Meters and Bridges

8/21/2019 Meters and Bridges

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Ch-4 part-2

ANALOG OHMMETER

Analog meter can be designed to measure

resistance by allowing unknown resistance to

complete a series circuit and calibrating the

ammeter scale to read the resistance

There are two types:

Series type ohmmeter

Shunt type ohmmeter

1- Series type ohmmeter

The current in the meter is inversely

proportional to the total resistance of the circuit

Rtot= Rzero adjust+ Rmeter + R Uk

ukmz RRR

VI

Zero adjustInternal

battery

Ammeter

Unknown

resistance

RUK


2/33

When the load is short circuit,

Ruk = 0

Ro= Rz+ Rm

Current is very high (maximum ammeter full deflection

Ifs) , so the pointer go to right , read 0

omz

fsR

V

RR

VI

When RUk= Rzero adjust+ Rmeter=Ro

The meter will read half scale deflection

22

/

2/

fso

oukoukmz

IRV

R

V

RR

V

RRR

VI

From the above scale it is clear that, the total internal

resistance is 10K

When RUnknown=

The meter will read zero

0

omz R

V

RR

VI

The scale is nonlinear and the resistance increase in the

left side


3/33

In general :

The deflection ratio of the meter (D) is expressed as

follows:

UKo

o

ukmz

mz

RR

R

RRR

RRD

Where

D fraction of full-scale meter deflection

Ro total internal resistance of the meter (Rm+Rz)

RUK the unknown resistance

Example-7

Find the value of the unknown resistance when the

Ifs=1mA, Rm=100 and Vbattery=3V for the followingcases

D= 20%, 40%, and 50%

zmfs

RR

VI

KmA

RI

VR m

fsz 9.2100

1

3

KKRRRzmo

39.2100

Unknown

resistance

RUK

Zero adjustInternal

battery

Ammeter


4/33

UKo

o

RR

RD

o

o

uk RD

R

R

For D=20% =.2 , so KRuk 123

2.0

3

For D=40%=.4 , so KRuk 5.43

4.0

3

For D=50%=.5 , so KRuk 33

5.0

3

The zero adjust is necessary to compensate for the

changing voltage from the internal battery due to its ages.

To make ohmmeter sensitive to small value, connected

some resistance in parallel as shown below, so the range

can change

(The shunt resistances reduces the total internal

resistance, so larger current can passes (which mean

measure a small unknown resistance).

Zero adjust

Internal

battery

Ammeter

Rs

Range

switch

Improved series ohmmeter circuit


5/33

2-Shunt type ohmmeter

This type is used to measure small resistance

values

The unknown resistor (Zuk) is connected inparallel with each of the meter internal

resistance and the range switch resistor

When Zuk=0 (short circuit), no current flow

through the meter, and all the current passes

through S.C

When Zuk is not connected (Open Circuit), all

the current flow through the shunt resistance,

meter resistance and range switch resistance.

The scale is nonlinear, and the resistance

increase to the right side ( reverse of series

ohmmeter)

The meter must be turned off, when it is not in

use , otherwise the battery will discharge

On-off

switch

Zero adjust

Internal

batteryA

Range

switchzUK

Shunt OhmmeterSeries Ohmmeter


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Using ohmmeter

Because the ohmmeter supplies a current to the

resistance to be tested, so it must used withun-energized circuit, otherwise the meter will

need a calibration.

The measured resistor must be isolated from

other resistance or disconnect one of its end

THE MEGAOHMMETER (MEGGER)

It is an ohmmeter used to measure very high

resistances such as insulation.

The megger is equipped with hand-crank

generator to turn generator to deliver a highenough voltage to force current in the

resistance to be measured. Or use AC source.

THE VOM35 T (10-kV model)


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VOM Voltmeter-Ohmmeter Milliammeter

It is a meter replace all the previous circuits

combined together.

Can be used to measure each of the voltage, current

and resistance.

Have different ranges for volt, ohms and amperes

Measure AC, DC voltage and current and measure

Ohms

To measuring Ac, it must have an internal rectifier

bridge


8/33

USING VOMs

The precautions for Voltmeter, ammeter and ohmmeter

are applicable for VOMs

For ohmmeter to measure a resistance, the

zero adjust should be

checked each time the

range is switch

For voltmeter/ammeter you must set the range

switch to higher than

expected reading, then

switch to lower to read

accurate reading.

To prevent arcing (damage), the switch range not

change while the DUT is connected.

How to Measure a Capacitor

step-1 (using Ohmmeter)

1-The capacitor is removed from the circuit and discharges

by connected it across resistance

2- Adjust the VOM to ohmmeter and adjust the scale tohigher resistance range

3-Take care of the capacitor polarity if it is electrolyte

4-The ohmmeter read small resistance then it increased asthe capacitor started charges


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5-If the increase of the resistance is small , due to slowlycharging, change ohmmeter to less range.

