unit-i block schematics of measuring …ac voltmeter : ac voltmeter using full wave rectifier •...

13-Aug-19

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SPHOORTHY ENGINEERING COLLEGEApproved by AICTE, Permanently Affiliated to JNTUH,

UNIT-IBLOCK SCHEMATICS OF MEASURING SYSTEMS

C.Lokanath ReddyAssistant Professor

Dept of ECE

13-Aug-19 2Electronics and Communication Engineering

Contents

• Performance Characteristics

• Measuring Instruments: DC Voltmeters

• D’ Arsonval Movement

• DC Current Meters

• AC Voltmeters and Current Meters

• Ohmmeters

• Multi meters

• True RMS Responding Voltmeters

• Specifications of Instruments.

Electronic Measurements & Instrumentation-EC743PE

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Performance characteristics

• The treatment of instrument & measurement system

characteristics can be divided into two distinct

categories.

1. Static characteristics

2. Dynamic characteristics



Static characteristics

1. Accuracy: Accuracy is the closeness with which an instrument

reading approaches the true value of the variable being

measured.

2. Precision: It is the measure of order or degree to which a

particular parameter is measured.

3. Resolution: Resolution is the smallest change in the measured

value to which the instrument can respond. It is the smallest

change the instrument can measure.


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Static characteristics

Example :

X : result

Centre circle : true value

XXX

XXXX

XXX

XXX

XXX

X X

X

xxHigh accuracy, high precision

Low accuracy, high precision

Low accuracy, low precision

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Electronics and Communication Engineering

Three targets with three arrows each to shoot.

Can you hit the bull's-eye?

Both accurate and precise

Precise but not accurate

Neither accurate nor precise

How do they compare?

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Electronics and Communication Engineering

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Dynamic characteristics

1. Fidelity: It is the degree to which an instrument indicates the

changes in the measured variable without dynamic error

(faithful reproduction).

2. Lag: It is the retardation or delay in the response of an

instrument to changes in the measured variable.

3. Repeatability: This is defined as the variation of scale reading

when the input is randomly applied.It is the closeness value of

same output for same input under same operating condition.



Dynamic characteristics

4. Reproducibility: This is a scale reading over a given period of

time when the input is continuously connected to the

instrument.It is the closeness value of same output for same

input under same operating condition over a period of time.


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Errors

• It is the numerical difference between the true value of a

quantity & its value as obtained by measurement.

• Error may be expressed as absolute or as percentage error.



Types of Errors:

• Gross Errors: These errors are mainly due to human mistakes

in reading or in using instruments or errors in recording

observations.

• Systematic Errors: These errors occur due to shortcomings of,

the instrument, such as defective or worn parts, or ageing or

effects of the environment on the instrument.

1. Instrumental Errors

2. Environmental Errors

3. Observational Errors


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Types of Errors:

• Random errors: These are errors that remain after gross &

systematic errors have been substantially reduced. No specific

reason can be assigned.



Problem:


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Statistical Analysis:



Gaussian Error :


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Root Sum Squares formula:



Problems:


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Problems:



Problems:


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D’ Arsonval Movement :

• The motor action is produced by the flow of a small current

through a moving coil which is positioned in the field of a

permanent magnet. This basic moving coil system is often

called the D’Arsonval galvanometer.



D’ Arsonval Movement :


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Permanent Magnetic Moving Coil Movement:



Permanent Magnetic Moving Coil Movement:• In this instrument we have a coil suspended in the magnetic

field of a permanent magnet in the shape of a horse shoe. Thecoil is suspended so that it can rotate freely in the magneticfield.

• When current flows in the coil, the developed(electromagnetic) torque causes the coil to rotate.

• The electromagnetic torque is counter balanced by amechanical torque of control springs attached to the movablecoil.

• The balance of torque, and therefore the angular position ofmovable coil is indicated by a pointer against a fixed referencecalled a scale.


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• The equation for the developed torque, derived from the basic

law for electromagnetic torque is

• The above equation shows that developed torque is

proportional to the flux density of the field in which coil

rotates, the current coil constants.


B A I N



• Since both flux density & coil constants are fixed for a given

instrument, the developed torque is direct indication of the

current in the coil. The pointer deflection can therefore be used

to measure current.

• The spring control provides a restoring torque


c

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Problem:



Problem:


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Indicators:

• Different instrument forms may be obtained by starting with

the basic meter movement & adding various elements as

follows

i) DC current by adding a shunt resistance, forming a micro

ammeter or a milliammeter.

ii) DC voltage by adding a multiplier resistance, forming a

millivoltmeter or a voltmeter.

iii) Resistance, by adding a battery & resistive network, forming

an ohmmeter.



