Arduino + Current Sensor -> Not always working
Arduino + Current Transformer sensor


The CT is a conventional transformer in all ways, it only works for AC currents.







simulate this circuit &ndash; Schematic created using CircuitLab

If the input current is sinusoidal, then the output voltage will be sinusoidal. The CT will work for non-sinusoidal wave shapes, but then to maintain accuracy you must calibrate the unit. 

The A/D input to an Arduino (or almost any other MCU) is uni-polar. In the case of a 5V Arduino, the signal range is 0-5V. 

If you feed the CT output directly into the Arduino you have a circuit like this:







simulate this circuit

As you can see from above you only get to measure the positive half cycle of the CT output.
Providing the positive and negative peak currents are the same (in the load you are measuring) you will measure the correct peak current for half of the output. This is probably what confuses you since you measured a lightbulb and got somewhat correct results. 

However, bad things are happening:


You are using the intrinsic diode in the Arduino MCU as a rectifier diode. YOU SHOULD NEVER EVER DO THIS!!!
You are only measuring current during one half of the AC cycle. This may be ok under some instances but is not a good design. 


Read the document I pointed to on the OpenEnergyMonitor site and try to understand the reasoning behind this schematic:



In the schematic above, they provide a bias offset to the mid range of the A/D range, then connect the CT to allow you to measure BOTH positve and negative half cycle currents. 
This is the correct way (though there are many variations) to measure AC current. 

There is no way to measure DC current with a CT (there actually is but it's very complicated) such as you have.
If you are measuring the AC current being used by an LCD power supply, then for most switching power supplies like this the peak current may be high and you are measuring all the losses included in the unit.
In the cases for devices that have non-sinusoidal AC currents you have to provide much higher resolution (number of conversions/second) of the AC current and calculate the power used based on a true RMS and Power Factor calculation. You cannot just measure the peak voltage out of the CT and expect to get accurate results.
This paper on calculating Power Factor with non-sinusoidal currents may help.