‫مختبر القياسات الكهربائية‬
Electrical Measurement Lab.
Experiment. No. (1)
AC Voltmeters
Objective
To design, construct, and calibrate a basic AC voltmeter circuit
using both half wave and full wave rectification.
Theory
There are several types of meters available in measuring
alternating currents and voltage, so far the most widely used is the
rectifier type of meters. This instrument can be constructed by using the
basic DC movement in conjunction with rectifiers. It is an average
responding type with scale calibrated in terms of the r.m.s value of a sin
wave.
It is important to review some basic definitions related to a.c quantities.
Average value of ac quantity is defined as that value which is obtained by
averaging all the instantaneous values over a period of a half cycle.
For the symmetrical a.c quantity, the average value over a complete cycle is
zero as both positive and negative half cycles are exactly identical. Hence
average value is calculated over a half cycle. If the a.c quantity is continuous
then average value can be expressed mathematically using integration as,
2
𝑣𝑎𝑣 =
𝑇
𝑇⁄
2
∫ 𝑣i𝑛 𝑑𝑡
0
Where the interval (T/2) indicates the average over half a cycle, "RMS"
stands for Root mean square, and is away of expressing an A.C quantity of
voltage or current in terms functionally equivalent to DC. For example, 10
volts A.C RMS is the amount of A.C voltage that would produce the same
amount of heat dissipation a cross a resistor of given value as a 10 volt D.C
power supply. Also known as the "equivalent" or D.C equivalent value of an
A.C voltage or current, mathematically the r.m.s value of the continuous as
voltage having time period T is given by.
𝑇
𝑣𝑟.𝑚.𝑠
1
= √ ∫ 𝑣2i𝑛𝑑𝑡
𝑇
𝑜
A.C Voltmeter using half wave Rectifier.
The A.C Voltmeter using half wave Rectifier is achieved by introducing a
diode in a basic d.c voltmeter.
This is shown in the fig (1). The diode D conducts only during positive half
cycle
And meter movement is by passed for another cycle. Hence it responds to
half the average value of the a.c input.
Eav = Em /π
Also Em = √2Er.m.s
Eav = 0.45Er.m.s
or Edc = 0.45Er.m.s , Edc = 0.318 Em
Thus the value of series multiplier can be obtained for a.c voltmeter as.
Rs = RT – (RD + Rm)
Where
RT = Ed.c / IFSD
RD = Forward resistance of the diode D.
Rm = Movement internal resistance.
A.C Voltmeter using full wave Rectifier.
The a.c voltmeter using full wave rectifier is achieved by using bridge
rectifier consisting of four diodes.
As shown in fig (2).
Where:
Eav = 2Em/π
Also Vm = √2Vr.m.s
Eav ≈ 0.9Er.m.s
Or Ed.c = 0.9 Er.m.s , also Ed.c = 0.636Em
The multiplier resistance can be obtained as.
RS = RT – (2RD+Rm)
It's obvious that the pointer will deflect to 90% of full scale.
The meter a.c sensitivity (Ω/V) equals 0.9 of d.c sensitivity for full scale.
Sensitivity (a.c) = 0.9 × sensitivity (d.c) for full wave
Procedure
1. Calculate theoretically the value of Rs, for the following r.m.s AC
voltage ranges (3, 5, 7 Vr.m.s) using half-wave rectification.
2. Construct the half-wave AC voltmeter circuit shown in Figure (1).
Using variable resistor as multiplier resistors (Rs).
3. Apply the r.m.s AC voltages range (3, 5, 7 Vr.m.s) to your AC
voltmeters and adjust the resistors Rs, for full-scale deflection on each
range.
4. With a full-scale reading on the 5V r.m.s range, connect the
oscilloscope across the movement and sketch the waveform.
5. Repeat the above steps for a bridge full wave rectifier.
6. Compute the percent error between the actual and computed value Rs
for each range of the half-wave and full-wave rectifier type AC voltage.
Discussion
Q1- What is the advantage of using permanent magnet moving coil
instruments for measurement on a.c instrument?
Q2:-what are the limitations of rectifier type instrument?
Q3:- could we use analog rectifier type a.c voltmeter in measuring d.c
voltage?
No. We cannot because the rectifier type instrument is average indicating
and will give Wrong answer. Much to low indication.
Q4:- Do you think that the value of the diode forward resistance RD is
fixed or variable through the entire range of the voltmeter?
Experiment No.(2)
Calibration of Ammeter
Objective
1- To learn how to calibration of ammeter.
2- To determine relative error.
Theory
There are many methods to calibrating the measuring devices and these
methods should have a high degree of accuracy during calibration procedure.
The Potentiometer is considered an important device in calibration process
due to its high ability in measurement small voltage in high accuracies
without drawing a sensible current from input terminal of measurement
system which used in calibration.
Calibrate of a dc ammeter can most easily carried out by the arrangement of
figure (1). The value of Ia through the ammeter to be calibrated is
determined by measuring the potential difference across a standard resister
Rs by the potentiometer method (digital voltmeter could be used for this
purpose) and then calculating the current Iaby Ohms law
Ia = E/Rs
Thus the results of this calculation is compared to the actual reading of
the ammeter under calibration and inserted in the circuit. The circuit
consist of a precision dc power supply and a resister Rs (which is the
internal resistance of the ammeter) and a variable resister R is placed in
the circuit to control the current to any desired value, so that different
points on the meter scale can be calibrated.
