current transformer
I have a SCT-013-000 current transformer that says it scales 100A:50mA, meaning it has a turns ratio of 1:2000, but the data sheet says that the turns ratio is 1:1800.
I'm pretty sure the turns ratio is 1:2000 but are there any test I could do to check?
Relevant part of the datasheet:
http://statics3.seeedstudio.com/assets/file/bazaar/product/101990029-SCT-013-000-Datasheet.pdf
RLC-50 is the series number. You can use that to look up the datasheet, especially if you know the manufacturer name, since more than one maker uses that series name (probably copied from ABB). The 50 refers to the inside diameter of the hole in the CT (~50mm).
PT is primary turns for the rated 400:5 ratio. One turn means that you just run the high-current wire through the hole and don't loop it more than once.
I am using 20:1 A CT with winding ratio 1:2500.
Not quite. You are using a 2500:1 transformer and this transforms a current of 1 amp in the primary to a current of 0.4mA in the secondary (providing you have an appropriate load).
I am interested in RMS value
If you are measuring load current of some AC appliance then the current is likely not to be very sinusoidal and the only realistic way of determining true RMS is to sample the output waveform at many times the AC power frequency.
The CT's output can be offset and biased so that the full waveform (at max current) is within the upper and lower bounds of your PIC's ADC. You then sample this at (say) 1kHz, square each sample, accumulate over a period (say 1 second) and then take the square root of the accumulated 1 second value.
If you don't want to go to that trouble, you can usually place a diode in series with the burden resistor to get a rectified half wave output (also add a clamping diode for the negative half cycle of the CT's output). You can also use a bridge rectifier between burden and CT but because there are two diode drops you need to check the data sheet to see if the extra volt-drop from two diodes will cause inaccuracies. This can of course be easily filtered into a relatively smooth DC value before feeding into your PIC's ADC input. It won't be exactly true-RMS but may be good enough for your needs.
Best Answer
The DS says the rated input is 0 to 100 amps and independantly (on a seperate line) it says the rated output is 0 to 50 mA. You should take the turns ratio as being 1:1800 as stated in the DS and not assume that at full rated primary current (100 amps) the device will output 50 mA.
Magnetic core saturation (or rather the onset of core non-linearities) will take the maximum current down a shade or two from 55.5 mA to 50 mA. You could reasonably expect that with a primary current of 50 amps the output will be much closer to 1:1800 than at 100 amps primary current.