Voltage scaler and shifter

Discussion in 'General Electronics Chat' started by kevclare, Apr 19, 2011.

  1. kevclare

    Thread Starter New Member

    Apr 19, 2011
    I have a 0-6v output from a current transducer which is clipped on to mains voltage.

    Essentially when the load changes on the mains the output from the ct fluctuations between 0-6volts as the current is put across a 10k resistor. I need to maintain an ac signal with 5volts peak on the output.

    So for example if the mains is drawing a low amount of current the voltage from the ct could be 1volt peak whereas I need it to stay a 5volts peak-peak so that i can read it into a microcontroller.

    I am trying to measure mains frequency and just need a stable input to the microcontoller. I have had a look at some op amp where they have a offset of 4volts which means for volts dc at 0volts in but i need to maintain ac to count the transistions so this is not.

    Any ideas how i can scale the voltage and maintain the frequency would be greatly appreciated.
  2. BillB3857

    Senior Member

    Feb 28, 2009
    Use the voltage to illuminate the LED in an optical isolator. The transistor output of the Opto can be used to feed the micro controller. Use an opto with a single LED and use an external diode to protect the opto LED from reverse voltage.
  3. Jaguarjoe

    Active Member

    Apr 7, 2010
    The problem with an LED is that at low mains current the output of the CT is also low. When the CT output goes below the Vf of the LED, the light goes out and the phototransistor output is 0.
    It would have been much easier to use a small step down transformer and a quick and dirty zener clipper to feed the processor directly. Mains voltage is normally very stiff and fluctuates very little with large changes in current. If the frequency of the current varies, the frequency of the voltage will be in lock step along with it.
  4. tom66

    Senior Member

    May 9, 2009
    If isolation is not crucial then measuring mains frequency can be done much easier than that. You can make a simple zero crossing circuit with a high impedance divider.

    The problem with the current transformer is voltage will vary under load as you have discovered. No load = no voltage.

    If isolation is important then you can use a low voltage (6-9V) transformer with the first method I mention. The bonus with this is you can also power your microcontroller from the same supply.