Indicators:

• The basic meter movement becomes an AC instrument,

measuring

i) AC voltage or current by adding a rectifier, forming a rectifier

type meter for power.

ii) RF voltage or current, by adding a thermocouple type meter

for RF.


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Indicators:



Types of Instruments:

• PMMC

• Moving Iron

• Electrodynamometer

• Hotwire

• Thermocouple

• Induction type

• Electro static

• Rectifier


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DC voltmeter:

• A basic D’Arsonval movement can be converted into a DC

voltmeter by adding a series resistor (multiplier) as shown in

Figure.

• Rs = multiplier resistance

• Rm = internal resistance of the movement

• V = full range voltage of the instrument

• Im= full scale deflection current of the movement (Ifsd)



DC voltmeter:

• From the circuit of above figure


m

m

s

m

mm

mms

msm

RI

VR

RI

V

I

RIVR

RRIV

)(

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Problem:



Problem:


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Problem:



Multirange voltmeter:

• The range of the basic DC voltmeter can be extended by using

number of multipliers and a selector switch. Such a meter is

called multirange voltmeter and is shown in the Fig. 1.


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Practical multirange voltmeter:




• The multiplier resistances can be calculated as :

• In position V4,

• In position V3, the multiplier is R3 + R4.


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• In position V2, the multiplier is R2 + R3 + R4. In position V4,

• In position V1, the multiplier is R1 + R2 + R3 + R4.



Extending Voltmeter Ranges:

• The range of a voltmeter can be extended to measure high

voltages by using a high voltage probe or by using multipier

resistors as shown in figure. However, great care must be used

not to exceed the voltage drop required for full scale deflection

of the basic movement.


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Precautions to be taken while using a voltmeter :

• The voltmeter resistance is very high and it should always be

connected across the circuit or component whose voltage is to

be measured.

• The polarities must be observed correctly.

• While using the multi Range voltmeter. First use the highest

range and then decrease the voltage range until the sufficient

deflections is obtained

• Take care of the loading effect.



DC voltmeter :

Globe Analogue Voltmeters

• Analogue Voltmeter- 0 to 30Vdc Measuring Range Accuracy2.5%

• Resolution- 0.5V• Moving Coil,• Grade B, 7.5cm Scale,• Easy to Read,• Inclined Display,• Fully Enclosed Plastic Case, Overload Protection, External

Screw Adjustment to Zero,• 135 Length, 100 Width, 85mm Height


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DC ammeter:

• Rm = internal resistance of the movement

• Rsh = shunt resistance

• Ish=shunt current

• Im = full scale deflection current of the movement

• I = full scale current of the ammeter + shunt (i.e. total current)



DC ammeter:


m

mmsh

msh

mmshsh

II

RIR

III

RIRI

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DC ammeter:

Properties of Shunt Resistor

• Resistance of the shunt should not vary with time

• Temperature co-efficient of shunt and instrument should be

low and should be same

Multirange Ammeter



Problem:


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Problem:



AC voltmeter Block Diagram:


RectifierVoltage divider

DC AmplifierPMMC meter

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AC voltmeter :

• Basic Rectifier Type AC Voltmeter

AC Voltmeter using Half Wave Rectifier

• The diode D conducts only during positive half cycle.



AC voltmeter :

AC Voltmeter using full Wave Rectifier

• The full-wave rectifier provides higher sensitivity rating

compare to the half-wave rectifier.

• Bridge type rectifier is the most commonly used


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AC voltmeter :

65 Moving Iron Instruments AC Voltmeter

• Type SD-65 AC V : Accuracy: Class 2.5

• Specification: 30V 50V 75V 100V 120V 150V 200V 250V 450V 500V 600V, if Over 600V, meters can be connected potential transformer with secondary Voltage 100V.



AC voltmeter :

Average responding voltmeter


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AC voltmeter :

Average responding voltmeter

• The AC input applied is first amplified with the help of high gain

stabilized amplifier, up to a required level. This voltage is the

rectified using the diodes D1 and D2. The rectified voltage is fed

to DC milliammeter used as a measuring meter.

• The blocking capacitor used at the input side blocks any DC

component in the input voltage. Large amount of negative

feedback is used for the amplifier to ensure stability for

measurement accuracy.



AC voltmeter :

Peak responding voltmeters

• As the name indicates, this type of meter responds to the peak

vale of the AC input signal

The two types of peak responding voltmeters are

1. D.C coupled peak responding voltmeter

2. A.C coupled peak responding voltmeter


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AC voltmeter :





Advantages• When AC input is strong, the pre amplification of AC input is

not necessary. In such case, the capacitor and diode can betaken out of the instrument and placed in a probe. Themeasured AC signal in this cased, travel no further than diode.The loading effect is minimized.

