### How to build an Arduino energy monitor - measuring mains current only = circuit explanation

Hi everybody,

In regard to this project :

http://openenergymonitor.org/emon/buildingblocks/how-to-build-an-arduino...

Can someone briefly explain what  the circuit in this project is doing and how it works ?

Thanks.

### Re: How to build an Arduino energy monitor - measuring mains current only = circuit explanation

Take a look at the Building Blocks section of the website

### Re: How to build an Arduino energy monitor - measuring mains current only = circuit explanation

But it is not what I asked for or I could not understand it, I need for explanation of what is the meaning of the resistors and capacitor in the circuit?

Why they built in this way, what is role of them there?  Sorry but I am very beginner in this area.

Thanks

### Re: How to build an Arduino energy monitor - measuring mains current only = circuit explanation

This site is for people interested in monitoring and controlling energy use. While it would be nice to write a customised basic electronics course for you, life is too short, and as you can see from that link, there are plenty - about 2 million - on offer. Not all will be at the level you need, and not all will be good. Come back when you understand the basics and then the Building Blocks section will mean much more to you.

### Re: How to build an Arduino energy monitor - measuring mains current only = circuit explanation

Hallo lolita!

I'll try to explain, you should look at the picture.

On the left side there is the CT sensor, this is the blue "ring" with the grey cable on the top of the webpage.

When it is clamped over the phase of the mains supply it does a transformation of the current running through this wire to a lower value that we can work with. But the arduino can't measure currents, so we need a resistor to transform the current to a voltage. This is done by the burden resistor.

Then we have this small circuit with the two resistors and the capacitor.

The resistors divide the 5V supply of the Arduino by two, so the voltage over one of them is 5V/2=2,5V. The capacitor is needed for stability because if the analog input of the Arduino is connected to the CT a very small current will flow and the connection point of R1 and R2 wouldn't be at 2,5V exactly. As the vurrent in the CT is alternating we would have errors, sometimes positive, sometimes negative compared to the real value. So the capacitor stabilizes the 2,5V at this point.

Why is this needed? Because the arduino can't measure negative voltages. When the voltage over the burden resistor is negative, the voltage at the other end connected to the arduino's analog port would be negative compared to the arduino's ground and we can't measure it. To get a correct measurement we only have to ensure that the voltage over the burden resistor never exceeds 2,5V, otherwise it would be higher then 5V or lower than 0V.

The Arduino then measures voltages between 0 and 5V. If the voltage that is measured is exactly 2,5V it would mean there is no voltage on the burden resistor, so the current is 0.