Basic CT circuit queries

I am looking at the basic implementation of the CT sensor for current measurement here: http://openenergymonitor.org/emon/buildingblocks/ct-sensors-interface

It mentions 'The capacitor C1 has a low reactance - a few hundred ohms - and provides an alternative path for the alternating current to bypass the resistor.'

Can someone explain the exact function of the capacitor in this circuit? What effect does the capacitor value have on this function?

Additionally, could someone explain why the DC bias voltage divider has the limits 10k-470k? I understand the function of the divider and that they must be equal, and that the higher the resistance the lower the parasitic power usage, but I don't quite understand why this stops at 470k, or what the advantage of using lower value resistors is.

Thanks for your help!

Robert Wall's picture

Re: Basic CT circuit queries

"It mentions 'The capacitor C1 has a low reactance - a few hundred ohms - and provides an alternative path for the alternating current to bypass the resistor.'

Can someone explain the exact function of the capacitor in this circuit?"
It does exactly as it says. It provides a low impedance path to GND to anchor the voltage at the midpoint.

"What effect does the capacitor value have on this function?"
Too large and a fault in the primary circuit could result in a failure of the protection diodes inside the analogue input due to overcurrent, too small and it will not have the desired effect, and it will appear as a significant impedance in series with the input.

"Additionally, could someone explain why the DC bias voltage divider has the limits 10k-470k? I understand the function of the divider and that they must be equal, and that the higher the resistance the lower the parasitic power usage, but I don't quite understand why this stops at 470k, or what the advantage of using lower value resistors is."
Too high and it will take a long time to charge that capacitor, too low and it costs power, and again there's a risk to the analogue input protection if the fault current is not limited. Contrary to intuition, the value won't affect the noise bandwidth presented to the ADC input.

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