EEE121Electric Circuit Analysis I

EEE-121 Electric Circuit Analysis I

Lab-02
Experimental verification of ohm’s law, Simulation Software
(LTSPICE/circuit maker)

Name Danial Abbasi

Registration Number FA20-BEE-044

Class/section 2A
Instructor’s Name Sir Khan Afsar

Lab Assessment

Total (10)
Pre-Lab (1) In-Lab (5)
Post-Lab (4)

Implementati Data Data Writing

Attendance/
on/Procedure/ Presentation Analysis Style
Task
Results

Comments:

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Objective
➢ To verify Ohm’s law experimentally and to find the relationship between voltage,
current, and resistance in a circuit.
➢ To learn how to create and simulate the circuits in LTSPICE

Equipment Required
Resistors, DMM, breadboard, DC power supply, and connecting wires.

Knowledge Level
➢ Before working on this lab, students should have good understanding of Ohm’s Law.
➢ Students should be able to theoretically solve the circuit shown in circuit diagram.

Pre-Lab Task:

EXPERIMENTAL VERIFICATION OF OHM’S LAW

Introduction:
Ohm’s law states that “the voltage v(or potential difference) across a resistor is directly
proportional to the current i flowing through it.” As the current increases the voltage drop also
increases provided that the resistance is kept constant; and the current decreases as the resistance
increases provided that the voltage across the resistor terminals remains the same. Mathematically,

v i
v = iR
WhereR is the constant of proportionality and is the
resistance of the resistor element.

Material that obeys Ohm's Law is called "ohmic" or

"linear" because the potential difference (or voltage)
across its terminals varies linearly with the value of
Figure 1
current flowing through it.

Task # 1:
Solve the circuit shown in figure below by hand before coming to the lab and find the Current
across the Resistor by varying voltages (from 0.5V to 5V in stepwise manner) for three values of

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resistance R1 as show in table 1 below and record these values in table. The values of resistors
should be taken as 1KΩ, 2.2KΩ and 5KΩ. (The values of resistor available in laboratory may
slightly vary.

Task # 2:

INTRODUCTION TO LTSPICE:
LTSPICE is an abbreviation for Linear Technology Simulation Program with Integrated Circuit
Emphasis. It uses mathematical models to describe circuit elements and allows DC and time
transient analysis of nonlinear circuits (transistors, diodes, capacitors, etc., also digital circuitry).

DOWNLOADINGANDINSTALLINGLTSPICE: -
LT spice can be downloaded fromhttp://www.linear.com/designtools/software/ltspice.jspThe
downloaded file is a .exe file which directly installs LTspice.

CREATINGA SIMPLE CIRCUIT: -

1. Open the LT spice software.
2. Choose File->New Schematic.
3. From Tools menu the color preferences can be changed, the grid can be turned on
or off from the view menu.
4. The tool bar is explained below

5. The component button can be used to put any circuit component on the schematic
diagram. The wire button can be used to connect different components.
6. The label button can be used to give labels to different nodes. Otherwise a default name
is given to each node.
7. To delete a component from the diagram either use F5 or click the scissors button
and click on the component to be deleted.
8. To make a simple circuit as shown below click on the component button.

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9. The following window appears.

10. This window contains a collection of basic component; to make the circuit as shown
above choose the voltage source and place it on the schematic diagram.
11. Place resistors on the schematic diagram and join those using wires to make the complete
circuit. To rotate a resistor so that it can be placed as in the given circuit, select the
resistor and press ctrl+r. Similarly, ctrl+eis used to mirror a resistor. Place the ground
at the lower node.
12. The circuit is complete. To set the values of observe that each component has two labels

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attached to it. One represents the name and other represents the value of the component.
To change the name or the value of any component left click on the corresponding label
e.g. each resistor comes with a label R1, R2 etc. that represents its name. Each resistor is
also accompanied by a label R that represents its value. To change the value of the resistor
use left click on the label. The following window appears.

13. Enter the value in the text field and click ok

14. Another way setting different properties of a component is by using left click on the
component itself e.g. if we use left click on the voltage source the following window
appears.

15. Now the DC value and the source internal resistance can be set from this window. The
advanced button can be used to change the voltage source from DC to other types which
shall be explored in other tasks.

TASK1: Simulating a Simple Circuit to Obtain DC Bias Point: -

1. Afterthecircuithasbeenmadeandvaluesaresetasexplainedabovewecansimulatethe circuit to
determine the DC bias point i.e. all node voltages and currents.

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2. Suppose we usetheDCvoltagesourceequalto5Vandbothresistorsaresetequalto1K. (The

symbolforprefixessuchaskiloandmilliandmegaarecaseinsensitivecanbeconfusinge.g.the
symbol for kilo is K or k, for milli it is M for mega it is MEG or Meg.
3. Now click Simulate-> run from the top menu or click the run button on the toolbar. The
following window appears

4. It shows the possible type of analyses LTspice can perform. At the moment we are only
interested in the DC bias point so click the DC oppnt button on the top menu of this
window and click ok.
5. The operating point is calculated and the following results appear.

6. Since we placed no label on the nodes so they are given names n001 and n002.The node
with ground connected is named 0.
7. Now we place our own label son the nodes by using the label net button on the tool bar
and run the simulation again.

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8. So the node voltages and current through each component are listed. Note that the current
through the resistor R1 is negative. The reason is that R1 was rotated before being placed
in the circuit .LTspicedefines a predetermined direction of current through each resistor.
A negative value shows that the actual direction of current is opposite to the predetermined
assumed direction. To check what direction LTspiceh as assumed click View->Spice
Netlist from the top menu. A net-list is at ext version of the schematic diagram. The
following window appears

9. Shows that R1 is connected between nodes N2 and N1 and hence the assumed direction of
current is from N2 to N1. Whereas the actual current flows from N1 to N2 and hence the
output generated a negative sign.

