# AC Current Sensing Problem : Nonzero Nocurrent Output

1. ## 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.

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2. ## 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.

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3. ## AC Current Sensing Problem : Nonzero Nocurrent Output

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

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4. ## 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. ## 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,

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