Finally the difference of the comparison between the exact readings with the
measured reading of the current determines the error in the meter.
Correction curves could be drowning between the measures current I and the
relative error Er as shown in Figure (2).
Procedure
1- Design a dc ammeter with (3mA, 10mA) ranges.
2- Make (VDC=4V, Ra=100  , Rs=1k ).
3- Change the current value from (0-3mA) in steps 0.5V.
4- Calibrate the similar device and fixed current on scale.
5- Calculate the error percent and draw correction curve.
6- Repeat the previews steps to range (10mA), and Vin=10V, Rs=1k
 , Ra=30  ).
7- Design a dc ammeter with (3mA, 10mA) ranges.
8- Make (VDC=4V, Ra=100  , Rs=1k ).
9- Change the current value from (0-3mA) in steps 0.5V.
10- Calibrate the similar device and fixed current on scale.
11- Calculate the error percent and draw correction curve.
12- Repeat the previews steps to range (10mA), and Vin=10V,
Rs=1k , Ra=30  ).
Discussion
1. Explain the function of the variable resistor R (Reheostat) used in the
circuit?
2. What is the use of a potentiometer? Did you use it in this experiment?
3. What is the most important difference between d.c and a.c
potentiometers?
Experiment. No. (3)
Photoconductive Cell
Objective
To study the Characteristics of Photoconductive Cell
Theory
Electrical conduction in semiconductor materials occurs when free charge
carriers e.g. electrons are available in the material when an electric filed is
applied .In certain semiconductors. Photoconductive cell are elements whose
conductivity is a function of incident electromagnetic radiation, since,
resistance of these materials decrease with increase in incident light,
therefore these materials are also called Light Dependent Resistor or LDR.
Commercially available photoconductive cell materials are cadmium sulfide
(CdSe) with band gap of 2.42 eV&1.74 eV respectively. On account of the
large energy bands, both the materials have a very high resistivity at ambient
temperature which gives a very high value of resistance for practical
purposes. The photoconductive cells use a special type of construction which
minimizes resistance while providing maximum surface. Photoconductive
cells are made by chemically sintering the required powder in to tablets of
the protective envelope of glass or plastic. Electrons are deposited on the
tablet surface and are made of materials which give an ohmic contact but
with low resistance compared with that of the photoconductor.
The electrodes are usually in the form of interlocked fingers as shown.
Photoconductive cell are made from cadmium sulfide doped with silver
antimony or indium chemically deposited on a substrate. Light falling on the
sensitive area breaks chemical bonds. The resulting electrons and holes
become available to increase the conductivity. These bonds are slow to reform when light is removed and the response time is sluggish. The resistance
of the ORP12 drops dramatically as the incident light increases. Is
characteristics are given in table given. The device requires a suitable load
resistor to provide a voltage output which then falls with increasing
illumination. The characteristics of a photoconductive cell vary considerably
depending upon the type of material used. When the cell is kept in darkness
its resistance is called Dark Resistance. The dark resistance may be as high
as 101o Ω. If the cell is illuminated its resistance decreases. The resistance
depends on the physical character of photoconductive layer as well as on the
dimensions of the cell and its geometric configuration. The current depends
upon the electricity voltage applied and its geometric configuration. The
current depends upon the electricity voltage applied and it is of the order of
the mA. When using photoconductive cell for a particular application it is
important to select the proper dark resistance, as well as suitable sensitivity.
The sensitivity is defined as:
S=∆R/∆H Ω/W.m^2
Where:
∆R=Change in resistance; Ω
∆H=Change in irradiation; W/m-2
The spectral response of the sensor must match that of the light source. A
photo conductor has a relatively large sensitive area. A small change in light
intensity causes a large change in resistance. The relationship between
irradiance and resistance is, however not linear. It is closely an exponential
relationship. The spectral response of cadmium sulfide cell closely matches
that of the human eye and the cell is often used in application where human
vision is a factor, much as street light control or automatic iris controls for
cameras, to alter the bias of transistor or change the gain of an amplifier.
Such circuits are used in automatic bridge circuit applications, and for
measurement of attenuation of light etc.
The device used on ST2301 is ORP12. Its characteristics are given below:
The photoconductive cell is already connected as shown below:
Procedure
1- Connect the circuit as shown in figure.
a. Socket C of Wire Wound potentiometer to + 12 V.
b. Socket A of Wire Wound potentiometer to 0 V.
c. Socket B of Wire Wound potentiometer to input of Power
Amplifier.
d. Output of Power Amplifier to input of Lamp Filament.
e. Other input of filament lamp to +ve input of Moving Coil Meter.
f. –ve of moving coil meter to 0v.
g. Connect C socket of Slide Potentiometer to +5v.
h. Output of Photoconductive cell to B input of Slide Potentiometer.
2-
345-
6-
i. Connect a Digital Multi-meter as voltmeter on 20V DC range to
measure the photoconductive cell output voltage between output of
photoconductive cell and 0 V
Set the 10kΩ Slide potentiometer setting to 3 so that the
photoconductive cell load resistance is approximately 3k. (can be
verified with DMM)
Place the opaque box over the plastic enclosure to exclude all ambient
light.
Switch ،On, Power Supply and set the Wire Wound potentiometer to
minimum for zero output voltage from the Power Amplifier.
Take readings of Photoconductive cell output voltage as indicated on
the Digital Multi-meter as the Lamp voltage is increased in 1V steps.
Record the results in below table.
Switch Off the Power Supply.
