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
    10
    0
    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
    539
    99
    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
    539
    99
    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.
     
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