convert 0-1ma to 0-20ma

Discussion in 'The Projects Forum' started by Dead Crow, Apr 22, 2010.

  1. Dead Crow

    Thread Starter Active Member

    Mar 6, 2010
    34
    1
    Hey Pls help
    I have a hydrocarbon analyser which provides a 0-1ma signal .
    Our Festo PLC modules only take a 0-20ma or a 4-20ma signal.Can i use a transistor of sorts to change the signal? I dont think its wise to increase the current through the existing line as it may be too much current for the existing circuit. I think perhaps i could use a transistor which isn't simply on/off (digital) a more analogue type transistor to switch a separate supply of 24VDC with a resistor may work? However i have no idea how to select the components :D
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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  3. SgtWookie

    Expert

    Jul 17, 2007
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    The attached circuit will do what you want, within the limits of what your Rload is and what your Vcc is.

    You'll need to use an opamp that can sense down to the negative rail, and who's output can be within about 0.6v of the negative rail.

    Not shown are a bypass cap across the opamp's Vcc/GND connections; it should be 0.1uF metal poly or ceramic. It would not hurt to use a 1nF-10nF cap to ground from the non-inverting (+) opamp input.

    If you use an opamp that has more than one amp in the package, you should wire the other opamps as voltage followers; output to the inverting input, and noninverting input to a voltage reference between Vcc/GND.

    You could use the output from the amp in use as a reference voltage.
     
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  4. hgmjr

    Moderator

    Jan 28, 2005
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    For those interested and not up on the circuit that sgtwookie has provided, the current gain for the circuit is based on the ratio 1000/50 which equals the required value of 20 requested by the OP. The figure 1000 is the "current-to-voltage converter" (a.k.a. resistor) while 50 is the value of the current sense resistor in the emitter of the transistor.

    hgmjr
     
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  5. Dead Crow

    Thread Starter Active Member

    Mar 6, 2010
    34
    1
    thanks very much guys I'm gonna try it do it correctly as shown
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Note that 50 Ohms is not a standard value of resistance.

    However, you can use two 100 Ohm resistors in parallel to get 50 Ohms, or four 200 Ohm resistors in parallel, etc.
     
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