Results and Discussion
1- Plot the graph of photoconductive cell output voltage against Lamp
Filament voltage. It should resemble the one given in Figure. Make a
comment on the results obtained.
2- What is luminance, state its units?
3- List any two practical application of LDR?
Experiment No. (4)
Tungsten Filament incandescent Lamp
Objective
To study the Characteristics of tungsten Filament Lamp
Theory
The light source used in ST2301 Trainer is a imported Tungsten filament
lamp with a mixture of nitrogen and argon gas under low pressure. The
lamp glows more brightly as the power feeding the lamp is increased.
Two factors will be affected as the lamp voltage is increased:
1 – The temperature of the filament is proportional to the input power.
Power varies with the square of the voltage and is also affected by the
resistance of the lamp, which increases as filament temperature increases.
2 – The spectral response of the lamp varies with the filament
temperature. At low temperatures the light is in the infrared region of the
visible spectrum and the light gradually increases in frequency (redorange-yellow…) as the temperature is raised. These factors make it
difficult to be precise about the response of the sensors which will be
investigated.
The lamp is connected as shown below
Procedure
1- Connect the circuit as shown in figure 3.
a. The socket C of wire wound potentiometer to +12V supply.
b. The socket A of wire wound potentiometer to 0V supply.
c. The socket B of wire wound potentiometer to I/P of power Amplifier.
d. Connect output of power Amplifier to input of filament lamp through
a Digital Multi-meter connected as an Ammeter at 200mA range.
e. Connect the other input of Filament Lamp to +ve input of moving
Coil Meter.
f. Connect the –ve input of moving Coil Meter to 0V.
2- Switch 'On' the power supply & set the 10 KΩ wire wound potentiometer
to minimum for zero voltage (on Moving Coil Meter) from the power
Amplifier.
3- Take readings of Filament Lamp current as indicated on the Digital Multimeter as lamp voltage is increased in 1V steps. Record the results in below
table.
4- Switch 'Off' the power supply.
Results and Discussion
1 - Calculate the corresponding values of Lamp Filament power (V×I) and
resistance (V÷I) record the results in the given table.
2 - Plot the graphs of lamp power and resistance against applied voltage. It
should resemble the one given in graph 1. Write down your comment.
3 - Plot the I-V characteristics and compare to an ideal resistor.
4- State the equation of temperature influence on metal resistance and write
a brief comments on filament resistance behavior on temp rise.
Experiment No. (5)
MEASUREMENT USING DC BRIDGES
WHEATSTONE BRIDGE
Objective
To investigate how D.C. bridges can be used to measure an unknown
resistances.
Apparatus
Wheatstone bridge circuit which is consists of four arms, galvanometer and
a power supply.
Theory
Precision measurements of component values have been made for many
years using various forms of bridges. The simplest form of bridge is for the
purpose of measuring resistance and is called the Wheatstone bridge.
Figure (1) shows the schematic of a Wheatstone bridge. The bridge has four
resistive arms, together with a source of emf (a battery) and a null detector,
usually a galvanometer or other sensitive current meter. The current through
the galvanometer depends on the potential difference
between points A and B. The bridge is said to be balanced when the
potential difference across the galvanometer is 0 V so that there is no current
through the galvanometer. This condition occurs when the voltage from
point A to C equals the voltage from point B to C; or by referring to the
other battery terminal, when the voltage from point A to point D equals the
voltage from point B to D. Hence the bridge is balanced when
𝐼1𝑅1 = 𝐼2𝑅2 ………………… (1)
If the galvanometer current is zero, the following conditions also exist:
𝐼1 = 𝐼3 =
𝐸
𝑅1+𝑅3
…………….. (2)
And
𝐼2 = 𝐼4 =
𝐸
𝑅2+𝑅4
…………….. (3)
Combining equations (1), (2) and (3) and simplifying, we obtain
𝑅1
=
𝑅1+𝑅3
From which
𝑅2
…….…………..(4)
𝑅2+𝑅4
𝑅1𝑅4 = 𝑅2𝑅3…………………. (5)
Equation (5) is the well-known expression for balance of the Wheatstone
bridge. If the three resistances have known values, the fourth may be
determined from equation (5). Hence, if the unknown resistor, its resistance
R4=Rx can be expressed in terms of the remaining resistors as follows:
𝑅𝑥 = 𝑅3 𝑅2 ……………….. (6)
𝑅1
Resistor R3 is called the standard arm of the bridge, and resistors R2 and R1
are called the ratio arms.
Procedure
1. Connect the Wheatstone bridge shown in Figure (1).
2. Set the value of R1=R2=1kΩ. Use a variable resistance box for R3 and
set an unknown resistor with relatively low resistance value.
3. Set the dc power supply to 10V.
4. The variable resistance R3 is varied until the galvanometer shows null
deflection.
5. When balance of the bridge is obtained, calculate the value of the
unknown resistance Rx from equation (6).
6. Replace R1=R2= 100Ω and repeat steps 2 to 5.
Discussions
1. What are the advantages of Wheatstone bridge?
2. State the sources of errors which are generally traceable in measuring
resistance with Wheatstone bridge?
3. What are the limitations of Wheatstone bridge?
4. State which is better, the high or low values of ratio arms (R 1 and R2)?
Why?
Experiment No. (6)
De Sauty’s bridge
Objective
To compare the capacities of two condensers (or) to find the
capacitance of the given condenser, by using De Sauty’s bridge.
