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Last Post 09/26/2012 12:00 PM by  Tony Hodges
Sense current with DC output
 1 Replies
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Mark Carlile
New Member
New Member

09/25/2012 4:59 PM
    I've got a model 19 CT and need to get a DC output signal for monitoring a 2hp single phase AC motor. Using application guide titled "Precision rectifier circuit for CT signal conditioning" has proven unsuccessful thus far.
    1) I'm assuming the two back to back zener diodes are to protect circuit from in-rush current when motor is starting.
    2) Line amps will be between 5 and 15 amps. Using 10 amps with a 230 turn model 19 gives .04A. Applied to a 22ohm resistor should give about 0.88Vac. R2 and R3 to first op amp are 470ohms. R4 is 220ohms. R5 is 1000ohms. For RC output Resistor is 5k ohm and Capacitor is 50 microFarad.

    a) Is the output DC supposed to be across the capacitor?
    b) Have I chosen the value of the resistors or capacitor correctly?
    c) Any suggestions or general guidelines?
    Tony Hodges
    CRM Staff
    Basic Member
    Basic Member

    09/26/2012 12:00 PM
    Dear Sir,

    Thank you for your interest in our product lines. Your design is probably getting too close to saturation. We recommend designing the burden resistor such that the voltage across the CT is 100 mV. So for a full scale of 15 amps, output is 65 mA, and the burden resistor should be .1/.065 = 1.5 ohms. Then gain up the op amp for the required output voltage.

    Thank you for your business.
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