+ Post New Thread
Results 1 to 5 of 5
  1. #1
    Newbie level 5
    Points: 1,204, Level: 7

    Join Date
    Jan 2007
    Posts
    8
    Helped
    0 / 0
    Points
    1,204
    Level
    7

    AC Current Sensing Problem : Nonzero Nocurrent Output

    I use a CT to do AC current sensing, when current passes, about 80mv output is generated on the sensing side of the transformer.
    The Problem is that when no current passes, and instead of 0V ouput, about 20mv exists ?
    What is the problem and is there an alternative to using CTs for AC current sensing ?

    Thanks.

    •   AltAdvertisment

        
       

  2. #2
    Advanced Member level 3
    Points: 6,081, Level: 18
    Achievements:
    7 years registered
    cameo_2007's Avatar
    Join Date
    Mar 2007
    Location
    12°58' N77°38'E
    Posts
    752
    Helped
    71 / 71
    Points
    6,081
    Level
    18

    Re: AC Current Sensing Problem : Nonzero Nocurrent Output

    May be that is because of the presence of some kind of noise signals in the i/p-o/p side or due to the inductive properties.By the way,where are you going to give the o/p of CT to...any microcontroller?
    Normally after such kind of sensing,its given to a signal conditionin ckts like diff.amplifier and then to microcontroller or other sensing devices.Then u'll have a total control of the aquired values,like u can adjust the offset(in this case- 20mV),set the gain to increase the resolution of sensing etc etc.



    •   AltAdvertisment

        
       

  3. #3
    Full Member level 4
    Points: 2,125, Level: 10

    Join Date
    Mar 2006
    Posts
    197
    Helped
    36 / 36
    Points
    2,125
    Level
    10

    AC Current Sensing Problem : Nonzero Nocurrent Output

    Yea -- it would be helpful if you could describe your measurement setup or your amplifier, etc...



    •   AltAdvertisment

        
       

  4. #4
    Member level 4
    Points: 2,599, Level: 11

    Join Date
    May 2005
    Posts
    75
    Helped
    8 / 8
    Points
    2,599
    Level
    11

    AC Current Sensing Problem : Nonzero Nocurrent Output

    CT need a resistor load (fixed value resistor specified by CT manufacturer), look at application circuit suggection or electrical specifications of your CT. Do you have that resitor load? Vithout proper termination your reading could be in error also with current passing if load side is open circuit by accident CT gets damaged.



  5. #5
    Newbie level 5
    Points: 1,204, Level: 7

    Join Date
    Jan 2007
    Posts
    8
    Helped
    0 / 0
    Points
    1,204
    Level
    7

    AC Current Sensing Problem : Nonzero Nocurrent Output

    I have the signal generated form the CT after rectified by some diode-capacitor rectification, I have it compared (by a LM339 Comparator) to a threshold value determined by a voltage divider, then the comparator output is connected to a microcontrtoller.

    The threshold value is set to about 35mv, if CT output is 20mv (Theoritically 0V) the comparator output is 5V(VCC), and if CT output is 80mv(Curren passing) the comparator output is 0V.

    So the threshold must be between 20mv and 80mv, but this is a small span the is afftected by noise in the circuit ip/op and the resistors tolerances.

    So, what is your opinion?
    Thanks,



--[[ ]]--