Theory
Bridges are the some of the most accurate measuring devices for
measuring impedance, capacitance, resistance, etc. For our purpose, i.e. for
measuring Capacitance, using a de Sauty’s is best. The De Sauty’s bridge is
an A.C Bridge works on the principle of Wheat stone’s bridge. This bridge is
used to determine the capacity of an unknown capacitor C 2 in terms of the
capacity of a standard known capacitor C1. Here R1 and R2 are non inductive resistors. R1, R2, C1 and C2 are connected in a Wheat stone’s
bridge as shown in the figure (1). When the bridge is balanced, the ratios of
impedances are equal as given below.
𝑍1
Z2
=
1
𝑗𝑤𝑐1
Z3 ..............................................................
=
R1
𝐶2
C1
=
(1)
Z4
1
jwc2 ..................................... (2)
R2
R1 ...................................................
(3)
R2
Procedure
1- The connections are made as per the circuit diagram shown in the figure
(1). The resistance R1 and a Condenser C1 are in series in one branch of the
bridge and a resistance R2 and a capacitor C2 are in series in another branch.
2 - Adjust the function generator to give a sinusoidal frequency of 1KHZ,
5Vpp.
3 – Set C1, R1 and C2 to the first values in table (1).
4- Now balance the bridge by varying the variable resistance R 2 until you get
the most stable and best null point.
5- Note down the value of R2 in the table.
6- Repeat the above procedure for different sets of values of C 1, R1 and C2 as
noted in table (1).
7- Calculate the unknown capacitance using the formula given in the table.
8- Repeat the above process for a frequency of 15KHZ.
M
Figure (1) De Sauty’s bridge
Discussion
1- What is the effect of the frequency on balancing the bridge?
2- Did you notice a change in the calculated value of C2in the two different
excitation frequencies.
3- State the difference between De Sauty and Schering Bridge.
Experiment No. (7)
Design and Construction of Analogue Multimeter
Objective
1. To understand the structure of the ammeter and voltmeter.
2. To learn how to use the meter and using them to measure the current and
voltage of an electric circuit.
Theory
D,Arsonval Galvanometer
This type of galvanometers is used in the various methods of resistance
measurement and in D.C. potentiometer work. D, Arsonval galvanometer
belongs to the group of moving coil instruments. So, this type of
galvanometer consists of a coil carrying the current to be measured and
swinging in the field of a permanent magnet.
Constructional features of the galvanometer are as follows:
1- Moving coil. The coil of many turns of fine wire wound on an
aluminum former may be either rectangular or circular. The coil is
suspended between the poles of a permanent magnet. There is a fixed
iron core inside the coil, the air gaps between the iron core and the
permanent magnet are usually about 1.5 mm. the iron core is cylindrical
in case of rectangular coil and is spherical in case of circular coil.
Figure (1): Moving coil galvanometer.
2- Damping. In case of a coil with a metal former, mainly aluminum, eddy
current damping is introduced. The damping torque which opposes the
motion is produced due to the reaction of eddy current with the permanent
magnet-filed. The eddy current is due to the e.m.f. induced in the former
by the filed of the permanent-magnet as the former rotates in it.
Alternatively, damping may be obtained by connecting a shunt resistance
across the galvanometer.
3- Indication. In most cases, a beam of light is projected to a small mirror
attached to the suspension strip, as shown in Fig. (1), through a glass
window in the outer case of the instrument. The beam of light is reflected
from the mirror on to a scale.
4- Zero-adjustment. A torsion head, to which the end of the suspension
strip is attached, is provided for adjusting the coil position and hence, the
zero setting.
5- Coil clamping. A suspension galvanometer has a delicate moving system
and hence they are protected from mechanical injury when it is moved.
D.C. Voltmeter
The voltmeter uses a high series resistance with the permanent magnet
moving coil meter.
The range of a voltmeter can be increased by resistance in series with the
voltmeter as shown in Fig.(2). If the instrument gives full scale deflection
when a current I is passed through it then
Where:
V= voltage to be measured
R= meter resistance
Rs= series resistance
V= iR= voltage drop across the instrument for full scale deflection
Figure (2): D.C. Voltmeter
D.C. Ammeter
The permanent magnet moving coil instrument is basically used for the
measurement of current. As the moving coil is small and light it can carry
very small current. Therefore, it is necessary, for measurement of high
current, to employ a device which allows a known fraction of the current to
be measured to pass through the coil. This known fraction must be within
the range of the instrument. A shunt resistance is used for this purpose.
Shunts are precise low resistances.
In the arrangement shown in Fig (3) an ammeter of resistance R is connected
in parallel with a shunt of resistance R s. Let i be the current of the ammeter
for full scale deflection and Is is the current through the shunt when the
current to be measured is I. Then
Thus, the shunt resistance required is given by:
Procedure
(1) Voltmeter
1. Connected the experiment circuit shown in figure (2).
2. Compute the resistance Rs theoretically to getting full scale deflection
(F.S.D.).
3. Change the source voltage to 10V and notice the deflection of the
galvanometer.
4. Vary the power supply voltage until scale deflection is reached.
Record the value of voltage.
5. Repeat step 1-4 to design a voltmeter that can measure the voltages
(15V and 20V).
6. Compute the absolute error and relative error.
(2) Ammeter
1. Connected the experiment circuit shown in figure (3).
2. Connected the ammeter to measure the current in the circuit.
3. Compute the resistance Rsh theoretically to getting full scale deflection
(F.S.D.).
