Digital Multi-meter is an electronics Instrument which is used in every laboratory for Several measurement in a single device like voltage , current, continuity and resistance etc. This very useful tools in electronics families. It is also known as voltage ohm meter (VOM).  Its provide a convenient means of making rapid and accurate measurement of a.c and d.c voltage,  a.c and d.c. current and resistance in wide range. The digital Multi-meters  indicate the value of the measurement in form of a decimal number.

The Digital multi-meter has two term one is digital that means indicate that the device has a digital or LCD output and second term is multi-meter that means that a single device can be used for multipurpose measurements.

Part of Digital multi-meter

Display : It has illuminated Display Screen for better visualization and it has five digit resolution display screen.

Selection Knob :  Multi-meter is used for several measurements Like Voltage , Current and Resistance. The Selection Knob allows  the user to select the different measurements.

Port : It has the port . One is mAVΩ Port, This port ( Positive probe or Red probe) Can used for current , voltage and resistance measurement  and second is common Port also called black probe. Third Port is used for measuring current greater than 10A. It is also called current port.

Block Diagram   

Here Draw the block diagram to easily  understand the all Circuit of multi-meter and its working.

The Block Diagram is shown in fig. In this Block Diagram Two input probes , one is positive probe and second is a common negative probe.

The Rotatory switch is rotated and connected to the point of circuit block by which is selected the input measurements. it can be voltage , current ans resistance.

The Rectifier is used for converting ac input to dc before applying to Analogue to Digital Converter  it is also filter ripple of  the ac signals.

All Block circuit operation is depend on applications.  A/D Converter ( Analog to Digital Converter) is a main function of digital multi-meter circuit. It Converts analog signal to digital signal which is operated for all measurements .The accuracy of the  Multi-meter depend on the accuracy of A/D Conversion and built in precision voltage references.

A/D Converter pass the signal into Digital Counter decoder and decoder converts the signal into digit and provide output in digit form on LCD display.

The performance of digital multi-meter largely depends on the A/D Converter. Low cost multi-meter use single chip digit A/D converter and high precision digit  multi-meters use microprocessor controlled discrete A/D converters.

Different Applications 

For Measurements of Voltage : To Measurements of ac voltage , The input is connected to the calibrated compensated attenuator by selecting Rotatory switch. The attenuator reduces the voltage and fed this ac quantities in rectifier and filtering circuits. The rectifier converted this ac  quantities into dc quantities and filter ripples of circuit. this signal fed into  A/D Converter.

To Measurements of DC voltage , The input is connected to the calibrated compensated attenuator by selecting Rotatory switch and afrer the reduction voltage directly paas to A/D Converter.

For Measurements of Current : To Measurements Current, the drop across an internal calibrated shunt (Current to Voltage converter) is measure directly the A/D converter in dc Current mode and after ac to dc conversion in ac Current mode.

For Measurements of Resistance : To Measurements of Resistance.  Digital multi-meter works on ohms law. here  Digital Multi-meter measures the voltage of across  Externally connected resistance and constant current source forced a current  through it and this quantities fed to buffer amplifier.

Here voltage of across  Externally connected resistance =V
                                            current source forced a current = I
                                                                               ohms law V =IR
                                                                                                R = V/I

Digital Multi-meter provide very high input impedance, better accuracy and resolution. Modern Multi-meter   have auto ranging auto polarity and auto zero facilities


Resolution , sensitivity and accuracy are some of specifications which is characterise a digital meter.

Resolution: The number of digit position used in a digital meter determines the resolution.

If n = number of full digits
Resolution = 1/10^n
Therefore a 5 digit display has a resolution of 1/10^5 = 0.00001

Sensitivity: The smallest change in input is a sensitivity which a digital meter able to detect. Therefore it is lowest value ranging full scale value multiple by meter resolution.
Sensitivity = ( f. s.)min x R ,  Here  ( f. s.)min   is the  lowest  full scale value of meter and  R is a resolution expressed as decimal.

Accuracy: Accuracy is usually expressed as a percentage of the reading plus a percentage of full scale.  Often the percentage of full scale part of the statement is given terms of digits. when this is done, the digits referred to are the least significant digits.

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