CT sensor output


I've only recently become interested in (solar) energy monitoring, so have not yet had time to look through all the posts.

I have been looking at the details of the use of Arduino to act as the brains of a system, and particularly the interfacing of CT's.  I'm just a bit puzzled by the schematic which shows a voltage divider (and a capacitor) with a burden resistor connected to the CT. The output from this is then connected directly to the ADC input of the microcontroller.  Now I must be missing something; I have not found the software that runs in the ATMega828, but the signal fed to the ADC must be  a (distorted) alternating voltage.  I was expecting to see some form of rectification, so that sampling speeds were then uncritical and asynchronous.

Hopefully someone with more experience here can put me back on track.  BTW, I am in the process of making an energy monitor for a friend from my rummage box, so any useful info is most welcome....


Robert Wall's picture

Re: CT sensor output

Hello Mike, welcome to the community.

You will find the software - called "sketches" - here: http://openenergymonitor.org/emon/buildingblocks  (towards the end of the Electricity monitoring section: Arduino sketch - current only and Arduino sketch - voltage and current ) but you can of  course customise the sketch to suit your needs.

The signal fed into the ADC is largely undistorted - unless the CT saturates, of course. Depending on how you configure the software, the waveform is sampled at around 50 times per cycle, and the rms value is calculated in the software. If you are simultaneously measuring voltage, then power is calculated by multiplying the instantaneous values of voltage and current and then averaged. In each case, the averaging period is usually chosen as 10 cycles, and then the measured values are transmitted for recording or display. By independently measuring current and voltage, we can know the direction of power flow, and obtain the apparent power (VA) and hence power factor.


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