4. Change the source voltage to 0.5 V and notice the deflection of the
galvanometer.
5. Vary the source voltage until full scale deflection is reached. Record
the value of current.
6. Repeat step 1-4 to design a voltmeter that can measure 1mA.
7. Compute the absolute error and relative error.
Discussion
1. What are galvanometers parts?
2. Could you increase the range of ammeter? How?
3. How could we extend the range of the a voltmeter?
Experiment No. (8)
Frequency Response of AC Voltmeter
Objective
To be able to analyze the working frequency response of AC
meters.
Theory
General propose AC voltmeters have a flat frequency response
curve over a limited range of frequencies, the meter response inaccurate
above or below this range of frequencies. This is due to the frequency
characteristics of components such as rectifier diodes; instrument-type
rectifiers, capacitors, and wire-wound resistors in voltmeter circuit. The
major effect is generally attributed to capacitance associated with the
rectifying elements in AC voltmeters that use a d'Arsonval meter
movement.
The frequency response is presented in a form of a graph that shows
output amplitude plotted versus frequency. Typical plot of the voltage
gain of an amplifier versus frequency is shown in Figure (1). The gain is
null at zero frequency, then rises as frequency increases, level off for
further increases in frequency, and then begins at high frequencies. The
frequency response of an amplifier can be divided into three frequency
regions.
The frequency response begins with the lower frequency region
designated between 0Hz and lower cutoff frequency, Fl, the gain is equal
to 0.707Amid is a constant mid-band gain obtained from the mid-band
frequency region. The upper frequency region covers frequency between
upper cutoff frequency and above. Similarly, at upper cutoff frequency,
FH, the gain is equal to 0.707Amid. After the upper cutoff frequency, the
gain decreases with frequency increases and dies off eventually.
Figure (1): Frequency response
Procedure
1. Connected the experimental circuit shown in Figure (2), use any one of
the two meter movement.
2. Set the sine wave generator output to 2Vrms at 1 KHz.
3. Decrease the generator frequency until the voltmeter reading
decreases to 1.4Vrms. Record this frequency until as FL.
4. Increase the generator frequency until the voltmeter reading
again decreases to 1.4Vrms record this frequency as FH.
5. Set the generator to each of frequency shown in the data table
and record the voltmeter reading.
6. Repeat steps 1 through 5 with the second voltmeter.
Discussion
Plot the frequency response curve for each meter movement. Discuss
the curves.
Experiment No. (9)
Integrated Circuit Temperature Transducer
Objective
To study the Characteristics of IC Temperature sensor (LM 335)
Theory
Each of the three popular transducers, i.e. RTDS, thermistor and
thermocouples have some significant limitations, e.g. thermocouples have a
low output signal which varies none linearly with temperature. Also, they
need some form of reference compensation. RTD's are more linear than
thermocouple, but the change in their resistance is very small even for large
change in input temperature i.e. they have low sensitivity. Thermistors have
high sensitivity but they exhibit highly nonlinear resistance temperature
characteristics.
For each of these transducers, electronic compensation circuits have to be
used in order to overcome their shortcomings. Also additional circuitry may
be needed to increase their voltage or current output. Usually this additional
electronics circuitry takes the form of monolithic integrated circuits. Thus it
requires combining temperature sensing element with signal conditioning
electronics to produce single monolithic IC package. The one used is LM
335
This is an IC containing 16 transistors 9 resistance and 2 capacitors
contained in a transistor type package. It provides an output of 10mV/°K,
measurements of output voltage therefore indicate the temperature directly
in degrees Kelvin e.g. at a temperature of 20°C(293°K) the output voltage
will be 2.93V.
The IC Temperature sensor is already connected as shown below:
Procedure
1- Connect just the digital multi-meter as voltmeter between output sockets
of IC temperature sensor. See Figure 2.
2- Switch 'On' the power supply. Connect +12 supplies to the heater input
socket and take the voltage reading every minute.
3- This (X100) representing the ambient temperature in °K (Record the
value in table below).
Time
(minutes)
Voltage (V)
Temperature
0
1
2
3
4
5
6
7
8
9
10
°K
Temperature
°C
Note: °C = (°K -273)
4- Switch 'Off ' power supply and disconnect heater element supply (+12V).
This exercise illustrates the Characteristics of the LM335 transducer,
indicates the maximum temperature rise possible using the heater supplied at
+12V, and also gives you an idea of the time scale required for the unit to
reach stable condition.
Results and Discussion
1- Construct the graph of output voltage against temperature in oC and oK.
And find the rate of change of the output voltage to temperature in oK
(∆V/∆oK).
2- State some advantages and disadvantages of the above sensor.
Experiment No. (10)
Maxwell Bridge
Objective
To measure the value of unknown inductance by using Maxwell
Bridge
Theory
The Maxwell's bridge is used to measured inductance by
comparison with a standard variable capacitance. One of the ratio arms
has a resistance and the capacitance in the parallel.
In this bridge at the balance condition there is no current flow in the
galvanometer. The general form of an AC bridge is shown in Figure (1).
The four bridge arms Z1, Z2, Z3, and Z4 are shown as unspecified
impedances. The bridge is side to be balanced when the detector response
is zero. Balance adjustment to obtain a null response is made by varying
one or more of the bridge arms. The condition for bridge balance requires
that the potential difference from A to C be zero. This occurs when the
voltage drop from B to A equals the voltage drop from B to C in both
magnitude and phase.
