Digital AC Ammeter using uC

Discussion in 'The Projects Forum' started by StarfleetRP, May 6, 2009.

  1. StarfleetRP

    Thread Starter Member

    Mar 23, 2009
    I have an AC voltage source of about 30V 4A, and would like to measure the current drawn by the load. The measuring uC would get power from the 30V (via rectification, and etc). What I am wondering is if the below method would work for this?

    I would assume that I should use a 0.025 ohm resistor if its going to be a 0.5W shunt for the 30V 4A.

    Then I would probably need a ???ohm voltage divider resistor of .25W?

    Code ( (Unknown Language)):
    1. [FONT=Courier New]Voltage Source-----------Rectifier-----------Resistor(.025)----------LOAD---Back to v. source
    2.                                        |                   |                         |
    3.                                    uC ADC1  ADC2    |                         |
    4.                                                             Diode                 Diode
    5.                                                             V. Divider           v. Divider
    6.                                                             To ADC1            To ADC2[/FONT]
    I am not sure if this would work, could you please tell me. Part of my concern is I dont know exactly how the ADC's would react to this. Thanks in advance for all of your help
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Since the uC gets its power from the same source that supplies the LOAD you will probably need a differential amplifier to "take" the shunt signal to the uC ADC input(s) for measurement. If the uC has a differential input mode - which I think is what your diagram is showing (???) - then it should work. Provided the shunt +/- signal lies within the common mode range of the ADC inputs.

    Another aaproach is to

    1. Use an isolation amplifier to get the shunt signal into the ADC input.


    2. Use a current transformer / transducer

    to isolate the current path from the measurement path.
  3. StarfleetRP

    Thread Starter Member

    Mar 23, 2009
    I am not sure what you mean by differential input. I am using a uC similar to a dspic30F4011. I thought about using a current transformer, but at $10.00 each it would be too expensive
    Last edited: May 7, 2009
  4. StarfleetRP

    Thread Starter Member

    Mar 23, 2009
    I would prefer to find a solution that did not need to use a current transformer, even if they are easy and cheap to make. I would welcome any other solutions, or explanation of what t_n_k meant by differential input mode.
    Thanks for all of your help so far.