AC Voltage Measurement Using PIC

Alternating Current Voltmeters are most common devices for Embedded and Hardware Engineers and are used on daily purpose for measuring AC RMS voltage values in their circuits. In this post i will show you how to make digital AC voltmeter using Microchip PIC micro-controller and then display voltage values on 16x2 Alphanumeric LCD. We have already posted a post earlier on DC Voltage measurement using PIC, those who are interested in DC Voltage measurement can read the post by clicking here.
In this post i will just concentrate on the AC RMS Voltage measurement technique using PIC Micro-controller, the process and sensing circuit is applicable for all other micro-controllers, those who are just interested in 16x2 Alphanumeric LCD, can read this post.
So let's begin our journey, in this post i will show you how measure AC RMS Voltage upto 240V RMS value.
As you know PIC micro-controller can only measure voltage upto 5V and if we give this 240V AC signal directly to PIC, then God knows what will happen to little PIC, but one thing is sure, its of no use to us.
So before proceeding further we have to drop down the AC Voltage to a level which PIC can withstand, and we can do this by simply using simple Voltage Divider circuit.
Let's consider the case, that we have to measure maximum Vrms of 240V, which means,

Vpeak = Vrms * sqrt(2)

Vpeak = 240 * 1.4142

Vpeak = 339.4 V

Vpeak = 340V (approximately)

Now let's see the circuit diagram which i am going to use for giving this demo.

Schematic Diagram

As you can see in the above circuit, Resistor R5 and R6 are used to drop down the AC voltage.

Scaled Down Peak Voltage = Vpeak * (R6) / (R6+R7)

Peak Voltage for PIC, Vp,uc = 340 * 1.5/(101.5)
Vp, uc = 340 / 67.67

Vp, uc = 5.0246V

So now we have dropped down the AC peak voltage to a level, which PIC micro-controller can detect easily, but their is one more problem, AC signal consist of negative and positive half cycles and we are, not in any mood of measuring the negative cycle, that's why the diode, D1 is used which will remove the negative half cycle of AC Signal as shown in the image below.

The negative Half Cycle is Removed

So now we have a very inexpensive circuit, which has scaled down the very high voltage value into a level suitable for the micro-controller to do the processing.
Now we will use this circuit and my PIC analog-to-digital converter to calculate the AC RMS Value from the ADC values received.

AC Cycle Time	ADC Count
0 millisecond	0
1 millisecond	0
2 millisecond	0
3 millisecond	0
4 millisecond	0
5 millisecond	0
6 millisecond	0
7 millisecond	0
8 millisecond	0
9 millisecond	0
10 millisecond	206
11 millisecond	492
12 millisecond	726
13 millisecond	900
14 millisecond	998
15 millisecond	997
16 millisecond	900
17 millisecond	711
18 millisecond	454
19 millisecond	150
20 millisecond	0
21 millisecond	0
22 millisecond	0
23 millisecond	0
24 millisecond	0
25 millisecond	0
26 millisecond	0
27 millisecond	0
28 millisecond	0
29 millisecond	0
30 millisecond	236
31 millisecond	517
32 millisecond	706
33 millisecond	886
34 millisecond	993
35 millisecond	1001
36 millisecond	914
37 millisecond	733
38 millisecond	482
39 millisecond	181

Table 1: ADC Count Values

Since I am from India and AC Frequency here is 50Hz, which means that the period of one complete cycle is 20millisecond time (1/50=20ms).
So if I take 1 sample of ADC per millisecond then I am in very good condition to calculate the AC RMS Voltage. In Table 1, ADC values for every millisecond for 2 complete cycles of AC Signal is present, as you can see clearly that for 0-9 milliseconds and for 20-29 milliseconds I am getting 0 ADC count, which is absolutely correct as with the help of diode negative half cycle of AC is clipped.
Now the next step is to convert this ADC count into Voltage and after that we will apply the mathematical formula for calculating RMS Voltage, which is as follow:

AC RMS Formula

So, after converting the ADC Count into Voltage (milliVolts) and then squaring each sample Voltage equivalent we get the following table.

ADC Count	Voltage(mV)	Square of Voltage
0	0	0
0	0	0
0	0	0
0	0	0
0	0	0
0	0	0
0	0	0
0	0	0
0	0	0
0	0	0
206	1005.859375	1011753.082
492	2402.34375	5771255.493
726	3544.921875	12566471.1
900	4394.53125	19311904.91
998	4873.046875	23746585.85
997	4868.164063	23699021.34
900	4394.53125	19311904.91
711	3471.679688	12052559.85
454	2216.796875	4914188.385
150	732.421875	536441.803
0	0	0
0	0	0
0	0	0
0	0	0
0	0	0
0	0	0
0	0	0
0	0	0
0	0	0
0	0	0
236	1152.34375	1327896.118
517	2524.414063	6372666.359
706	3447.265625	11883640.29
886	4326.171875	18715763.09
993	4848.632813	23509240.15
1001	4887.695313	23889565.47
914	4462.890625	19917392.73
733	3579.101563	12809967.99
482	2353.515625	5539035.797
181	883.7890625	781083.107

Now as per the above formula we will take the sum of all square values, which will be equal to, 247668337.8, you can use your calculator for this calculation and later use your micro-controller for automating this calculation.