Figure (1): General AC Bridge
In complex notation we can write:
I1Z1=I2Z2 … (1)
Also:
I3Z3=I4Z4 … (2)
And:
I1Z3=I2Z4 … (3)
Since:
I1=I3
and I2=I4
Dividing Eq. 1 by 2 yields:
𝑍1 𝑍2
=
𝑍3 𝑍4
The Maxwell's bridge shown in Figure (2) measures an unknown
inductance in terms of a known capacitance.
Figure (2): Maxwell's bridge
Observing the bridge we can see that:
Z1 =
(𝑅1 )(−jw𝑐1)
Z2=R2
Z3=R3
𝑅1−jw𝑐1
Z4=Rx+jwl
The real and imaginary terms are calculated by following equations 5 and 6
𝑅𝑥 =
(𝑅2 )(𝑅3)
𝑅1
… (5)
Lx=R2R3C1 … (6)
Thus we have two variables R1, C1 which appear in one of the two balance
equations and hence are independent.
The advantages of this bridge are:
1. The two balance equations are independent if we choose R1 and C1 as
variable elements
2. The frequency dose not appearing in any of the two equations.
3. This bridge yield simple expression for unknowns Lx and Rx in terms of
known bridge element.
4. The Maxwell's inductance–capacitance bridge is very useful for
measurement of a wide range of inductance power and audio frequency.
Procedure
1. Connection is made as per the circuit diagram in Figure (2).
2. Set the value of R2=1kΩ and C1=100nF also select position 1 in the
unknown coil.
3. Apply a sinusoidal signal of 5Vpp amplitude and fixed frequency of 1
KHz using function generator.
4. Balance the bridge by adjusting R1 and R3 consequently till the DMM
inchoate as near as null.
5. Note down the value of R1 and R3 in the table.
6. Repeat the same procedure for different sets of values of R2 and C1 and
position in the unknown coil.
7. Calculate the unknown inductance Lx and its resistance Rx using the
formulas given in the table.
Discussion
1. What is the Q factor of the coil?
2. What are the other methods used for measurement of inductance?
3. Expline with the aid of necessary equations the limitation of using
Maxwell Bridge.
Experiment No. (11)
Characteristics of NTC Thermistor
Objective
To study Characteristics of NTC Thermistor
Theory
Thermistor is a contraction of a term thermal resistor. Although positive
temperature co-efficient (P.T.C) of unit exhibit an increase in the value of
resistance with increase in temperature are available, most a negative
temperature coefficient i.e. their resistance decrease with increase in
temperature. In some materials the resistance of Thermistor at room
temperature may decrease as much as 6% for 1°C rise in temperature.
This high sensitivity to temperature change make the Thermistor extremely
well suited to precision temperature measurement, control & compensation.
Therefore, especially in lower temperature range of -100°C too 300°C, or to
detect very small changes in temperature which cannot be observed with an
RTD or a thermocouple Thermistor are composed of a sintered mixture of
metallic oxides, such as Mn, Ni, Co, Cu, Fe, & U. Their resistance range
from 0.5Ω to 75MΩ and they are available in wide variety of shapes and
sizes.
Figure (1)
Smallest in size are the beads with a diameter of 0.15mm to 1.25mm.
In conventional temperature measurement application, the Thermistor forms
of one of the arms of the wheat-stone bridge. Any change in the Thermistor
resistance as a result of temperature change is reflected in the readout
device.
Other application of thermistor includes:
1. Measurement of power at high frequencies.
2. Measurement of thermal conductivity.
3. Measurement of level, flow and pressure of liquids.
4. Measurement of composition of gases.
5. Vacuum measurement.
Two negative temperature coefficients Thermistor are provided in the
trainer. TH2 is connected to 0V and TH1to +5V. Each requires a suitable
load resistor when connected to the power and the wire wound potentiometer
is recommended for this purpose. Both the Thermistor is located inside the
heater compartment. Their resistance will be about 4.7 K Ohm at room
temperature and this will fall in a nonlinear manner as the temperature rises.
Parameter
Minimum
Type
Maximum
Resistance
3900 Ohm
4700 Ohm
5170 Ohm
Characteristic
4350K
Temp.
They are already connected as under.
Figure (2)
The resistance of the NTC Thermistor varies over a wide range for the
temperature rang available within the heated enclosure. If resistance reading
is to be taken at regular interval of 1 minute, the reading must be obtained
very quickly. The method selected connects the Thermistor in series with a
calibrated resistance to the +5 supply.
For each reading the variable Thermistor is adjusted until the voltage at the
junction of the Thermistor and resistance is half of the supply voltage. For
this setting there will be the same voltage drop across the Thermistor and the
resistance and the since the same Current flows in each their resistance must
be equal. Hence, the value of the resistance read from the calibrated
resistance scale is the same as the resistance of the Thermistor.
Procedure
1- Connect the circuit as shown in figure (4).
a. The A output of NTC Thermistor to C socket of 10 turn
potentiometer.
b. Connect a digital multi-meter as voltmeter between socket B of 10
turn potentiometer and ground.
c. Connect socket A of 10 turn potentiometer to Gnd.
2- Switch 'On' the power supply and note the temperature by connecting
the voltmeter temporarily to the IC temperature sensor output adjust the
10 turn potentiometer until the voltage indicated by the voltmeter is 2.5V
and then note the dial reading.
Note: Since there is a 1K resistance in the output lead of the
potentiometer the total
Resistance will be 10×Dial reading +1K Ohms.
