very-low AC to DC Conversion Project

Discussion in 'General Electronics Chat' started by JoeLucia, Nov 5, 2007.

  1. JoeLucia

    Thread Starter New Member

    Nov 5, 2007
    2
    0
    I have a very small AC signal coming off of a Current Transformer. I get 0 to 0.333VAC from the Current Transformer output (which relates to 0 to 50 Amps detected on the input coil).

    I need to convert the AC signal to DC to run it into a PIC for Analog to Digital Conversion. I haven't found a straight-forward circuit/explanation to do this so I'm looking for some more suggestions.

    Ideally I would like to get the 0-0.333vac into a 0-3vdc range for a full-range Analog-to-Digital conversion with a PIC16F690 running at 3vdc.

    Maybe someone has a way to get a PIC or other A2D converter to be able to read the AC voltage directly without converting to DC first?

    Any suggestions or recommendations?
     
  2. JohnBoy

    New Member

    Oct 30, 2007
    7
    0
    Joe,

    I'm not sure what your turns ratio is, but you could take the CT's output into another transformer's primary (or secondary) and step up your voltage.

    John
     
  3. JoeLucia

    Thread Starter New Member

    Nov 5, 2007
    2
    0
    This is a MeasurLogic Current Transformer (MLG-SCT-1250-050) made for monitoring the current passing through standard AC wiring. So I have a 60Hz 0.280VAC signal right now which is measuring a 120V AC line running at about 30Amps.
    0..50Amps = 0...0.333VAC ouptut.
    I'll play around with the Transformer idea. That was suggested to me by someone else also, but I was looking for a more op-amp/transistor based solution.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Got one for you, as soon as I can get the files off the old computer and converted to a jpeg for the schematic.

    It's an amplifier to scale the AC voltage, followed by a precision rectifier and RMS converter to make DC for the ADC input.

    Be a bit patient and I'll post the circuit.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Here it is. The input op amp gives a about 10 to 1 voltage gain. The dual op amp does the rectifying and RMS conversion. That is straight out of the old National Semiconductor op amp circuit collection. If the output changes too slowly, decrease the size of C1, but don't go below 1 microfarad, or you'll see ripple. The R1 trimmer will let you scale the output to 3 volts for 333 mv in.
     
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