Using a current transformer

Question

I have to measure a current of 500 A (50 Hz) using a 500 A/5 A CT with a uC. In my circuit, I have a 2.5 V reference/offset and the voltage will be measured about this offset value. There are low cost 5 A/5 mA PCB mount CTs available. Is possible to short the output (5 A) of 500 A/5 A, through the 5 A/5 mA, then use a burden resistor of 1k on the 5 mA and thus measure the 500 A current?

Answer 1

Is possible to short the output(5A) of 500A/5A, through the 5A/5mA , then use a burden resistor of 1k on the 5mA and thus measure the 500A current ?

Yes, you can do this but you are compounding errors. I suspect that the main CT (500/5) will have a fairly decent specification regarding errors/tolerance. But, the 5 A/5 mA might significantly degrade the overall accuracy.

I understand the problem; to get a 2.5 volt peak with 5 amp peak output requires a resistor of 0.5 Ω and, if the AC is a sinewave, the RMS will be 3.536 amps. This will lead to a power dissipation in the 0.5 Ω burden of 6.25 watts.

So, my preference would be to use a significantly smaller burden such as 0.1 Ω and live with the 1.25 watts of dissipation. Of course, the peak voltage will now only be 0.5 volts but, a half-decent op-amp can restore this towards 5 volts p-p.

In my circuit, I have a 2.5V reference/offset and the voltage will be measured about this offset value.

You say you want to use a 1 kΩ burden resistor with a current of 5 mA. If that current is RMS then, into a 1 kΩ resistor, the signal voltage would be between +7.071 volts and -7.071 volts superimposed on the 2.5 volt reference i.e. well beyond your MCU input range.