3- Connect the +12V supply to the heater element input socket and at 1
minute intervals note the values the dial reading to produce 2.5V across
the resistance and also the temperature from the IC temperature sensor.
Record the values in above Table.
4- Record the values of dial reading & temperature in below Table.
5- Switch 'Of' the power supply and disconnect the Heater element supply
(+12V).
Figure (3)
Results and Discussion
1- Plot the graph the Thermistor against temperature. It should resemble
the graph below in Figure (3). Make a comment on the results obtained.
2- Explain the difference between Thermistor and resistance thermometer
(RTD).
3- Discussion the various limitations of Thermistor.
Figure (4)
Experiment No. (12)
Study and Calibration of a Single – Phase kilo watt hour Meter
(Energy Meter)
Objective
1. To study the principal operation of single – phase kilo watt hour meter.
2. To draw the model graph of the percentage error versus a unity power
factor load current.
Theory
Energy is the total power taken through a period of time:
Energy= power* time
The electrical energy converted to work or consumed as a heat
could be expressed as:
------ (1)
Where:
W= work
v= Voltage in volt.
i= Current in Amp.
t= Time in second.
The unit of energy is Joule or watt – second. It is one watt during a
period of time equal to (1) second and, if the unit of time is hour then the
energy was measured is watt hour. In high energy loads, a kilo watt –hour
units is used which is the value of (1000) watt in one hour.
The watt hour meter is not often found in a laboratory situation but
it is widely used in commercial measurement of electrical energy. In fact,
it is evident wherever a power company supplies an industrial or domestic
consumer with electrical energy.
Energy meter is an integrating instrument and takes into account
both quantities i.e the power, time and the product of them, which is the
energy. An energy meter keeps a record of the total energy consumed in a
circuit during a particular period of time but it does not give any idea
about the variation in the rate of energy consumption during that period.
The inductive energy meter device consists of two electrical
magnets. One of them is the load current coil, and the other is the voltage
coil connected across the line potential.
A light aluminum disk is suspended in the air gap between current
coil and potential coil. Both electromagnets coil produces a flux passes
through the disc. The two fluxes induce e.m.f.s., in the disc which further
produces the circulatory eddy currents. The reaction of the eddy currents
create a torque on the disc, causing it to rotate. The developed torque is
proportional to the field strength of the voltage coil and the field strength
of the current coil.
Braking of the disk is provided by two small permanent magnets
located opposite each other at the rim of the disk.
Now let the power to the load equals:
P = I.V. cosø
-------- (2)
At a constant power delivered to the load circuit the disc of the
meter rotates at a constant speed i.e. the speed of the disc is proportional
to the power i.e.
P 𝖺 n; p = k.n
Where n = speed of the disc in rev/ time.
k = constant of proportionality.
Hence in a given period of time, the total number of revolution
𝑡2
∫t1 ndt
Is proportional to ∫t1𝑡2 pdt i.e. the electrical energy consumed,
Calibrating the energy meter means to find out the error in the
measurement of energy by energy meter .every energy meter has its own
characteristics constant specified by the manufacturer which relates the
energy measured in KWh and the number of revolutions of the disc. But
practically the revolutions are very large and can not measure in the
laboratory. Hence using this constant energy recorded for certain less
number of revolutions say 5, is calculated in the lab. For the calibration
purpose, this energy is denoted as EA (Actual energy).
To have zero error the Actual energy consumed by the load for the
time corresponding to the 5 revolutions must be same as EA. This energy
is called the true energy denoted as Et. For various loads, the time
required to complete the 5 revolutions of disc is measured with the help
of stop watch. The percentage of error can be calculated by:
%(ε) Error = [Actual energy (EA) – True energy (Et) / True energy
(Et)]×100%
True energy measured in watt.sec = power (p) in watt × time (t) in seconds
Actual energy measured in watt.sec = (3.6×106 × no. of revolutions in time
(t)) / N
Where:t = time in seconds for n cycles of meter under test
p = power measured in wattmeter
N = Energy meter constant (Gearing constant) in revolution / KWh
If error = ± 3% from (1/10) from full load then the meter will be
used for commercial purposes. For secondary standard wattmeter the error
is less from ± 0.5%.
Procedure
1- Observe and record the Energy meter constant (N) in revolution /
KWh
2- Connect the circuit as shown in Figure (1). Select the suitable range
for the voltmeter, ammeter and watt meter.
3- Change the load current in steps of 0.5 A to maximum current of the
KWh meter (current coil) and write down (A, V, W) of each step, also
the time taken by the disc for 5 revolutions (use stop watch).
4- Calculate the relative error for every value of load current.
Discussion
1- Draw error percentage curve against current.
2- Discuss the results.
3- Could we use the watt meter device in this experiment for
commercial purpose or secondary stander purpose?
4- What is the meaning of (Creeping) in energy meter?
5- In the case of slowly or rapidly rotating disc. Explain how it can be
corrected.
6- What are the types of energy meter?
Experiment N0. (13)
Voltage Measurements by Using the Oscilloscope
Object
Employing the oscilloscope to measure AC voltage, and description signal
waveform.
Theory
Firstly the cathode ray oscilloscope as shown in Figure (1) is an instrument
which enables alternating voltages to be exhibited accurately and instantly
on a screen. A stream of electrons from a heated cathode inside an exhausted
glass envelope is focused on to fluorescent screen, giving to bright spot.
