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Current transformer resolution

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gehan_s

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Hey all,

I need to measure an AC current of 0-6A rms using a CT. I have searched for CTs in the internet and found out that many of them go upto about 30A (some of them higher). If I use one of them (max 30A) will it reduce the resolution of the detected current ?

If so how can I increase the resolution ?(I am thinking of measuring atleast 50mA increments)

Also please tell me how to interface the CT to the PIC

thanks in advance !!!!!!!!!!!!!!!
 
Do you need the isolation that a CT provides? If not, you could use a shunt.

With a CT or shunt, you will have to rectify and filter to get a DC level that a PIC AD can read.
 
Hey all,

I need to measure an AC current of 0-6A rms using a CT. I have searched for CTs in the internet and found out that many of them go upto about 30A (some of them higher). If I use one of them (max 30A) will it reduce the resolution of the detected current ?

If so how can I increase the resolution ?(I am thinking of measuring atleast 50mA increments)

Also please tell me how to interface the CT to the PIC

thanks in advance !!!!!!!!!!!!!!!


Hello,

The resolution of a current transformer is basically infinitesimal. But your ADC has much worse resolution, so to increase resolution use a current transformer that is linear within the range you need and use an appropriate load resistor to get the proper range for your application. To get much better resolution you need to increase the number of bits of your ADC. If you are now using 8 then maybe 10 will be enough, or perhaps 12 bits. 10 bits gives you a count of 1024 which is roughly 1000, so that means with a 10amp full scale setup you'll see a resolution of about 10ma.
 
You can wrap the conductor around the ct up to a few turns to increase the apparent current, ie if you make 5 wraps a 1 amp current will appear as 5 amps.
Of course the maximum current will be reduced by the same factor.
 
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thank you all for replying.

The pic has a 10bit AD so it is the CTs accuracy that I am worried about.

I don't want to rectify the current for this is really for a power measuring device and it will be hard when I am doing the calculations + there will be diode drops. So I am thinking of shifting the signal within 0 to 5V. For this I have to chose the load resistor properly. How do I do this. (For shifting I am thinking of using an single supply inverting amplifier with 2.5V given to the + terminal).
 
thank you all for replying.

The pic has a 10bit AD so it is the CTs accuracy that I am worried about.

I don't want to rectify the current for this is really for a power measuring device and it will be hard when I am doing the calculations + there will be diode drops. So I am thinking of shifting the signal within 0 to 5V. For this I have to chose the load resistor properly. How do I do this. (For shifting I am thinking of using an single supply inverting amplifier with 2.5V given to the + terminal).

Hi,

You dont have to worry as much about rectifier drops in a current transformer output as you do with an ordinary transformer output. That's because it's a current transformer, not a voltage transformer so it works a little differently even though similar. If you input 1 amp and you get out 10ma for example it doesnt care whether it is driving a diode or a resistor, it will still put out 10ma (within reason). In fact, many current transformers are made with a center tap output so that you only need two diodes to rectify the output. The idea however is not to connect the resistor directly to the output, but rather connect the resistor after the diodes. This way that 10ma gets through the resistor in spite of the extra 0.7v drop by each diode. So with say a 100 ohm resistor that 10ma turns into 1v with or without the diodes, but of course it will be pulsating DC with the diodes and smoother if you use a filter cap.

Precision rectification wont buy you much for the trouble. The main source of error will be in the transformer non linearity. You'll want to check the manufacturers web site and possibly make some of your own measurements too. You can then scale the readings by a factor depending on what you read. For example, for currents between 100ma and 1 amp you may need a scale of 1.1, while currents from 1 amp to 5 amps a scale of 1.02 or something like that. The idea here is to correct the transformer non linearity using an algorithm that has the transformer curve built in, in the form of a table or a formula that gets computed every time a reading is made. For example of an algorithm, you may find that you can scale it by 0.01*i^2, giving the higher currents a slight boost (that's just a rough idea what you might do, a real correction like this would probably involve A*i^2+B*x+C or maybe even a third order polynomial.
 
thank you very much MrAl.

i am soooo sorry for i have rated your post as -1. i was going to give you a +1 but accidently pressed the other and now i can't see a way to undo that (they haven't enabled that option). so i am soooooooooooo SORY !!!!!!!!!!!!!!!!!!!!!! if you know a way to undo it please let me now !!!!!!!!!!!!!!

its a good thing that you mentioned that there were center taped CTs as i was wondering why some of them had 3 pins.

sorry again for the negative rating !!!!!!!!!!!!!
 
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thank you very much MrAl.

i am soooo sorry for i have rated your post as -1. i was going to give you a +1 but accidently pressed the other and now i can't see a way to undo that (they haven't enabled that option). so i am soooooooooooo SORY !!!!!!!!!!!!!!!!!!!!!! if you know a way to undo it please let me now !!!!!!!!!!!!!!

its a good thing that you mentioned that there were center taped CTs as i was wondering why some of them had 3 pins.

sorry again for the negative rating !!!!!!!!!!!!!

Hi,

Oh well maybe it's good that this happened so that we can find out how to change it. We'll have to ask about that.
 
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