Current Shunt - Dc amplifier

Discussion in 'The Projects Forum' started by H man, Mar 24, 2006.

  1. H man

    Thread Starter New Member

    Mar 24, 2006
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    I am developing a battery monitoring system for duel batteries in my 4x4. Using an Atmel AVR micro I intend to monitor the discharge/charge current going into the batteries so I can calculate the remaining charge in the batteries to maximise their usage. (When hundreds of Km from anywhere it is important to keep the 12v system healthy) The code for the micro is the easy bit, the problem I am encountering is getting the data into the micro.
    I have a current shunt that gives me current flow readings from 0 – 50 amps in millivolts 0 – 50 mv. 50mv being 50A through the shunt. I require a DC amplifier to increase the shunt voltage into the ADC on the micro to around 5v (a 100x amplifier). Whilst this does not seem too difficult the big hurdle for me arrises when I have to monitor current in and out of the system. For instance if 10 amps is being drawn out of the battery I will have a positive 5 mv being read across the shunt, but if 10 amps is being delivered to the battery from the cars charging system I will register negative 5mv.

    What I am asking help with is building a DC amplifier that sits at 2.5v when there is neither current being delivered or drawn. So if I am using 50A I will get 5v Dc into the micro but when I am charging the batteries I will get a reading below 2.5 volts. Can this be done using a single rail supply Op Amp? Any help would be appreciated.


    Thanks in advance.

    H man...
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,587
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    Yes, it is possible. I need to know if the sensing resistor is connected in the positive or negative part of the circuit.
    Then you use a differential amp to get the voltage on the floating sense resistor and by another amp you push it to the +2.5V point.
     
  3. H man

    Thread Starter New Member

    Mar 24, 2006
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    The sense resistor is on the negative side of the battery.
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    So you use a H-bridge to control the motor? Do you use PWM regulation or some shunt regulator?
    Does the car recuperate (charge when brakeing)?
     
  5. Spoggles

    Well-Known Member

    Dec 2, 2005
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  6. Spoggles

    Well-Known Member

    Dec 2, 2005
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  7. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
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    The circuit on this page will do the level shifting and amplification using only one opamp. Note that to get an output that goes up to 5V you need more than 5V supply to the opamp (the 12V will do). You need to calculate the appropriate values for your resistances. On that circuit it uses 3.3V supply for maximum output of 3.3V which would be impossible without an ideal rail-to-rail opamp.

    Opss.. forgot to put the link... it's here.
     
  8. H man

    Thread Starter New Member

    Mar 24, 2006
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    Thanks for these 2 options, they both do exactly what i am after. The allegro current sensor appears to be the easiest option with only a few conections required.

    There always seems to be a device made for every application the only trouble is finding it.


    Thanks again.

    H man.
     
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