AC Current Measurement Using ATmega8

For AC current measurements we use CT’s. If you never worked with CT you will think like this must be something which will hurt you badly. But if you already used CT’s you can understand that these types of things are only reason that AC systems are still alive. Yes, CT’s are just simple transformers which step ups the voltage. Moreover, working on CT is simple if you learn some important basics.

What are Current Transformers?

Current Transformers are simple transformers which step ups the voltage and steps down the current. In live wires, currents range can be vary from some milli-amps to hundreds of amperes. Measuring devices like Atmega8 / Atmega16 are not able to handle such high current. So CT’s steps down the currents and then give that small current to the micro-controller. There are two main types of CT’s

Toroid type

Toroid Type CT
Figure 1: Toroid Type CT

Toroid type CT is those which are easy to use with current sensing because in this type of CT you don’t have to cut the live wire. The primary winding of CT  is  the live wire itself and  secondary  winding  has  hundreds  of  turns  so  that  a  proper  voltage  will  induce  in  the secondary.  We have to just put the live wire inside the hole and a corresponding voltage will induce on the secondary terminal.

Simple Core type

Simple Core Type Current Transformer
Figure 2: Simple Core Type CT

Simple core types of CT are those in which you have to break the live wire and put a CT in between the live wire of the load. The primary of this kind of CT is mostly single high current bearing wire because whole current of load have to pass that wire. In our project we used this type of CT because they are cheap and easily insert in PCB’s.

Note: If it is hard for you to find this type of CT then go to some electronics repairing shop asked him something like this photo. He will surely find this from his scrap.

How to connect a CT with your ATmega8?

AC current measurement using Atmel AVR Microcontrollers
Figure 3: Complete Circuit Diagram/Simulation (AC Ammeter)

To understand this circuit completely, follow the link below:

AC voltage Measurement Using Atmel AVR Microcontroller

The difference in circuit of voltmeter and ammeter is just of a burden resistor. We have to put a burden resistor in parallel with the secondary of CT. If we left open circuited the secondary then a large voltage will induce across it and will put stress or may be burn out the windings.  So there must be an optimum valued resistor parallel with the secondary of CT.

After burden resistor part the remaining components have same working as voltmeter. So there is no need to explain that.

To get the whole code of Atmega8 and simulation file click on the buttons below.

Complete Code Proteus File



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Electrical engineering is my profession, my hobby and my passion. I completed bachelors of electrical engineering in 2015. Currently I am working with a utility company which provides electricity to its consumers. Power Electronics, Embedded Systems and Energy Metering are my fields of specialization. In free time I listen music and watch movies.

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