• When capacitor and diode are placed in the probe, thefrequencies up to hundreds of MHZ can be measured.

Disadvantages• The harmonic distortion in the input causes the errors.• The instrument has limited sensitivity due to imperfect and

nonlinear diode characteristics.• The error is introduced if input waveform is not symmetrical.


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AC ammeter :

Thermocouple type RF ammeter

• Thermocouple consists of a junction of two dissimilar wires, so

chosen that a voltage is generated by heating the junction.

• The output of thermocouple is delivered to a sensitive dc

micrometer.

• Thermocouple instruments are standard means for measuring

currents at radio frequencies.



AC ammeter :



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AC ammeter :




AC ammeter :Thermocouple type RF ammeterAdvantages:• Thermocouples are cheaper than the resistance thermometers.• These instruments are not affected by stray magnetic fields.• These instruments are free from frequency errors.• These instruments have high sensitivity.• These are very useful when used as transfer instruments to

calibrate dc instruments by potentiometer.

Disadvantages:

• They have lower accuracy & cannot be used for precision work.• To ensure long life of thermocouples in their operating

environments, they should be protected in an open or closedend metal protecting tube.


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True RMS responding voltmeter:




• The heating power of a given input signal is proportional to the

square of the rms value of the voltage. Sensing this power, this

meter produces a deflection using a thermocouple.

Thermocouple outputs are non-linear, in general.

• Therefore, DC current is directly proportional to the effective

or rms value of the input voltage.

• This is indicated on the meter movement in the output circuit

of the DC amplifier. Thus, the true rms value of the AC input can

be measured irrespective of the shape of the input.


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Advantages:

• These instruments are not affected by stray magnetic fields.• These instruments are free from frequency errors.• These instruments have high sensitivity.• These are very useful when used as transfer instruments to

calibrate dc instruments by potentiometer.• Thermocouples are cheaper than the resistance thermometers.

Disadvantages:

• They have lower accuracy & cannot be used for precision work.• To ensure long life of thermocouples in their operating

environments, they should be protected in an open or closedend metal protecting tube.





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Series type Ohmmeter :


Fig. Series type Ohmmeter Dial of series ohmmeter



• Values of R1 and R2 can be determined from the value of Rx,

which gives half the full scale deflection.

• The total current of the circuit,


m

mmh

RR

RRRRRRR

2

2121 )//(

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• The shunt current through R2 is I2=It-Ifsd

• The voltage across the shunt, Vsh= Vm

• So, I2 R2=Ifsd Rm

• Since I2=It-Ifsd

• Then,


fsdt

mfsd

II

RIR

2



• Since It=V/Rh


hfsd

hmfsd

RIV

RRIR

2

V

RRIRR

hmfsd

h 1

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Shunt type Ohmmeter :


Fig. Shunt type Ohmmeter Dial of series ohmmeter



• From fig, Rx=∞, the full scale meter current is


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• With any resistance Rx to be measured;

• The meter current




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Multimeter :

• A multimeter or a multitester, also known as a VOM (Volt-Ohm

meter), is an electronic measuring instrument that combines

several measurement functions in one unit.

• A multimeter is basically a PMMC meter. To measure dc current

the meter acts as an ammeter with a low resistance.

• Range changing is accomplished by shunts in such a way that

the current passing through the meter does not exceed the

maximum rated value.



Multimeter :


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Meter Protection:



Extension of Range:


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Extension of Range:



Extension of Range:


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Extension of Range:



Digital Voltmeter:Ramp type DVM


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Digital Voltmeter:Successive Approximation type DVM



Digital Voltmeter:Successive Approximation type DVM• At the start of conversion cycle both the control register & the

ring counter are set with 1 in the MSB and 0 in all bits of lesssignificance.

• Thus the ring counter shows that the cycle has started & theprocess is in its first phase.

• The control register shows 1000(4 bit converter) & this causesan output voltage at D/A converter section of one half ofreference voltage supply.

• At the same time a pulse enters the time delay circuitry. By thetime that the D/A converter & comparator have settled thisdelay pulse is gated with the comparator output.


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Digital Voltmeter:Dual Slope Integrating type DVM



Digital Voltmeter:Dual Slope Integrating type DVM

• In this technique, an integrator is used to integrate an accurate

reference voltage for a fixed period of time. The same

integrator is then used to integrate with the reverse slope, the

input voltage. The time required to return to the starting

voltage is measured.


unit-i block schematics of measuring …ac voltmeter : ac voltmeter using full wave rectifier •...

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