In order to use the oscilloscope to make AC voltage measurements, which
means the observation and measurement of time varying voltage signals. In
Figure (2) define some of properties of periodic waveforms, in this Figure
for sinusoid. One defining property of sinusoid is its peak amplitude (value).
This value can obtain on the scope by measuring the `vertical (voltage)
distance from zero voltage (horizontal) axes to either the positive peak or the
negative peak. In case the sinusoidal, these peaks are equal in amplitude.
Figure (2): Components of a Sinusoid
Procedure
1.
2.
3.
4.
5.
6.
Connect the oscilloscope with function generator.
Take sinusoidal waveform and frequency at 2kHz.
Measured and record the period of the sinusoidal waveform.
Compute and record the frequency.
Plot the waveform.
Repeat the procedure for a frequency of (1kHz) and (1.5kHz).
Discussion
1. Compare between measurement frequencies and compute frequencies.
2. Compute and plot the time for square wave has 20kHz.
Experiment No. (14)
Calibration of Voltmeter
Objective
1. To learn the procedure of calibration of a voltmeter.
2. To determine relative error.
Theory
There are many method to calibrating the measuring devices and these
method should have a high degree of accuracy during calibration procedure.
The Potentiometer is considered important device in calibration process due
to high ability to measuring small voltage in high accuracies without
drawing a sensible current from input terminal of measurement system
which used in calibration.
R1
E
Voltmeter
R
V
Potentiometer
R2
Figure (1): Calibration of Voltmeter Circuit
A simple method of calibrating a dc voltmeter is shown in Figure (1), where
the voltage (E) across dropping resistor (R2) is accurately measured with
potentiometer or digital voltmeter. The meter to be calibrated is connected
across the same two points as the potentiometer and should therefore
indicate the same voltage. The exact value of the voltage V on the terminals
of the calibrated voltmeter is Figure (1) can be
By changing the variable resistor (R) so that several point on the voltmeter
scale can be calibrated. Voltmeter tested with the method of calibrated with
accuracy of ±0.01percent.
Procedure
1- Design a dc voltmeter with (3V, 10V) ranges.
2- Make VDC = 5V, RS = 3K .
3- Change voltage value from (0-3V)in steps (0.5 V).
4- Calibrate the similar device and fixed voltages on scale.
5- Calculate the error percent and draw the correction curve.
6- Repeat the previews steps to range (10V), and Vin =10V, RS=10k  .
Discussion
1- Explain other way to calibrate voltmeter.
2- What is the potentiometer consisting of? Explain its theory of
operation.
3- A Digital Voltmeter is used in this experiment, explain briefly the
operation and the specifications of digital devices and compare with
analogue device.
Experiment No. (15)
Wien Bridge
Objective
Study of the Wien Bridge Oscillator and effect output frequency with
variation in RC combination.
Theory
Wien Bridge is one of the simplest and best-known oscillators and is
basically used extensively in circuits for audio applications. Figure (1)
shows the Wien bridge circuit configuration. On the positive side, this circuit
has only a few components and good frequency stability.
Because of its simplicity and stability, it is the most commonly used audio
frequency oscillator. In the figure1 shown the Wien Bridge circuit is
connected between the amplifier input terminals and the output terminal.
The bridge has a series RC network in one arm and a parallel RC network in
the adjoining arm. In the remaining two arms of the bridge, resistor R 3 and
RF are connected.
The phase angle criterion for oscillation is that the total phase shift around
the circuit must be 0°. This condition occurs only when the bridge is
balanced, that is at resonance. The frequency of oscillation F0 is exactly the
resonant frequency of the balanced Wien Bridge and is given by
𝐹𝑜 =
1
2𝜋√𝑅1 𝑅2𝐶1𝐶2
If R1= R2=R
and C1 = =C
C2
F0 = 1/2πRC = 0.159/RC
Assuming that the resistors are equal in the value, and the capacitors are
equal in the value in the reactive leg of the Wien Bridge. At this frequency
the gain required for sustained oscillation is given by
Av = 1/B =3
i.e., 1+ RF /R3 = 3, or RF = 2R3
Figure (1) Wien-Bridge Circuit Schematic
Equipments Needed:
1. Oscilloscope
2. Patch Cords
Circuit diagram:
Circuit used to study Wien Bridge Oscillator is shown in Figure (2).
Procedure:
1. Connect +12V, -12V DC power supplies and GND in the circuit at their
indicated position using patch cords.
2. Connect a patch cord between terminals 6 and 7.
3. Connect a 10K resistance between terminals 1 and 2.
4. Connect other 10K resistance between terminals 3 and 4.
5. Switch “on” the power supply.
6. Vary RF potentiometer to make gain (RF / R3) greater than 2.
7. Now vary the pot of 470K (RF) to adjust gain of the amplifier in case of
clipped waveform.
8. Now observe the output signal across the terminals 5 and GND on
oscilloscope.
9. Record the frequency of output waveform into the observation table.
10. Calculate the theoretical frequency using equation (1).
11. Compare measured frequency with the theoretically calculated
Frequency.
12. Switch “off” the power supply.
13. Now connect 20K resistance between terminals 1 and 2, and other 20K
resistance between the terminals 3 and 4 using patch cords.
14. Repeat the above steps from step 5 to 12.
15. Similarly connect the resistance of 39K and 100K instead of 20K and see
the response.
Discussion
1. What is the reason of the frequency limitation in Wien Bridge?
2. State other two useful application of the Bridge.
3. Could we use the bridge for capacitance measurement? If yes, explain
the main condition to achieve that.