Electronic Instrumentation & Automation ET-7th...

RCET/Rahul Sharma/ET VII/2011RCET/Rahul Sharma/ET VII/2011RCET/Rahul Sharma/ET VII/2011

ET-7th semester

Electronic Instrumentation & Automation

By : Rahul SharmaET & TC Deptt.RCET, Bhilai


UNIT: IIIVoltage and Current Measurements

Digital Voltmeters:Non-Integrating type, Integrating Type, Using counting circuits.

Detectors: Principles of AC voltage measurement, Average and Peak responding detectors, Peak to Peak detector, Root mean square detectors, DC and AC probes.

Multimeters:Basic Hand-held Multimeter, Bench type Digital Multimeters, Comparison of Analog and Digital Multimeters, Digital LCR meters.


Digital Voltmeters

It displays the value of voltage, AC or DC, being measured in digital number system.

It eliminates observational errors like parallax or approximations completely.

It provides output with a better speed and also fed it directly to memory devices for storage.


Reference

ADCSignal

Processing

Data Transmission

Element

Display


Classification of DVM

Non Integrating Type Ramp Type Voltmeter Potentiometric Type Voltmeter Successive Approximation Type Voltmeter

Integrating Type Voltage to Frequency Conversion Type Voltmeter Dual Slope or Voltage to Time Conversion Type

Voltmeter


Ramp Type Voltmeter

Linear Ramp Type Voltmeter


Its principle is to measure the time that a linear ramp voltage takes from level of input voltage to zero voltage.

The time interval is measured in terms of counts, which is displayed as a number of digits.

A Sample Rate Multivibrator is used for initiating pulse for ramp voltage.


Advantages of Linear Ramp Type Voltmeter

It has easy circuit design. The cost is low. Output pulse can be transferred over long

lines without data loss. Input signal is converted to time which is

easy to digitize.


Disadvantages of Linear Ramp Type Voltmeter

The accuracy depends only on ramp. Large errors are possible, if noise is

superimposed with input. The speed of measurement is low.


Staircase Ramp Type VoltmeterStart Stop Gate Open

Gate Closed


DAC

Clock

Ref.Voltage

Counter Display

Comparator

Reset/Start


It compares the input signal with internally generated staircase ramp voltage which increases starting from zero.

The total number of staircase is counted and displayed as the output.

The step height of staircase voltage can be varied by appropriate reference voltage.


Advantages & Disadvantages of Staircase Ramp Type Voltmeter

Advantages: Comparatively greater accuracy. Circuit is simple & of low cost. Input impedance of DAC is high when

compensation is reached. Disadvantages:

Accuracy depends only on DAC. Speed is limited up to 10 readings per seconds.


Potentiometric Type Voltmeter

Comparator

ErrorAmplifier

PotentiometerAdj. Device

ErrorSignal

InputVoltage

Readout

FeedbackVoltage

SlidingContact

Ref. Voltage


Successive Approximation Type Voltmeter

DAC

Reference

Logic Control& Sequencer

Digital Display

UnknownVoltage

High

Low


Advantages & Disadvantages of Successive Approximation Type Voltmeter

Advantages: Accuracy is very high. High speed of 100 readings per second is possible. Display up to 5 digits is possible.

Disadvantages: Circuitry is complex. Input impedance is variable. Comparator can make errors if noise is introduced.


Integrating Type DVM

Voltage to Frequency Conversion:


It measures the average value of the input voltage over a fixed measuring period.

It employs the voltage to frequency conversion as an integration technique.

For any input voltage, the integrator produces a ramp voltage of opposite polarity.


Dual Slope Type DVM:


Advantages of Dual Slope Type DVM

Excellent noise reduction. RC time constant not affect the input

measurement. Integrator response to average value, so no

sample & hold circuit is required. Accuracy is high & can be varied by

requirement.


Principles of AC Voltage Measurement

AC voltmeters are classified as: Average responding. Peak responding. RMS responding.

Average & peak responding voltmeters are calibrated to read the rms value of sine wave.

RMS calibration is done because DC voltage generates same amount of heat as AC voltage.

RCET/Rahul Sharma/ET VII/2011RCET/Rahul Sharma/ET VII/2011

The rms value of sine wave is given as,

Vrms = V2

n

The average value of sine wave is given as,

Vav = V n

1/2


Average Responding Detectors


The multimeter is calibrated for rms input voltage which is filtered, averaged & then displayed.

The capacitors act as filter capacitors for rectifier diodes & coupling capacitors for the feedback signals.

The diodes act as switches to maintain unidirectional meter current despite of instantaneous polarity changes of the input signal.


Peak Responding Detectors


The capacitor charges through the diode to the peak value & the meter circuit responds to the capacitor voltage.

The capacitor discharges due to high impedance of amplifier so it charges to peak voltage.


Advantages of Peak Responding Detectors

The diode & capacitor can be removed & placed in the probe when no pre amplification is required.

The peak responding voltmeter is able to measure frequencies up to hundred of MHz with a minimum of circuit loading.


Disadvantages of Peak Responding Detectors

Peak responding detector is susceptibility to errors caused by harmonic distortion in the input waveform.

Due to imperfect diode characterization, instrument is limited sensitive.

Input waveform is not symmetrical, due to which different output is received when voltmeter leads are reversed.


Peak to Peak detector


It is also called as Voltage Doubler Circuit. The capacitors used should be large enough,

so that voltage across them should not change appreciably if other capacitor is recharging.


Root Mean Square Detectors


DC Probe Jaw:

DC Probes


AC Probe


AC Probe Jaw:


Multimeters

It usually measures Current AC or DC Voltage Resistance

It is further classified as Analog Multimeters Digital Multimeters


Analog Multimeters

It works on PMMC or D’Arsonval principle. It consists of a set of resistors

& a rectifier unit. It is provided with a rotary

selector switch.


Digital Multimeters

Modern multimeters are often digital due to their accuracy, durability and extra features.

They provide high accuracy, have high input impedance & are of smaller size.

They employ analog to digital converter & have a digital readout display as output.


Bench Type Multimeter

They are used to measure high degree of accuracy. They can measure voltage

down to 10 nV, current downto 1pA and resistanceas high as 200 T.


Comparison Between AnaolgMultimeters & Digital Multimeters

Analog multimeter is less costly compared to digital due to simple construction & as no power supply required.

Visual indication of changes in analog multimeters is not good due to damping torque effect. While in digital multimeter, excellent visual indication is acquired.

Analog multimeters are not as accurate as the digital multimeter as the magnetic effect in the coil degrades.

In an analog multimeter, the range has to be adjusted by the user. Whereas in a digital multimeter, the range is adjusted automatically.

In a digital multimeter, the rotary switch can be easily rotated to tune the circuit to its peak.

RCET/Rahul Sharma/ET VII/2011

LCR Meters

RCET/Rahul Sharma/ET VII/2011

Digital LCR Meter

Electronic Instrumentation & Automation ET-7th...

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