Smart power monitoring is getting increasingly popular to improve energy efficiency in medium/small scale industries, multi-story buildings, etc. We have previously built a smart energy meter which can measure power and report usage online. While this type of system is popularly used in energy meters, another most commonly available technique to measure current is by using a current transformer (CT). CT can detect and measure AC current in machines and other AC appliances to monitor the current consumption and thus the health of machine or AC appliance. 

How does a Current Transformer Work?

According to Faradays Ampere law, “If a magnetic field is integrated around a close loop of wire, the value of that integral is equal to the net current enclosed by the loop”.

The primary winding of CT, the main loop has the wire with the current we wish to measure passed through the center of the core. The primary winding that carries the main current is said to have a single loop or winding. The wire produces the magnetic field that drives the current on the secondary winding, which is used as output of the CT.

Circuit Diagram to measure AC Current using Arduino

The below schematic shows the Circuit diagram for current measurement using the current transformer.

In above circuit, we used 16 * 2 LCD but in project we have used I2C LCD display. For I2C LCD display simply connect. 

Arduiono Gnd – I2C Gnd

Arduino 5V - I2C Vcc

Pin A4 - I2C SDA

Pin A5 - I2C SCL

We have used AC voltage source as a Secondary winding voltage in above circuit diagram. This is how Rectifier and amplifier circuit looks like in breadboard.

As we can see in this circuit, a red color wire passing through the current transformer, we are going to measure current value passing through this wire. 20k + 2k resistances and Zener diodes are connecting parallel with CT. This resistance is burden resistance of CT (22k) and Zener diodes improve the safety of circuit. Here, we also connected one Schottky diode that will behave as half wave rectifier.

How does Current Measurement Circuit Work?

First of all, both the terminals of the bulb are connected to both the terminal of the transformer and one wire of the bulb passed through the hole present in the middle of the current transformer, this wire acts as the primary winding for the current transformer. 

Due to the AC current in the primary winding, AC voltage is induced in the secondary winding as well. Now to convert this AC voltage into DC voltage, it is sent to half wave rectifier, half wave rectifier is forward biased in positive half cycle and reverse biased in negative half cycle. In figure, Vi is input sine wave pulse and Vo is output sine wave pulse.

An electrolytic Capacitor is added to the output of the have wave rectifier to receive only the DC voltage. This capacitor only forwards the DC voltage in the circuit. The output voltage that is obtained from the rectifier circuit is in very less magnitude, so we use amplifier. we have used non inverting operational amplifier with 10k and 1k feedback resistor value.