Alternating Current Measurement using Pic Microcontroller

Alternating current or AC current measurement using a pic microcontroller. In this article, we will discuss how to measure alternating current using the current transformer/CT and pic microcontroller. After reading this, you will be able to design AC Ammeter using PIC16F877A microcontroller. It is a very important task in pic microcontroller projects. I have already posted a dc current measurement circuit with code. you can also read it.

What you will learn in this tutorial

  • How to use a current transformer to measure ac current
  • What is the best way to interface CT with PIC16F877A microcontroller
  • Writing your first code to measure AC current

In the first section, we will provide details of the current transformer and in the next section, we will talk about measuring voltage from the secondary side of CT and after that working of code will be explained.

How to use Current transformer for AC current measurement

  • CT is used to measure an alternating high current of the order of thousand Amperes.
  • It steps down ac current to lower value so that it can be easily read with the help of a microcontroller. The step-down the ability of the current transformer depends on the rating of the current transformer and its current step down ratio.
  • Suppose you have a current transformer with a current ratio of 100:10 Ampere. Its mean primary current of the transformer is 100 Amp and the secondary current is 10 Amp. you can not use this current transformer to measure current more than 100 Amp.
  • By measuring the secondary side low current, we can easily convert it into primary current value by using the current ratio formula. I will discuss in the latter part of this discussion how to use this step-down ratio in the programming part.

How to convert current into voltage?

We cannot measure current directly. Firstly, we convert the secondary side current into voltage. We can use a known value of resistor load. We measure the voltage across this known resistor. After that, we can convert this measured voltage into the current. We can use ohm’s law formula to convert the voltage into the current.

V=IR
I = V/R

As you can see in this circuit diagram, we use an R1 load resistor to convert current into voltage. In this circuit, a voltage divider is used. But we can also use an operational amplifier to step down the voltage across the load resistor.

current transformer interfacing circuit

How to Measure the Secondary Side current of CT

Now we will see how to sense ac current with the secondary side. There are many methods to measure low alternating current you can also measure it using AC ammeter. But if you want to perform some control operation and want to send measured current value to another place. You have to use some kind of intelligent system.
For example, you want to make a current protection circuit and circuit specification are followings
if the current flowing through a line is greater then 100 Ampere, a control action  should be performed to operate a rely
Current > 100 Amplere , Relay = open
Otherwise if current < 100 Ampere , Relay = close
To make such kind of intelligent system, we may use analog and digital electronics, but it is better to use digital electronics as far as cost issue is a concern. In this tutorial, we will use a PIC microcontroller to measure the alternating current that is PIC16F877A.
You can check about PIC16F877A pinout and its features on this link.

Hardware components

  1. PIC16f877A
  2. CT
  3. An operational amplifier as a Difference amplifier

AC current measurement circuit using pic microcontroller

The operational amplifier acts as a voltage level shifting circuits or difference amplifier. You can go through this guide on AC voltage measurement to understand its working.

Difference amplifier Circuit

To measure ac current with a pic microcontroller, we have to use the ADC module of PIC microcontroller. To use the ADC module,  we will convert current into voltage form by using a .1 ohm shunt resistor across CT and we will measure this voltage drop across the shunt resistor. Then this voltage drop can be easily converted into current again. For example voltage drop across .1 ohm shunt resistor =8v
then-current according to ohm law
                                                            V=IR

                                                            I=V/R

                                                          I=8/.1=8A
but the problem is ADC of the pic microcontroller can never measure voltage greater than 5 volts. Therefore,  to solve this problem, we can use the difference amplifiers. Because, by adjusting the gain of the difference amplifier,  we can reduce voltage lower than 5 volts. The following diagram shows the circuit of CT and difference amplifier interfacing. you can use any op-amp such as LM741, TL074
differene amplifier interfacing with current transformer

Circuit diagram with pic microcontroller

This is a circuit diagram of interfacing the current sensor with a pic microcontroller. Connect the secondary side of  CT to the points shown on the schematic. After that, Connect the shunt resistor in parallel with the current sensor. The difference amplifier circuit converts voltage below 5-volt magnitude.  It  also shifts the level of ac voltage from the negative side to the positive side. Connect output of difference amplifier with RA1 or analog channel one of PIC16F877A.

alternating current measurement using pic microcontroller
alternating current measurement using pic microcontroller circuit

The liquid crystal display shows the values of the current. We use PORTB to connect LCD. You can go through this LCD interfacing guide.

program of AC Current measurement using PIC16F877A

// These lines define that LCD is connected
// with PORTB pins from RB2 to RB7
sbit LCD_RS at RB2_bit;
sbit LCD_EN at RB3_bit;
sbit LCD_D4 at RB4_bit;
sbit LCD_D5 at RB5_bit;
sbit LCD_D6 at RB6_bit;
sbit LCD_D7 at RB7_bit;

