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Convert 1-5V to 4-20mA: Which op-amp ?

  1. May 7, 2012 #1
    So i found this post about voltage to current conversion.

    http://www.allaboutcircuits.com/vol_3/chpt_8/7.htmlhttp://www.allaboutcircuits.com/vol_3/chpt_8/7.html

    It talks about how to convert 1-5V to 4-20mA which is exactly what I am trying to do. I have attached the circuit which I am referring to.

    I have a small question which well, might be a bit dumb but please help!

    Can I use an op-amp such as LM741 from TI in this circuit or is that amp some special kind of amp? Please help.
     

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  3. May 7, 2012 #2

    gneill

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    I may be misremembering, but I think that most versions of the LM741 can source or sink about 30 mA max. So while it may run warm, it should work if other concerns (like switching speed) are not an issue; the 741 is not the last word in op-amps, and there are many faster and higher power capable ones to choose from. what sort of data rates are you expecting?

    You could always buffer the 741 output with an npn transistor if you're concerned about overtaxing the beast, or choose a beefier op-amp that won't sweat the current. you might even be able to get away with a single supply (only positive supply voltage required) capable op-amp for your application, if that's an issue.
     
  4. May 7, 2012 #3
    By switching speed what are you referring to ?

    Actually I have only used the LM741 amp once before and actually have no experience with using amps apart from that one so I asked if I could use that one. Do you have any suggestions of other amps that I can look at ?
     
  5. May 7, 2012 #4

    Bobbywhy

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    A little searching shows three ICs that are commonly used for DC to 4-20 mA converters. They are LM10, LM163, and LH0045. You would have to look at them and their application notes to decide which.
     
  6. May 7, 2012 #5

    gneill

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    Essentially, how fast is your input voltage changing? Is it sine waves or pulses? If pulses, the speed (bandwidth) of the amplifier will determine how well the current waveform retains the "square side" shape of the pulse.
    I see that Bobbywhy has provided some suggestions.
     
  7. May 7, 2012 #6
    Thank you! I shall follow that up. :D
     
  8. May 8, 2012 #7
    Oh no! It s an analog voltage that I will control and will not be abrupt at all.
     
  9. May 8, 2012 #8

    jim hardy

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    I've used TI's XTR110 in industrial conrollers
    it's made for just that job and saves alot of work..

    see datasheet here
    http://www.ti.com/lit/gpn/xtr110

    see page 6 for input ranges and recommended pass transistor.
     
  10. May 8, 2012 #9
    Jim, actually I did read through that but could not make out head or tail of the circuits. (sorry my electrical is rather poor being a mech person). The circuit with the amp and resistor seemed really easy to understand so I thought I d try that out first.....
     
  11. May 8, 2012 #10

    jim hardy

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    Sounds great -- we learn 10X more by doing than by just reading about it.

    if the 741 isn't quite stout enough just add a transistor driver stage to output, 2n3904 or something...

    Have fun !!
     
  12. May 8, 2012 #11
    True. Things look easier on paper. It s a mess getting some things working! haha!

    Ok! thanx a lot for your help !! :) everyone!
     
  13. Mar 22, 2014 #12
    If you use the common current drive setup, the opamp drives a transistor which will drive the current and you use a resistor as feedback, any opamp which matches your voltage, frequncy, bandwith, budget needs will do. Also a LM741.

    see: for a example setup [noparse]http://www.divize.com/techinfo/4-20ma-potentiometer.html[/noparse] [Broken]

    Regards,
    Heinrich
     
    Last edited by a moderator: May 6, 2017
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