6- If the resistance not increase, the capacitor is bad

Step-2 (using Voltmeter)

1-Change the VOM to voltage measuring

2-The capacitor is connected in series with voltmeter, seefigure

3-When the voltage is applied , it will divide betweeninternal voltmeter resistance and the capacitor

4- As the capacitor start to charges , the voltage across thevoltmeter is decease.

5-If the voltage not increase, the capacitor is bad

For large electrolyte capacitors may it have leakage current,

so they can appear bad especially with high impedance

meter, to check, compare with similar and good capacitor

Disadvantages of PMMC voltmeter

Low input impedance: Loading effect

Insufficient sensitivity to detect lowlevel signal

Supply

Cap


10/33

Approach

Utilized electronic devices such as

Op-amp ,BJT or FET to solve the above problems

Operational Amplifier (op-amp)

Inverting Amplifier

f

oin

R

V

R

v

00

1

1R

R

v

v f

in

o


11/33

Non-inverting Amplifier

inA vv

f

outinin

R

Vv

R

v

1

0

1

1R

R

v

v f

in

out

1-Using Op-amp to Measure voltage and current

This circuit can be used as ammeter or

Voltmeter


12/33

Ammeter

When function switch at A, current passes through

Rs, so voltage appear on V+

and current can bereading through the meter

Voltmeter

When function switch at V, based on the selected

range, a voltage appears on V

+

Vo=A*V+

Where A is the op-amp gain

Example-8

For the circuit shown, if Ifs=50A, Rm=1K and the Op-

Amp gain is 101, calculate the value of RS

Vo= A*Vin= 101*0.1=10.1 V

Vo = Ifs*( Rs+Rm)

10.1 = 50*10-3

mA*(Rs+ 1K)

Rs= 201 K


13/33

Transistor

The output voltage is related to input current, so it

is called :

Trans-Resistor Transistor

IE= IC+IB IC= IB Ic = IE

1

1 =(100-200)


14/33

It is clear that the common collector (Emitter

follower) have the largest input impedance, so it

can be used as VOA


15/33

Electronic meter

1-Transistor Emitter Follower Voltmeter

Example-9

The simple emitter-follower

circuit has VCC= 20 V,Rs+Rm=

9.3 k, Im= 1mA at full scale,and transistor hFE= 100

(a) Calculate the meter current

when Vin= 10 V

(b) Determine the voltmeter

input resistance with and

without the transistor.


16/33

Solution

When Vin=10V

(a) Im= (10-0.7)/9.3 =1 mA

Disadvantages of this cir cui ts

1-Can't measure voltage less than 0.5V, because

the junction between B, E will be off

2- When the input voltage is 5v(half the input),the meter reading must be 0.5mA, but in fact

I =(5-0.7)/9.3 =0.4624 mA

(b) The input impedance with transistor

KRRI

V

I

VR ms

E

in

B

inin 930)3.9(*100)(

(b) The input impedance without transistor

KRRI

VR ms

fs

inin )3.9()(

Based on this example, the above circuit has some

drawback


17/33

2- Emitter Follower Bridge Configuration

Voltmeter

Vm refer to the voltage o the meter

Vm=VE1-VE2

VE1=Vin-VBE1

VE2=Vp-VBE2

When Q1 and Q2 are identical , then VBE1=VBE2

Vm=VE1-VE2=(Vin-VBE1)-(Vp-VBE2) =Vin-Vp

When Vin=0, R5is adjusted such that Vp=0

Consequently, if Vp is set properly, Vm will be the

same as Vin


18/33

Junction Field Effect Transistor


19/33

FET VOMs and Electronic Meters

With using FET amplifier, the sensitivity can

increase from 20K/V to 10M /V

The resistance loading effect will nearly have no

effect.

The capacitive loading still needs to be considered

at high frequencies

The FET amplifier is sensitive to noise than thepassive VOM

3- FET Voltmeter

Vm = VE1-VE2

EG= VGS+VBE1+VE1So VE1 =EG- VGS-VBE1

VE2 =Vp-VBE2,

Vm = VE1-VE2= EG- VGS-VBE1-Vp+VBE2

= EG- VGS-Vp


20/33

The JEFT added a very high input resistance which

is need for voltmeter to minimize the loading effect

and make sensitivity is good

4- Electronic Ohmmeter (series connection)

1RR

REE

x

xB

1

1

E

E

RR

Bx

Example-10For the electronic ohmmeter shown in the Figure,

determine the resistance scale marking at 1/3 and 2/3 of

full scale

1

1

E

E

RR

B

x

At (1/3) full scale deflection E=EB/3

21

3/1

111 R

E

E

R

E

E

RR

B

BB

x


21/33

At (2/3) full scale deflection E=2EB/3

1

112

13/2

1

R

E

E

R

E

E

RR

B

BB

x

At (1/2) full scale deflection E=EB/2

1

11

12/

1

R

E

E

R

E

E

RR

B

BB

x

5-Electronic Ohmmeter (parallel connection)

