simple circuit for measuring current

Discussion in 'General Electronics Chat' started by mscerri, Mar 21, 2015.

  1. mscerri

    Thread Starter New Member

    Nov 22, 2012
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    Could you help me out in this setup please? How should I connect it to determine actual current? I am using opamp OPA335

    Thanks in advance!
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    We frown on hijacking someone else's thread. Now you have your own.
     
  3. MrChips

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    You will notice that that circuit is an inverting amplifier. Hence a positive voltage on the input will result in the output trying to go negative if the power supply will allow it to do so.

    What you want is a difference amplifier that will amplify the DC voltage across a resistor.
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Often it is done like this using a single supply.
     
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  5. PeterCoxSmith

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    Feb 23, 2015
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    normally this circuit is used to measure low currents such as from a photo diode...
    e.g.

    [​IMG]
     
  6. MCU88

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    Mar 12, 2015
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    This is called an transconductance amplifier true?
     
  7. PeterCoxSmith

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    Feb 23, 2015
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    transconductance amplifiers are voltage controlled current sources e.g.

    http://www.ti.com/product/opa861
     
  8. MCU88

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    Transimpedance amplifier sorry.
     
  9. PeterCoxSmith

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    Feb 23, 2015
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  10. MCU88

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    Mar 12, 2015
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    So it is like the mosfet Vs the BJT. The mosfet is an voltage controlled current source, whereas the BJT is an current controlled voltage source. I've never used an MOSFET (Metal Oxide Semiconductor Field Effect Transistor) -- before. Memorized the anocrym for an exam years ago and have never forgotten it.

    Seems a bit strange / expensive / specialized using the OPA335 OPAMP for such an fundamental building block. The more common TL072 JFET dual OPAMP is perhaps more appropriate?
     
  11. PeterCoxSmith

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    Feb 23, 2015
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    there are 4500 different types of opamp on the Farnell website. You could choose any one of them, as appropriate, for your application.
     
  12. MCU88

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    That's enough to send you mad. Farnell in Australia is called Element 14 now. Don't think business is so crash hot today. Old money, BIG profits from the 80's and the 90's maybe paying the bills.
     
  13. mscerri

    Thread Starter New Member

    Nov 22, 2012
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    Sorry for that. Didn't know.
     
  14. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    IMHO, the easiest way of measuring current in DC circuits is to use a dedicated IC like a ZXCT1009 and its cousins.
     
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  15. mscerri

    Thread Starter New Member

    Nov 22, 2012
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    Hi Peter,

    First of all thank you for yor interest to reply to my difficulty.

    I have just realised what you have just said, that a transimpedance amplifier are not intended for supply line current measurement.

    I intend to build a circuit that is able to capture the current drawn in the ranges on mA and uA. The main reason for this is to observe how power is consumed in battery powered device.

    Would you be so kind to guide me please seeing that you are proficient in this area? I have modified my circuit to match the circuit which you provided earlier. [http://forum.allaboutcircuits.com/b...ed-an-amplifier-difference-or-isolation.693/]

    I am using OPA335 opamp. R1 0.8ohm. R2,R3,R4 and R5 are all 10Kohm .1% resistors. R6 is a 220ohm resistor in series with a red LED (for testing purposes).

    I am measuring the output voltage by hooking the probe to Vout and Gnd.

    I appreaciate your help.

    Thank you in advance!

    Malcolm.
     
  16. mscerri

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    Nov 22, 2012
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  17. PeterCoxSmith

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    Feb 23, 2015
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    The resistance of the current shunt RShunt produces 50mV at full scale current. So:
    RShunt = 50mV/FullScaleCurrent

    for 100A, RShunt = 0.50mR
    1A, RShunt = 50mR
    10mA, RShunt = 5.0R

    In each case, at full current, the shunt drops 50mV; you need to remember that this volts drop may affect your circuit. Take a look at the TI webpage you found and another blog I wrote http://forum.allaboutcircuits.com/blog/cursns2-current-sense-resistors.696/
     
  18. crutschow

    Expert

    Mar 14, 2008
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    No. For large signals (switching and DC bias calculations) a BJT can be considered a current-controlled current-source (the collector has a high impedance).
    For small AC signal applications, the voltage-controlled current-source (transconductance) model is often used.
     
  19. mscerri

    Thread Starter New Member

    Nov 22, 2012
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    I have a 5V 1amp power supply. So does that imply that I need to use a 0.05ohm resistor please?
     
  20. PeterCoxSmith

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    Feb 23, 2015
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    for 1A yes. But how low a current do you want to measure, I think you mentioned microamps?
     
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