Current Transformer Output Voltage

[Suraj143]

In a current transformer it has mentioned 1A/1mA that means 1000:1 ratio.

My question is If I feed 230V AC/ 1A on primary side how much voltage will show on secondary side?

That means if I load a 1K resister on its secondary will it show 1V?But still its AC voltage..!!

I want to access this output to a micro controller with ADC input.

 

[MikeMl]

There have been many previous threads on this topic on these forums.

 

[crutschow]

As MikeMl stated, there have been many previous posts on this topic. Typically a precision rectifier is used to convert the AC to DC that can be read by the ADC.

 

[dougy83]

You can just connect a current-limiting resistor from the 1VAC signal to the PIC ADC input and read the voltage of the positive half-cycle at multiple points.

 

[MrAl]

In a current transformer it has mentioned 1A/1mA that means 1000:1 ratio.

My question is If I feed 230V AC/ 1A on primary side how much voltage will show on secondary side?

That means if I load a 1K resister on its secondary will it show 1V?But still its AC voltage..!!

I want to access this output to a micro controller with ADC input.

Hi,


Yes it is AC voltage coming out of the current transformer, that's how they work. But to read this with the ADC you have several choices.

1. Direct input to the ADC pin. Using a current limiting resistor (like 10k) you might be able to use the lower protection diode on the chip to clip the negative peaks of the AC wave. This results in a positive portion only which you can then measure. You have to be able to sample the wave enough times to average the result however or at least hunt for the peak.
2. Diode plus filter cap. Using a diode and capacitor means you get near DC so you can sample it anywhere in the waveform and get usable results with just one or two samples.
3. Precision rectifier. Requires an op amp.
4. Full wave rectifier and cap. This means you will be sampling both positive and negative portions of the AC which is a little better. Many current transformers have a center tapped output so you can use two diodes and get full wave rectification.
5. Add a DC offset. Using two resistors and a capacitor, you can AC couple the signal into the ADC input pin. The two resistor provide a DC bias for the pin so that the AC waveform appears above zero everywhere. This allows you to sample the true AC waveform anywhere in the cycle. The advantage is there is no delay. You do have to sample multiple times though to get an average over the entire wave.

So you really have several choices and the best one is the simplest that you can get away with for your application. The simplest is the diode and cap because you only have to sample once whenever you need a reading. If you instead care to sync to the incoming sine wave (with offset) though you can get pretty fast dynamic results because you can use an algorithm to compute the peak even if the wave is not at the peak at the time of measurement.

 

[Suraj143]

Hi,


Yes it is AC voltage coming out of the current transformer, that's how they work. But to read this with the ADC you have several choices.

1. Direct input to the ADC pin. Using a current limiting resistor (like 10k) you might be able to use the lower protection diode on the chip to clip the negative peaks of the AC wave. This results in a positive portion only which you can then measure. You have to be able to sample the wave enough times to average the result however or at least hunt for the peak.
2. Diode plus filter cap. Using a diode and capacitor means you get near DC so you can sample it anywhere in the waveform and get usable results with just one or two samples.
3. Precision rectifier. Requires an op amp.
4. Full wave rectifier and cap. This means you will be sampling both positive and negative portions of the AC which is a little better. Many current transformers have a center tapped output so you can use two diodes and get full wave rectification.
5. Add a DC offset. Using two resistors and a capacitor, you can AC couple the signal into the ADC input pin. The two resistor provide a DC bias for the pin so that the AC waveform appears above zero everywhere. This allows you to sample the true AC waveform anywhere in the cycle. The advantage is there is no delay. You do have to sample multiple times though to get an average over the entire wave.

So you really have several choices and the best one is the simplest that you can get away with for your application. The simplest is the diode and cap because you only have to sample once whenever you need a reading. If you instead care to sync to the incoming sine wave (with offset) though you can get pretty fast dynamic results because you can use an algorithm to compute the peak even if the wave is not at the peak at the time of measurement.

Very nice information.Got it nicely.Thanks for the reply.

I'm aware with diode thing because.If CT primary side the load reaches 1A then the CT output will be 1V if 1K load resister there.
But whenever the primary load reaches 500mA then the CT out will 0.5V, then the diode will never conduct