Design of Digital Voltmeter By using Microcontroller

Tutorial for Atmeag8 /AVR based Voltmeter

               It measures the Ac voltage and shows readings in RMS value.

Basics Of AC signal 

The AC signal ,its amplitude varying in accordance with time. In india AC frequency is 50Hz.

Then the Time period one full cycle ,
  T = 1/frequency 
  T = 1/50Hz = .002second 
                     = 20milli Second
                     = 10 Milli second for Half Cycle

There are a lot of methods for measuring AC volt digitally

we could find the RMS value by multipliying the Peak Voltage with 0.707. 

   Vrms = Peak Voltage * 0.707 .

Approximatly Indian Line AC Peak volatge is 325v.

  Example : 
                 325v *0.707 = 229.775Volt      .(appro = 230Vrms).

But Microcontroller could only sense  up to 5 volt  as well as it couldnt sense negative values

so we cannot  give 230 volt directly to It .
It can be rectify by using a Stepdown transformer ,which has an advantage of safety.
Ac voltage can be reduce by using transformer to comfortable range . (ie 0 to 5volt DC)

make the calculation  as   ,
 The voltage Divider would give a 5 volt when the Vrms 230v. [will be more give details on the way ].

The full wave rectifier output will give to ADC of micro controller . Then do corresponding coding.

Microcontroller Section 
I am using here Amega8. 
we need to mesure only one half cycles repeatedly because  the waves same are repeating 

I selected , the F_cpu frequency is the internal RC Oscillator 8MHz.
Atmega8 ADC uses Prescalar [frequency reducer or divider by 2th,4th etc]
Atmega ADC  Prescale Frequency is in between 50Khz to 200Kz
ADC conversion time is in between 13 to 260 micro second.

ADC frequency = F_cpu / Prescaler

                         = 800,000HZ  / 8
                        = 62.5KHz

At high frequency ADC conversion would be fast while at lower frequency the conversion will be more accuracy.

ADC conversion time Calculation 

Conversion Time = 1/ADC frequency

                           = 1/62500 
                           = 0.000016 Second
                           = 0.016 Milli Second
                           = 16 Micro Second.     [At 62.500KHz]

So 16 mirco second would taken for one cycle of  conversion.with 62,5KHz.

13 ADC cycles required for one conversion

    13 * 16Micro Second = 230 Micro Second [approximately]
                                      = 230us.

The Half wave has 10milli second time duration.[100000microsecond].

There the available conversion times is 

                                        =  Total time period for a half cycle /one ADC conversion time  
                                        = 10000 micro second / 230 micro second
                                        = 43.4 samples  [ for one half cycleie 10 milli second]

    Image Shows only 7 Samples per half cycles .in our case it is 43

And again reduced the sampling to 40 .

calculation of Vpeak

 Takes the largest Vpeak from the 40 samples.
Vrms = Vpeak * .707

Atmega8 ADC StepSize Calculation

The Atmega8 ADC is a 10 Bit /[ie the Maximum high value is 1023 =1111111111] at which the ADC will give fullscale Value .
The ADC  require a clock as well as a reference voltage .
The resolution of ADC output decide the Step size.

ADC o/p = Vin * 1024 / Vref

  Vin =5volt , Vref =5volt 

                 ADC o/p = 5 * 1023 /5 = 1023 [ADC highest Value]

               Step Size  voltage = Vref / 1023 = 0.004887 volt =4.88millivolt
             One binary digit changes when 4.88volt changes takes place



Image's Step Size is 1.25 volt


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