to current converters, and the voltage being converted is set by the potentiometer (thus giving you the desired output current).
     
  3. rperch

    rperch New Member

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    Thanks for the reply, this looks to be what I want. Could you advise where I could look, for information for the parts and design of the circuit.

    Thanks R. Perch
     
  4. dave

    Dave New Member

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  5. Leftyretro

    Leftyretro New Member

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    Yes, a simple 24vdc source and a variable 5k pot (suggest using a 10 turn precision pot) can power a loop powered 4-20ma device. However you will have to have a accurate DVM meter set to milliamps in series with the loop to see and measure what you are sending to the device as you adjust the pot. Lose the 1.2K resistor as you will never reach 20ma because of the added resistance of the loop connected device.

    Lefty
     
  6. Mikebits

    Mikebits Well-Known Member

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  7. rperch

    rperch New Member

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    Great information guys, I'm a plant electrician at a new Ethanol plant, and have never been involved with instrumentation before thanks for the support.

    R. Perch
     
  8. Grossel

    Grossel Member

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    I have this very simple solution using only one bjt.

    With your window og 4 - 25 mA, you'll have to find out the voltage over the resistor. What value, let me guess 100 ohms. Lowest voltage must be U_R.low = I.low * R = 4mA * 100ohms = 400mV. The highest voltage is U_R.high = I.high * R = 20mA * 100ohms = 2V.

    Because of the voltage follower, we don't care about the voltage drop from base-emitter, the opamp will handle that.

    Then you'tt have to put your 5k potmeter in series with two resistors. One above and one belove. The potmeter will work in a voltage window of 2v - 0.4v = 1,6v. That means the current through R.var = 1.6V/5kΩ = 0.32mA.

    Value of R2 must then be 0.4v/0.32mA = 1250Ω. Value of R1 must be: (24v-2V)/0.32mA = 68.75kΩ.

    That means you can use your 1.2kΩ resistor get litle less then 4mA minimum, and you can use a 68k (or a 82k) resistor.
     

    Attached Files:

    Last edited: Jan 20, 2009
  9. rperch

    rperch New Member

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    Thanks Grossel, I 'm going to try experimenting with the circuit you provided, I had something else that I was thinking of using but have been unable to attach a copy of it to a reply to get another opinion, its a bmp file.

    regards R.Perch
     
  10. Boncuk

    Boncuk New Member

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    Hi,

    with this circuit you don't have to experiment.

    I suppose the valve is driven by 24V and requires a control signal from 4 to 20mA connected to a burden resistor in the control circuit converting the 4 to 20mA to a control voltage of 2 to 10V.

    This circuit does the same.

    Regards

    Boncuk
     

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    • 20mA.gif
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  11. rperch

    rperch New Member

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    Just a note the voltage output needs to remain constant, close to 24VDC and the current needs to be variable 4-20ma or greater in range. I've opted to use an LM317 and as stated before and have made up a schematic using MicroSoft Paint but I don't know how to attach it to a message so I can get some opinions on it.

    Thanks R. Perch
     
  12. blueroomelectronics

    blueroomelectronics Well-Known Member

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    If the load is constant the voltage will vary when you adjust the current. What are you building?
     
  13. rperch

    rperch New Member

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    I'm trying to build a 4-20ma supply to manually control an actuated control valve while we have the automatic controller repaired. the device is in a 24VDC self powered control loop. When I've checked the voltage from the analog output card it has been close to 22-23 volts. I've checked the current in the loop but I never looked at the voltage / current values at the same time.

    R.Perch
     
    Last edited: Jan 22, 2009
  14. blueroomelectronics

    blueroomelectronics Well-Known Member

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    Is it just a solenoid or is there some electronics on the valve?
     
  15. rperch

    rperch New Member

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    The valve has an electronic current transducer that controls the air pressure to the pneumatic actuator that controls the position of the valve.

    R.perch
     
    Last edited: Jan 22, 2009
  16. blueroomelectronics

    blueroomelectronics Well-Known Member

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    Do you have a part number or datasheet for the valve. Usually 4-20ma is a current loop for communications over long range.
     
  17. mvs sarma

    mvs sarma Well-Known Member

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    I presume, if to be preset constant current source, the setting range being 4 to 20mA. In such case, the voltage will vary.
     
  18. rperch

    rperch New Member

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    We have about six diferent types and this way of control is an industry standard. They are called current to pressure transducers, they are Mfg. by ABB, Foxboro, to name a few. As mentioned before they are rated for 24VDC /4-20ma analog input. The ABB Model is a TZID-C, Type V18345-20222420001.

    R. Perch
     
  19. Analog

    Analog New Member

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    Mr. Perch,

    As I had mentioned before, and somebody posted, an op amp with a bjt in the feedback loop is one way to do this.

    A more efficient method would be to simply use an IC that does this internally for you. The XTR-110 from Burr-Brown (now TI), is just such a circuit. You merely connect a pot to it and power, and it will then provide the current loop for you. Here is a link to the data sheet:

    http://www.electro-tech-online.com/custompdfs/2009/01/xtr110.pdf
     
  20. KanLok

    KanLok New Member

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    Hi Boncuk,

    I have the same application as you had mentioned. I am being beginner in electronics, could you please give a line diagram (schematic) for how to exactly connect the LM317, R1(62.5ohms) and R2 (Pot,250Ohms).

    Thanks & Regards,
    KanLok
     
  21. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    For a commercial application I like using these units. However, if you want a roll your own circuit later today when I get to work I'll post part of a drawing for a small voltage driven (ten turn pot) with a 4 to 20 mA out loop I have been using on valves. I also like the Calex units I linked to.

    <Edit> I found the attached. The negative can be dropped and just the op amp and one transistor can be used. The values would need modified but the general idea is here. </EDIT>

    Ron
     

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    Last edited: Mar 27, 2012

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