Choosing the ideal current shunt value

Discussion in 'General Electronics Chat' started by Mad Professor, Jan 19, 2013.

  1. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009
    133
    1
    Good day all.

    I am looking at doing some voltage and current monitoring with my Arduino Uno.

    The voltage range is 0-15vdc, and 0-5amp.

    I know I can monitor the voltage by just using simple voltage divider into the Arduino ADC port.

    Regarding monitoring the current I know I can buy current/voltage monitoring ic's that I can interface with the Arduino, but I would like to keep away from that.

    So what I would like to know is how do I go about sizing the right current shunt resistor vavlue for a 0-5amp load, so that I can use the Arduino adc port.

    Looking in my used parts bin, I have found a bunch of "DALE LVR-3 0R01 1% 3W" Resistors.

    Would this be suitable?

    Thanks for your time.

    Best Regards.
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    Typical current shunts give 0-50mv or 0-100mv full scale output.

    a .01 ohm 3W
    .01 x 5^2 =.25W
    That will work just fine

    5 x .01 = 50mV
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    You likely will want to amplify the 50mV to a higher voltage to get reasonable resolution in the Arduino.

    And if you can't use the shunt in the ground line then you will need a differential amp that can work at the supply rail to do the amplification.
     
  4. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009
    133
    1
    Thanks for your replys.

    I am indeed looking to use the current shunt on the ground side.

    As I have not used op-amps before, can someone recommended me a suitable op-amp to convert 0-50mv to 0-5v.

    Or a link to where I can find the required information.

    Thanks again.

    Best Regards.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    You can use the non-inverting op amp circuit shown here. If you only have a single power supply then you will need to use a single-supply op amp such as an LM358.

    What is the supply voltage(s) you have to power the op amp?

    Edit: The LM358 and many common op amps have a typical input offset of several mV which would be a significant portion of your signal. If that's a problem for you, then you would need a low-offset rail-to-rail op amp.
     
    Last edited: Jan 19, 2013
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  6. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009
    133
    1
    Thanks again for your reply.

    My circuit has a +12vdc input, then a 7805 to supply +5vdc for the microcontroller.

    I am just reading the Operational Amplifiers section of the allaboutcircuits website.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    Is the 12V regulated or from a rectifier and filter?
     
  8. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009
    133
    1
    I am using a mains power adaptor with a fixed 12vdc regulated output.
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    So you could use that or the 5V to power the op amp, depending upon the amp limits.
     
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