Digital Voltmeter, Digital

Ammeter and Digital Multimeter

Done by,

C. Balaji 15E105

S. Deepak Arumugam 15E108

J. Karthickraja 15E117

S. Praveenkumar 15E137

Y. Elangovan 16E501

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Contents

Advantages of Digital meters Digital Components Digital Voltmeter(DC) Digital Voltmeter(AC) Measurement of Current Measurement of Resistance A complete DMM Measurement of hfe A digital voltmeter and ohmmeter using

Arduino.

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Advantages of Digital meters

Very high accuracy (5 to 0.05%)

Has very input impedance (2MΩ)

which ensures less loading effect on

the input.

The numeric display of digital meters

provide zero parallax error.

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Digital Components

Crystal Oscillator

Counter

A/D Converter

Voltage-Frequency Converter

Phase Locked Loop

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Crystal Oscillator

A piezoelectric crystal is used.

Generally, a1MHz crystal is used

which produce a clock pulse of 1MHz.

Used to provide the clock pulses.

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Counters

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A/D Converters

Converts Analog data to Digital data

using a dual slope converter.

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Voltage-Frequency Converter

Produces a frequency signal which is

proportional to the input voltage.

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Digital Voltmeter(DC)

The voltage is first attenuated and

then converted into a digital word

which is displayed.

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Role of ADC

The lines supplied by the ADC to the

display consists of

◦ The digital word

◦ A latch

◦ Start bit

◦ Polarity information

◦ Floating point information

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Features

Polarity detection

Auto ranging

Auto zeroing

Polarity detection:

The information is given by the ADC.

When the voltage is +ve, the characteristic

curve of the converter is in first quadrant.

The characteristic curve will be in the fourth

quadrant, if the voltage is –ve.

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Auto Ranging

Two comparators are used.

The typical ranges are,

◦ 0 to 100 mV

◦ 100 mV to 1 V

◦ 1 V to 100 V

◦ 100 V to 1000V

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Auto Zeroing

Zero is displayed during the start of

conversion.

This is achieved by discharging the

capacitance to the ground properly.

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DVM (AC)

The AC is processed first i.e., phase

compensated attenuation with

rectification and filtering is done.

The dc voltage obtained is converted

into a digital word, which is displayed.

For high frequencies, a precision

diode with small reverse recovery time

is required.

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DVM(Low voltage AC)

For measurement of voltages in the

range between 1 nV and 10 nV, the

voltage is converted into a frequency

signal by a voltage-frequency

converter or phase locked loop.

The frequency is measured by proper

networks.

This reduces the effect of noise

through ADC.

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Phase locked loop

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Measurement of current by DMM

Typical ranges

◦ 0 to 10 mA

◦ 10 mA to 100 mA

◦ 100 mA to 1 A

◦ 1 A to 10 A

Auto ranging is available.

Auto zeroing is also available.

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Two ways of measurement

◦ Measuring the voltage drop across a

resistance.

◦ Current frequency converter.

In the first method, current is passed

through a constant resistance. The

voltage drop across it is measured

using a DVM.

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In the second method, a current-

frequency converter is used.

The output of comparator is a

frequency which is proportional to the

current.

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Measurement of Resistance using DMM

A constant current is passed through

the unknown resistance and the drop

across it is measured by a DVM.

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Complete circuit of a DMMThe switches are coupled to each other.

The display is generally 3 digits (-1999 to 1999)

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Measuring hfe of a transistor

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Hfe measurement

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A small project: Digital Voltmeter

using ATMEGA328P

A0 GND

ATMEGA328P

Direct voltage measurement:

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RANGE EXTENDED VOLTMETER

A0 GND

ATMEGA328P

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CODE:

float voltage, previousvoltage=0;

void setup()

{

serial.begin(9600);

}

void loop()

{

voltage=analogRead(A0)*0.00488758553;

if(voltage!=previousvoltage)

{

Serial.println("the voltage is");

Serial.println(voltage);

previousvoltage=voltage;

}

delay(500);

}

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Ohmmeter using arduino

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Arduino code

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int analogPin= 0;

int a;

int Vin= 5;

float Vout= 0;

float R1= 1200;

float R2= 0;

float buffer= 0;

void setup()

{

Serial.begin(9600);

}

void loop()

{

a= analogRead(analogPin);

if(a)

{

buffer= a * Vin;

Serial.println(a);

delay(1000);

}

}

Challenges

If you use a small value of known

resistance to measure a large value of

resistance the value gives up an error,

So use known resistance which is

some what closer to the measuring

resistance to give an output with

greater accuracy.

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References

Sawhney A K, “A Course in Electrical and

Electronic Measurement and Insrtumentation”,

Dhanapat Rai & Sons, New Delhi, 2011.

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Thank You!!

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