.Hello everyone. im building a project using current sensor to measure ac.
what current sensor should i use? that can measure even the smallest amount of current/ampere. thankyou
that can measure even the smallest amount of current/ampere
Not a very useful specification.
Please state the range of current that you wish to measure, in Amperes.
The project is to measure standby power/current of loads. as we know standby power/current is a very small current. the project is also capable of measuring overload current to maximum of 12amps so the sensor should be higher than 12amps. but importantly i want to focus on measuring the standby current. what ac current sensor is suitable for accurate measurement? thank you for your help 
Inexpensive sensors are typically accurate to 1 to 2% of full scale reading.
So, a sensor capable of reading 12A full scale might be able to read a current of 0.12A.
But I would not count on that.
Please continue to think about and attempt to define your actual requirements. You might also do a little research of your own, such as looking at the current sensors (and their specifications) from Pololu.
OP mentioned AC.
If mains AC, then the only safe option is a (clip-on) current transformer.
AC measured with an Arduino 10-bit A/D means 1024/2 *0.7= ~358 A/D values.
A max resolution (steps) of 15/358= ~42mA.
Probably not enough to measure standby current of mains powered appliances.
Seems you have to choose between measuring one or the other.
Leo..
Perhaps the best isolated current sensors are the hall-effect ones with a secondary winding which generate a nulling
current in the secondary, keeping the hall sensor at zero. These are more resistant to issues with
hysteresis and temperature changes in the ferrite core. I can't recall the make of ones I've used in
the past, but they were PCB mount with several single-turn primaries pinned out so you could
vary the sensitivity.
Failing that a current transformer is nearly as good since the magnetic field in the core is kept
very low. (Assuming the secondary is correctly terminated, which it has to be for safety). But
of course they are strictly AC only, with a limited frequency range.
Hall sensors without a transformer (like the ACS711, ACS712, etc etc) are much more rough
and ready - they don't have a ferrite core to compensate, but they are very noisy and pickup
stray magnetic fields very easily, as well as being fairly insensitive.