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Current TransformerOutput Current Conversion To Voltage

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vinhove

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Hi Guys;

I am having a current sensing transformer which is measuring the current drawn by a particular device connected to a wall socket. It gives an output ac current which I want to convert to voltage so as to use it as one of the inputs to the pico-controller. From the researches I have done I am supposed to connect a burden resistor across the output pins so as to get the voltage equivalent to the output current depending on the calculations done on the relationship between input current and the voltage. In my circuit, am getting the same output voltage for each particular device even if I change the burden resistance. My question now is what causes that and what might be done to solve that problem.

Kind regards

Vincent
 

In my circuit, am getting the same output voltage for each particular device even if I change the burden resistance.
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You should tell about the curren transformer type and operating conditions. On possible reason is, that the current transformer is already in saturation, then the output voltage can't further increase.
 

Thanks Fvm for your reply. The current transformer is a Murata 56100C which can measure currents up to 10A and on its datasheet it is stated that it can measure currents at 20kHz-200kHz. So am not sure if it is caused by the frequency range which it is designed to measure instead of the 50Hz for the mains.
 

vinhove said:
Thanks Fvm for your reply. The current transformer is a Murata 56100C which can measure currents up to 10A and on its datasheet it is stated that it can measure currents at 20kHz-200kHz. So am not sure if it is caused by the frequency range which it is designed to measure instead of the 50Hz for the mains.
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If you're trying to use it for 50 mains, then the burden resistor will have to be very low in order to give an accurate reading. In general you want the burden impedance to be much less than your magnetizing impedance. Your CT has a rated magnetizing inductance of 20mH min, so at 50Hz your CT will have a magnetizing impedance of ~6.3 ohms. You'll probably need to set your burden resistor to 1 ohm or less to get any decent results. But even then you may risk overheathing or saturation in the CT. I would try to find a CT which is actually rated for 50Hz.
 

A high frequency current transformer will be in fact difficult to use for 50 Hz. I don't see a saturation Vt area specification in the datasheet, but you can easily calculate, that the low secondary winding reactance of about 6 to 11 ohm will basically restrict the reasonable shunt resistance range. A value below 1 ohm seems necessary.

You can get 50/60 Hz current transformers of similar case size.
 

Hello FvM

Thanks very much for your response, will try to get some 50/60Hz CTs for the correct results.
 

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