How to check current in AC line with Arduino?
Thanks.
What voltage is this AC line?
For high voltage and high current (such as the mains supply in a property) you would normally use a non-intrusive hall-effect sensor that fits over your power line
For lower voltage devices (such as small electric vehicles) you might use an all in one current sensing chip such as this:
Both ways are essentially the same but the second requires the wires to be cut and are typically used for lower voltage systems. The second also provides you with an analog voltage output with a certain number of millivolts per amp so it is very easy to use with a microcontroller such as the arduino.
Mowcius
Mowcius
How to connect CST306-1A to Arduiuno board? I need to sence current 0-3 Amp 110V 60 Hz in power line.
;)
You cannot connect this directly to an Arduino since it gives out an AC current and MUST be loaded to prevent excessive voltage generation.
You'd be better using a hall type AC current sensor which can be conditioned to give a 0-5 volt DC signal suitable for direct connection to the Arduino.
These look very like the sensor you've shown.
jack
I just noticed that the CST306 is intended for high-frequency switching power supplies, not for 50-60Hz power mains.
Suggest that this is a better way to sense up to 5A of mains current. This is a hall-effect sensor as jackrae suggests.
SparkFun Current Sensor Breakout - ACS723 (Low Current) - SEN-14544 - SparkFun Electronics
It has everything you need on the board. That page also shows the "Wiring Example". Wiring is almost identical to Arduino, and the hardware connections and software should be virtually identical.
I afraid to use this sparkfun break out board with power line. 
Is it ok to use it with 110v 0-5Amp?
It isn't "non-invasive", and it needs to be done with CARE (and a knowledge of what you are doing), but one "answer", depending on your needs/ circumstances is to fit a relay in parallel with the power you are monitoring. Relays with coils for "mains" electricity to exist. (110v, 60hz USA, often 230v elsewhere).
Then you just "watch" the contacts of the switch opened and closed as the coil is energized/ not.
The wiring from the relay to your Arduino wants to be done by someone knowledgeable... mistakes here can let dangerous electricity get to places it mustn't. Yes, it will be low voltage... but it will be NEAR wires carrying high voltage.