Implimintation of Current Transformer with Microcontroller

Discussion in 'Embedded Systems and Microcontrollers' started by gpm, Nov 27, 2011.

  1. gpm

    Thread Starter New Member

    Nov 23, 2011
    Dear Sir
    Please help if there any way to connect CT (0 to 5 A 2500 Turn Trodial) to Microcontroller for reading AC Current without using precision rectifier .
    I want some cheap solloution but accurate..
  2. jimkeith

    Active Member

    Oct 26, 2011
    The transformer is not clearly identified here--I assume that the primary current is 5A and the secondary current is 5 ÷ 2500 or 2mA.

    Current transformers like this have inherently high source resistance--therefore, it is practical to rectify the signal with a simple full-wave bridge rectifier--waveform distortion and loss of accuracy is minimal. I have personally viewed on an oscilloscope both rectifier input and output--the input waveform is very interesting as the step (rectifier forward bias) voltage change is very apparent as the current makes the transition through zero. The rectifier load would be a fixed resistor--the addition of a shunt capacitor will reduce the ripple to near zero so that the A-D can easily handle the signal. Remember to apply the RMS to Average conversion factor 0.637 /0.707 or 0.9000.

    Now let me qualify the above--the transformer in question had a 50mA secondary as it was intended for feedback applications.

    Another way to specify it would be 5A = secondary current--current transformers with 5A secondaries are intended for metering applications and may not work well into a rectifier as it tends to increase the transformer flux into the non-linear region.

    Note that when using connectors with current transformers, connect the load ahead of the connector as the transformer can develop astronomical voltages into an open circuit--perhaps a silicon TVS diode could suffice as a default load.
    Current transformers are strange animals indeed.
    Last edited: Nov 27, 2011
  3. jimkeith

    Active Member

    Oct 26, 2011
    Another curious detail about rectifying the output of a current transformer:

    When a capacitor is added to the output of the bridge, the capacitor does not peak detect as it does with a voltage source--the peak voltage actually decreases and approaches the average DC value. Excessive capacitance can make the response sluggish.