// These lines set the direction PORTB pins 
// All pins as a digital output pins
sbit LCD_RS_Direction at TRISB2_bit;
sbit LCD_EN_Direction at TRISB3_bit;
sbit LCD_D4_Direction at TRISB4_bit;
sbit LCD_D5_Direction at TRISB5_bit;
sbit LCD_D6_Direction at TRISB6_bit;
sbit LCD_D7_Direction at TRISB7_bit;

float v;  // variable to store results

// This function reads the maximum voltage
// across shunt resistor and finds the peack
// of ac voltage which will appear 
// across the shunt resistor. 
void current_READ(void)
{
float max;
int i,current;
int t[40];
ADCON0.ADON=1;
for(i=0; i<=39; i++)
{
v= ADC_Read(1);
v =v*(10.0/1023.0);
v=(v-5.0);
t[i]=v*10;
}

ADCON0.ADON=0;
max=t[0];
for(i=0; i<=39; i++)
{
if(max<t[i])
max=t[i];
}
max=max*.707106781;   // convert peak into RMS
intToStr(max, txt1);         // convert integer value into string 
Lcd_out(2,8,txt1);          // displays measured current on LCD
delay_ms(1000);
}

void main()
{
Lcd_Init();      // It will initialize the library of LCD which is included in Mikro C for Pic
ADCON0.ADCS0=1;     
ADCON0.ADCS1=1;
ADCON1.ADCS2=1;
ADCON0.ADON=0;

while(1)
{
Lcd_out(2,1, "Current:");   // it will display " Current " in second line of LCD
current_READ();               // call current_READ() function to measure current 
delay_ms(1000);
}
}
  • This code is used to measure the secondary current of CT. you can easily calculate primary current according to your CT current ratio by using the current ration formula.
  • .You just have to add one more line in a current function written above if you have a basic understanding of microcontrollers programming.
  • This code is written using Mikro c for pic compiler.
  • Just upload this code to your PIC16F877A device and make connections according to the circuit diagram given above.

Video lecture on alternating current measurement

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49 thoughts on “Alternating Current Measurement using Pic Microcontroller”

  2. Hi
    Thanx for this blog
    My project is to measure current of the output of the ups (solar and ac utility joined together) and according to this measurement i will close or open switch
    What i will add to the code
    جزاك الله خيرا

    Reply

    • مرحبا استاذ وائل جزاك الله كل خير ، ان امكن ولديك معلومات بخصوص حساب التيار والقوات والحراره والضوء في pic اتمنى ان تساعدني في ذلك بارك الله فيك 🌹✨

      Reply

  12. hi, thanx for the blog. I need helpon how to monitor current drawn by a 1Hp submersible pump using a PIC uC. its a single phase pump 220-240Hz. I want to monitor the current drawn so that I can protect the pump from dry running and from overload.
    thanks in advance

    Reply

  15. Hello Bilal,

    I have been given a project recently to measure primary current 10-300A using Pic 16f1787 & 3.6V battery.
    My CT is having turns ratio of 2000.It means my secondary current will be between 0.005 to 0.15A rms.
    I am told to use 3.6V lithium battery for powering the circuit. Your article was a bit relief for me.By reading your article I have understood that I can use difference amplifier to rectify the signal.please guide me.

    Reply

  20. i am a 2nd year b.tech student don’t know how to calculate primary current according to CT current ratio by using current ration formula & on’t know high level of microcontrollers programming.please help me

    Reply

  24. nice project work, i doing same work but in this project i want to measure ac source of 220v 20A, what modification i should do,

    Reply

  28. v= ADC_Read(1);
    v =v*(10.0/1023.0); //reference has been shifted from 5v to 10v
    v=(v-5.0); // offset voltage is 5v
    t[i]=v*10; //inverse of (2.2k/22k) is
    }

    Reply

  29. Why did you choose to go with the diff amp, which is a lot more complex? Why didnt you just use the simple voltage divider? Is there any particular advantage of the diff amp method over te divider?

    Reply

  30. WOULD YOU LIKE TO HELP ME BY SENDING HOW TO CONTROL POWER THEFT WHO STOLE POWER BEFORE ENERGY METER . I NEED TO DO PROJECT ABOUT THUS .SO WOULD YOU LIKE TO HELP ME BY SENDING MICRO C CODE ABOUT IT

    Reply

  33. Please pardon me for my ignorance. As we all know that ADC of mcu can take input as voltage and current measurement can be done by I=(V/R) where R=shunt resistor. In this code I didn’t find any thing like dividing voltage by resistor value.

    I also have some difficulty in understanding this piece of code

    v= ADC_Read(1);
    v =v*(10.0/1023.0); // aren’t we supposed to use 5 since it can tolerate 5
    v=(v-5.0); // why we subtracting by 5 here
    t[i]=v*10; // is it for applying 100:10 ratio ?
    If some made me understand that would be helpful.
    Thank you.

    Reply

  34. The code is giving error like undefined identifiers for AD0N0.ADON & many more plz help me sir reply as soon as as possible on email plz sir

    Reply

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