Now as per the above formula we have to divide the sum of square value using the number of samples, which is 40 in our case, but we will divide the equation by 20 (removing zero value samples, as they are not participating in RMS calculations).

After doing averaging, Averaged Squared Sum of Voltage Value is = 12383416.89, and now the last step as per the formula above, is to take the square root, which will give the AC RMS value at the input of micro-controller as, 3519.007941mV.

This is value at the input ADC pin of the micro-controller, so now to get the actual value we have to multiply this value by 67.67, I hope you remember this value from the above equation when we scaled down the AC Voltage using Voltage Divider.

Actual AC RMS Voltage, Vrms = 3519.007941 * 66.67 = 238119.53 mV = 238.119V

So guys we calculated our AC RMS Voltage, you can use this method for any other micro-controller you want.

You can see the results of the above method in the following images or in the video given below.

238V AC Measured

91V AC Measured

67V AC Measured

Note: AC is very dangerous and the circuit shown here is just non-isolated circuit and can cause serious damage if not used properly, use it at your own risk.

The main file of the project is as follows:

/*  

  * File:  main.c  

  * Author: Embedded Laboratory

  *  

  * Created on November 10, 2016, 10:24 PM  

  */  

 #include "config.h"  

 #include "lcd_16x2.h"  

 uint32_t led_timestamp = 0;  

 static uint8_t led_state = FALSE;  

 uint32_t adcUpdate_timestamp = 0;  

 char lcd_msg[LCD_BUFFER_LEN] = { 0 };  

 #define ADC_BUFFER_LENGTH  40u  

 uint16_t adc_data[ADC_BUFFER_LENGTH] = {0};  

 uint8_t adc_data_index = 0;  

 void main()  

 {  

  // Select 16MHz Internal Oscillator  

  OSCCONbits.IRCF = 0x07;       // From 1MHz to 16MHz  

  InitTimer0 ();  

  LCD_Init ();  

  // Configure ADC  

  ANSELAbits.ANSA0 = 1;   // Disable the Digital Input Buffer on AN0  

  TRISAbits.RA0 = 1;    // AN0 as Input Pin  

  ADCON1bits.PVCFG = 0x00; // +Vref = AVDD  

  ADCON1bits.NVCFG = 0x00; // -Vref = AVSS  

  ADCON2bits.ADFM = 0x01;  // Right Justified  

  ADCON2bits.ACQT = 0x01;  // 2TAD  

  ADCON2bits.ADCS = 0x05;  // FOSC/16  

  ADCON0bits.CHS = 0x00;  // AN0 Selected  

  ADCON0bits.ADON = 1;   // Enable ADC  

  sprintf (lcd_msg, "  Embedded");  

  LCD_Print_Line (0, lcd_msg);  

  sprintf ( lcd_msg, "AC RMS = %dV", 0u);  

  LCD_Print_Line (1, lcd_msg);  

  LCD_Update ();  

  while(1)  

  {  

   // LCD Update Task  

   if( millis() - led_timestamp > 1000u )  

   {  

    led_timestamp = millis();  

    if( led_state )  

    {  

     led_state = FALSE;  

     sprintf (lcd_msg, "  Laboratory");  

    }  

    else  

    {  

     led_state = TRUE;  

     sprintf (lcd_msg, "  Embedded");  

    }  

    LCD_Print_Line (0, lcd_msg);  

    LCD_Update ();  

   }  

   // ADC Update Task  

   if( millis() - adcUpdate_timestamp >= 1u )  

   {  

    uint32_t ac_value = 0;  

    adcUpdate_timestamp = millis ();  

    ADCON0bits.CHS = 0x00;  // AN0 Selected  

    ADCON0bits.GODONE = 1; // Start Conversion  

    while(ADCON0bits.GO == 1);  

    uint8_t adc_lo = ADRESL;  

    uint8_t adc_hi = ADRESH;  

    adc_data[adc_data_index] = (uint16_t)(adc_hi << 8) | (uint16_t)adc_lo;  

    adc_data_index++;  

    if( adc_data_index >= ADC_BUFFER_LENGTH )  

    {  

     adc_data_index = 0u;  

     // Time to Process and Display Data  

     uint32_t adc_filter_val = 0;  

     uint16_t adc_buf_local = 0;  

     for( uint8_t i=0; i<ADC_BUFFER_LENGTH; i++ )  

     {  

      adc_buf_local = adc_data[i];  

      adc_filter_val += ((uint32_t)(adc_buf_local*(uint32_t)adc_buf_local ));  

     }  

     // 625*625 = 390625  

     // 128*128 = 16384  

     // 625^2 / 128^2 = 23.8414  

     adc_filter_val = (uint32_t)((float)adc_filter_val * 23.84);  

     adc_filter_val /= ADC_BUFFER_LENGTH;   // mean squared sum  

     adc_filter_val *= 2;  

     ac_value = (uint16_t)sqrt(adc_filter_val);// root mean square  

     ac_value *= 68u;  

     ac_value /= 1000u;  

     sprintf ( lcd_msg, "AC RMS = %luV", ac_value);  

     LCD_Print_Line (1, lcd_msg);  

     LCD_Update();  

    }  

   }  

  }  

 }

Click here to download the complete source code (16x2 LCD with busy flag check also present) along with the Proteus simulation file.