How to make a high voltage AC voltmeter

In our previous articles, we learned how to make an AC voltmeter and how to measure high voltage AC voltage. In this article, we will learn how to make a high voltage AC voltmeter. Yes, you can measure 500V+ or higher easily with this circuitry.

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Before you read further, I’ll request you to read the other articles

1. How to make an AC voltmeter
2. How to measure high voltage AC

Different ways to measure high voltage AC:

There are different ways to measure high voltage AC. A transformer is the most common device in this case. But when we need to consider the size and weight, we have to find other ways to do the job.

An Op-Amp can be used in this case. Op-Amps are cheap and small in size and weight is also negligible comparing with transformers. A circuit like this is very very helpful for this purpose:

Voltmeter circuit diagram:

Coding:

/*******************************************************************************
* Program for "High voltage AC voltmeter"                                      *
* Program written by_ Engr. Mithun K. Das                                      *
* MCU:PIC16F73; X-Tal:8MHz; mikroC pro for PIC v7.6.0                          *
* Date:05-05-2020                                                              *
*******************************************************************************/
char segment_array[]={0x3F,0x06,0x5B,0x4F,0x66,0x6D,0x7D,0x07,0x7F,0x6F};//cmmon cathode_non dot

sbit digit0 at RC0_bit;
sbit digit1 at RC1_bit;
sbit digit2 at RC2_bit;
sbit digit3 at RC3_bit;

char digits[5];
void display_7segment(int number)
{
   digits[3]=number/1000u;
   digits[2]=(number/100u)%10u;
   digits[1]=(number/10u)%10u;
   digits[0]=(number/1u)%10u;
}

void InitTimer0()
{
  OPTION_REG     = 0x85;
  TMR0           = 100;
  INTCON         = 0xA0;
}

int position=0;
void Interrupt() iv 0x0004 ics ICS_AUTO
{
  if (TMR0IF_bit)
  {
    TMR0IF_bit   = 0;
    TMR0         = 100;

    digit0 = 1;
    digit1 = 1;
    digit2 = 1;
    digit3 = 1;
    if(position>3)position=0;

    if(position==1)PORTB = segment_array[digits[position]]+128; //dot point
    else PORTB = segment_array[digits[position]];

    if(position==3)
    {
        digit0 = 0;
        digit1 = 1;
        digit2 = 1;
        digit3 = 1;
    }
    else if(position==2)
    {
        digit0 = 1;
        digit1 = 0;
        digit2 = 1;
        digit3 = 1;
    }
    else if(position==1)
    {
        digit0 = 1;
        digit1 = 1;
        digit2 = 0;
        digit3 = 1;
    }
    else if(position==0)
    {
        digit0 = 1;
        digit1 = 1;
        digit2 = 1;
        digit3 = 0;
    }
    position++;
  }
}



void Get_AC_Voltage(void);
unsigned int max_point=0,mm=0,k=0,temp=0,adc_rd0=0,ac_voltage;
void main()
{
 TRISA=0xFF;//all input
 TRISB=0x00;//all output
 TRISC=0x00;//all output
 PORTB=0x00;
 PORTC=0x00;//clear ports
 ADCON1=0x00;//all analog in
 InitTimer0();//5ms timer
 while(1)
 {

    Get_AC_Voltage(void);
    display_7segment(ac_voltage);

 }
}

void Get_AC_Voltage(void)
{
      int mm;
      max_point = 0;//clear all data
      adc_rd0 = 0;//clear data
      for(mm=0;mm<10;mm++)
      {
          for(k=0;k<500;k++)
          {
               ADCON0 = 0b00000001;//AN0 selected
              if(temp = ADC_Read(0),temp>max_point)
              {
                  max_point = temp;
              }
          }
          max_point = abs(ceil((long)(max_point-128)));
          adc_rd0+= max_point*41.8;
      }
      adc_rd0/=10;

      ac_voltage = adc_rd0;
}

Code explanation:

char segment_array[]={0x3F,0x06,0x5B,0x4F,0x66,0x6D,0x7D,0x07,0x7F,0x6F};//cmmon cathode_non dot

sbit digit0 at RC0_bit;
sbit digit1 at RC1_bit;
sbit digit2 at RC2_bit;
sbit digit3 at RC3_bit;

In this part, we took an array for different digits of 7-Segment. And later on, set our MCU’s pins for different digits of the 4×1 7-Segment so that we can call the digit directly later.

char digits[5];
void display_7segment(int number)
{
   digits[3]=number/1000u;
   digits[2]=(number/100u)%10u;
   digits[1]=(number/10u)%10u;
   digits[0]=(number/1u)%10u;
}

In this part, display_7segment is just taking each digit of a number and saves in array digits[]. This will be called in ISR later.

void Interrupt() iv 0x0004 ics ICS_AUTO
{
  if (TMR0IF_bit)
  {
    TMR0IF_bit   = 0;
    TMR0         = 100;

    digit0 = 1;
    digit1 = 1;
    digit2 = 1;
    digit3 = 1;
    if(position>3)position=0;

    if(position==1)PORTB = segment_array[digits[position]]+128; //dot point
    else PORTB = segment_array[digits[position]];

    if(position==3)
    {
        digit0 = 0;
        digit1 = 1;
        digit2 = 1;
        digit3 = 1;
    }
    else if(position==2)
    {
        digit0 = 1;
        digit1 = 0;
        digit2 = 1;
        digit3 = 1;
    }
    else if(position==1)
    {
        digit0 = 1;
        digit1 = 1;
        digit2 = 0;
        digit3 = 1;
    }
    else if(position==0)
    {
        digit0 = 1;
        digit1 = 1;
        digit2 = 1;
        digit3 = 0;
    }
    position++;
  }
}

In this ISR, we just displaying our multiplexed 7-segment with the help of timer interrupt. Timer0 is used in this case here. A dot point is positioned too here.

void Get_AC_Voltage(void)
{
      int mm;      
      max_point = 0;//clear all data
      adc_rd0 = 0;//clear data
      for(mm=0;mm<10;mm++)
      {
          for(k=0;k<500;k++)
          {
               ADCON0 = 0b00000001;
              if(temp = ADC_Read(0),temp>max_point)
              {
                  max_point = temp;
              }
          }
          max_point = abs(ceil((long)(max_point-128)));// subtract 2.5V
          adc_rd0+= max_point*41.8; //after calculation,calibration factor. 
      }
      adc_rd0/=10;

      ac_voltage = adc_rd0;
}

And finally, in the AC voltage measurement section, we used the same peak finding method with the average result like our AC voltmeter’s code. Only one thing is changed here is the calibration factor. Also note that as the output of Op-Amp circuitry is 2.5V at no input voltage so we have to subtract 2.5V from our calculation.

Test result:

Here is the test result of our high voltage AC voltmeter:

As you can see our voltmeter is displaying almost the exact voltage it is supplied from the POT-VR. A small error in the result is not so bad at all. Although this error can be eliminated in professional design by tuning the circuit and code.

This way, we can measure 500V+(!!!) even more if the resistors are selected properly. Remember, electricity is always dangerous. Work carefully at your own risk!!!

A non-polar 0.01uF capacitor can be used just at the input terminals of our Op-Amp to reduce noise. Note that, this may reduce the response time but it will filter most of the noises. R-C filter also can be added after output which will make the output more noise-free.

Anyway, I hope you enjoyed the article and will be able to make a voltmeter for yourself which can measure high voltage. If you need any help, feel free to ask. Thank you, Enjoy!

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