When Rx= or open circuit

21

2

RR

REE B

which is the full scale deflection

When Rx//R2=R1

2//

//

11

1

21

2 BB

x

xB

E

RR

RE

RRR

RREE

which is the half- scale deflection


22/33

When Rx= 0 or short circuit

E =0

When Rx= any value

x

xB

RRR

RREE

//

//

21

2

AC Electronic Voltmeter

The scale on ac voltmeters are ordinarily calibrated in

rms volts

It should be noted that the rms value is calculated from

Vin, while the average value is calculated from the output

of ac-dc converter

Form factor is the ratio of the rms value to the average

value of the wave form

Crest factor is the ratio of peak value to the rms value

of the wave form


23/33

Average-Responding Voltmeter

=================================


24/33

Peak-Responding Voltmeter

The primary difference between the peak-

responding voltmeter and the average responding

voltmeter is the use of a detector (storage capacitor

with the rectifying diode)

In the first positive cycle: VC tracks Vin with the

difference of VD, until Vinreaches its peak value.

In the negative half cycle: diode is reversed bias

and the circuit keeps VCat VpVD.

The effect of discharging through R will be

minimized if its value is large enough to yield that

RC >> T.


25/33

RMS-Responding Voltmeter

1-RMS value definition: Mathematics

2-RMS value definition: Physical

rms voltage is equivalent to a dc voltage which generatesthe same amount of heat power in a resistive load that the

ac voltage does.

Digital Multimeter


26/33

Digital Voltmeter (DV)

DVM is essentially an Analog to digital

converter (A/D) with a digital display

Comparison between Digital and Analog Meters


27/33

Digital Multimeter Specification

1-Resolution

The number of digits shown for a

digital multimeter is generally

expressed as a mixed number such as

31/2, 33/4 or 5

1/2 .

31/2or 33/4 mean total # of digit is 4

5 1/2 mean total # of digit is 6

1/2 mean that the most significant digit (MSD)

may be 0 or 1

31/2 can display number from 0000-1999

So, the resolution is 1 part from 2000 or .05%

3/4 mean that MSD can be either 0,1,2,3

43/4 can display number from 00000-39999

So, the resolution is 1 part from 40000 or .0025%

Example-11

For 31/2 meter , calculate the resolution for 20V,

10V, 2V, 1V ranges

Resolution = Vrange*Resolution of 31/2

For 20 v R= 20*.05/100 = 10mV


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For 10v R=10*.05/100 = 5 mV

For 2V R=2*.05/100 =1 mV

For 1V R= 1*.05/100 =0.5 mV

As voltage range decrease as the resolution increase

2-Accuracy

The accuracy of digital meter is specified in variousways.

(a) As a percentage of the reading plus or minus a

number of contents

Manufactures specify the accuracy as a

percentage of the reading plus a percentage of the

full-scale-reading

(b) Parts per million (ppm)

Example-12

Two meter reading are given as follows:

Meter-1 accuracy : 0.05% rdg. +1 digit at 250

Meter -2 accuracy 0.05% rdg.0.03% (full-scale at 250)

Compute the possible error for a 31/2 digit meter operating at

250with an input of 100V.

Due to 31/2 digit, both meter will read four digit

The optimum reading will be 100.0V


29/33

For meter-1

0.05% rdg+1 digit ==0.05%*100+0.1=0.15

For meter-2

The meter is specified for both percentage of reading

and a full-scale. Since the full-scale reading for this

range will by 199.9V

0.05% rgd+0.03% FS 0.05%*100+0.03*199.9=0.109=0.11V

So the second meter have better accuracy than

meter-1

3-Sensitivity

Is the ratio of output response to the input causeunder static conditions

For meter , the sensitivity is related to the lowest range

available for a given function

ElectrometerIs a specialized multimeter with the capability of

measuring voltage, current, resistance, or charge

Has extremely high input impedance, above

100T


30/33

Designed with low input offset current, low drift

and low current noise

Picometer measure small current 10-12

A

Nanovoltmeter measure small voltage10-9V

Power Measurements

1-Low frequency power measurements

There are three types of power

a-True power I2R

b-Reactive power I(VL-VC)

c-Apparent power IVs


31/33

P =VI*Cos()

I current in Ampere (A)

V voltage in Volt (V)

P power in Watt (W)

phase between voltage and current

Cos() power factor (varies between 0 and 1)

For Single Phase

Measure V, I and the phase angle in between will

described later


32/33

Dynamometer wattmeter

It is device used to measure V and I simultaneously

This instrument is of the electrodynamics type.

It consists of:

1- pair of fixed coils, known as current coils

2- movable coil known as the potential coil.

The fixed coils are made up of a few turns of acomparatively large conductor.

The potential coil consists of many turns of

fine wire. It is mounted on a shaft, carried in

jeweled bearings, so that it may turn inside the

stationary coils.

The movable coil carries a needle which

moves over a suitably marked scale. Spiral coil

springs hold the needle to a zero position.


33/33

Power factor measurement

Use the previous dynamometer with adding anextra moving coil, The moving coils are

connected together at right angle to each others

One of the coils is connected in series to

inductor to respond to current that is out of

phase with the line current

The other is connected is series with a resistor

and respond to current that in phase with the

line current

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Meters and Bridges