2-5V to 4-20mA converter

Discussion in 'The Projects Forum' started by dhooker, Jul 10, 2009.

  1. dhooker

    Thread Starter Member

    Jun 9, 2009
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    I know there are loads of designs out there but a quick, simple (few components) solution is what I am after.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    That'll take a couple of opamps, a half-dozen resistors or so (mostly fixed), a transistor and a few caps.

    Is that too compex?
     
  3. millwood

    Guest

    one opamp, one transistor and a few resistors.

    you can also do it with a lm317-type device.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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  5. SgtWookie

    Expert

    Jul 17, 2007
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  6. ELECTRONERD

    Senior Member

    May 26, 2009
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    Yikes! :eek:
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    Wow, Gunny - no wonder I'm so impressed. Op amps it is.
     
  8. pilko

    Active Member

    Dec 8, 2008
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    20
  9. SgtWookie

    Expert

    Jul 17, 2007
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    The attached should do it.

    R1 through R4 and Vee (-5v) are critical values.

    R5 is just there to limit maximum output current from the opamp in case Q1 fails.

    Q1 is used as a voltage follower. Although the LM358/LM2904 opamp used can source a minimum of 20mA, that's pushing the specifications. Just about any general-purpose NPN transistor could be used instead of a 2N2222.

    Maximum load impedance is about 728 Ohms with a 20v supply.
     
  10. millwood

    Guest

    if you use a chip like tda2030, or lm1875, and your load can be floated, you don't need that transistor. just get rid of R5 and put the load between the opamp's output, and tie the other end to the feedback, then you have gotten yourself some CCS!
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    LM1875's run around $3/ea.
    TDA2030's run around $2/ea.
    LM2904's run around $0.40 each, and the transistors are around $0.10 if you're buying singles.
     
  12. dhooker

    Thread Starter Member

    Jun 9, 2009
    11
    0
    Many thanks for all your ideas
     
  13. BillB3857

    Senior Member

    Feb 28, 2009
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    My guess is that $20 is for the 4-20 conversion and $100 is for the isolation.:)
     
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