CT Coil Interface
The main purpose of this project is to provide an open source solution to non invasive energy monitoring. Basing our design off the Atmel 465 application note, we will use 2 analog front ends(3 for both phases of split phase) one each for voltage and current measurement. The basic idea is to use a CT coil as our current measurement device, and a wall wart (AC output) as out voltage measurement device. In order to get accurate measurement we must capture the voltage waveform and phase, this will allow us to calculate our power factor. Most of this is covered quite well in the app note, so be sure to take a look!
Lets Get Started
So how do we measure (AC) current
A CT coil is basically a secondary of a transformer. When placed around an AC carrying line a transformer is created between the (primary) the Line and the (secondary) the CT coil. Some CT coil contain a burden resistor and others do not. The burden resistor converts the current into a voltage (drop) against the burden resistor. The Analog front end should then provide an offset so that the waveform is always positive and also amplify the signal is needed.
More on the App Note
There is a lot of information buried in App Notes please read them and datasheets carefully. I know from experience RTFM!!
Lets start by taking a quick look at our Analog Front End, we know that we will be capturing AC signals from both a CT coil and a transformer/wall wart. The main difference between our usage and the app note is pure isolation. Our main design consideration is an non-invasive or fully isolated measurement system. In short we will isolate our circuitry from mains, making this a safe and fairly easy installation.
The Voltage front end is very simple, we will use a transformer to isolate and step down our voltage. Because most wall warts/transformers will still net a higher voltage then our ADC can accept we will use a voltage divider to step it down and a single stage of an op amp to offset and bring the wave into positives(note that crosstalk with the current AFE can cause significant error when both current and voltage and put through a dual op-amp).
The Current front end is a bit more challenging, since we must amplify AC signals without much DC gain. If we get too much DC gain we will just saturate our output and wont see anything but a full ADC reading. The best way around this is to use an op-amp in an inverting configuration, this allows us to decouple the signal while maintaining lower capacitance on the input. (The shunt resistor, decoupling cap and gain stage resistor form a High pass filter and care must be taken in choosing these values)
CT Coil Interface by Sparky’s Widgets is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.Based on a work at http://www.sparkyswidgets.com/portfolio-item/ct-coil-interface/.