10. To connect R1 i.e. the assumed direction is from N1 to N2 select the resistor by using the
move or drag button (the buttons with the symbol of open or closed hand) from the tool bar
and press ctrl +e to mirror the resistor. Now run the simulation and view the Spice Netlist.
11. The current through Voltage source is negative as it should be by passive sign convention

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DC SWEEP ANALYSIS: -Students have no idea

What diodes are…they study them in electronics 1…

DC Sweep is a type of simulation in LTSpice where the DC voltage of one or more than one
source(s) is varied in a step-wise manner. At each step the DC bias point is calculated, there ults
are usually represented in the graphs. This type of analysis is most suitable when plotting the V-I
curves of different devices or when designing a specific DC bias point for a particular circuit.

TASK 2: Plotting the V-I curve of a real diode

1. Create a new schematic and draw the following circuit. Remember to label them
asV1andV2 as shown.

2. To perform the DC sweep analysis, click the run command and choose the ‘DC Sweep’
button on the window that appears.

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3. Provide the name of the DC source which will perform the sweep i.e. V1 in our case.
Provide a start value let’s say 0V and an end value let’s say 2V and an increment (i.e. Step)
value letssay0.05V.
4. The simulation will be performed and a graphical window would appear.

GRAPHICAL ANALYSIS (TRACE): -

1. Since the results of the DC Sweep are best viewed using a graphical utility so shall use the
graphical analysis of LTSPICE also called TRACE. When use a DC Sweep analysis the

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value of the DC voltage source varies from the initial to the final value in the form of
incremental step, at each step all voltages and currents are measured and stored. Here we
want to plot the diode current as the value of diode voltage varies.
2. Maximize the graph window; take them cursor over the horizontal axis, Use the ‘right-
click’ button and a window would appear.

3. This window tells us that the quantity plotted on the horizontal-axis is the DC source
voltage V1. It also tells what the maximum and minimum value on the axis is and where
the ticks are placed. We can change all these values. Since we want to plot diode current vs
diode voltages we should place diode voltage on the horizontal-axis. To do so change the
value of ‘Quantity plotted’ from ‘V1’ to V(V1)–V(V2).
4. Now move the mouse cursor somewhere on the graphical screen and use ‘left click’, from
the drop-down menu that appears click ‘Add Trace’. The following window appears

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5. It lists all the voltages and current which have been calculated during the DC sweep
simulation. Choose I(D1) which is the diode current.
6. The V-I curve is plotted on the screen.
7. A number of mathematical operations can be performed on the graphs. A constant may be
added, subtracted, multiplied or dived from the graph. Two or more graphs may be added,
subtracted, multiplied or divided. Similarly, the logarithm or some trigonometric function
of the graph may be plotted as well.

8. In this window any algebraic expression may be written.

9. By using right click on the graph, then numerical values at different points can be observed.

Note: Now you have learned how to simulate the circuits on LTspice and plot Graphs.

Task # 3:
Create the circuit shown in figure 1 below on LTSPICE and simulate using DC Sweep Analysis
by varying voltages (from 0.5V to 5V in stepwise manner) for three values of resistance R1 and
plots the V-I curves of a Resistor R1 before coming to the lab and bring the screen shorts with you
in lab. The values of resistors should be taken as 1KΩ, 2.2KΩ and 5KΩrespectively. (The values
of resistor available in laboratory may slightly vary. Also simulate the circuit using DC Bias-Point
and observe the values of corresponding current ‘Simulated Value’. Record these values along
with calculated in the table below:

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Figure 2

In-Lab Task:

Task# 1: Procedure

Task 1 Assemble on the bread-board the circuit shown in Figure 2.6 using the same voltage
setting on the power supply and the same resistor as shown. Set the multi-meter to
measure dc current, make sure that the leads are correctly set for current
measurement. Measure the current flowing through the resistor. Does this value
 v
agree with the calculations using Ohm's Law  i =  ?
 R

Task 2 Measure the current flowing through the resistor in the opposite direction. This is
done by reversing the leads of the ammeter. Does this value agree with the
 v
calculations using Ohm's Law  i =  ?. Record these Values and Complete the
 R
table shown in Measurements Section

R
1kΩ

10V

Figure 3: A simple resistive circuit to verify Ohm’s law

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Lab Exercise:

Safety Precautions

 Look at each exercise carefully before connecting the circuits.

 Make sure all power is off before connecting or disconnecting components.
 Ask your TA to check the circuit before turning on the power.
 When measuring voltage or current, make sure the DMM is correctly set for what
you need to measure.

Measurement Section: Task-1

V R = 1K Ω R = 2.2K Ω R = 5K Ω
(volts)
Calculated

Calculated

Calculated
Simulated

Simulated

Simulated
Measured

Measured

Measured
I (mA)

I (mA)

I (mA)

I (mA)

I (mA)

I (mA)

I (mA)

I (mA)

I(mA)
0.5 0.6 0.5 0.5 0.2 0.2 0.2 0.1 0.1 0.1

1.0 1.0 1.0 1.0 0.5 0.5 0.5 0.2 0.2 0.2

1.5 1.5 1.5 1.5 0.6 0.6 0.6 0.3 0.3 0.3

2.0 1.9 2.0 2.0 0.9 0.9 0.9 0.4 0.4 0.4

2.5 2.7 2.5 2.5 1.1 1.1 1.1 0.5 0.5 0.5

3.0 3.1 3.0 3.0 1.3 1.3 1.3 0.6 0.6 0.6

3.5 3.7 3.5 3.5 1.6 1.5 1.5 0.7 0.7 0.7

4.0 4.3 4.0 4.0 1.8 1.8 1.8 0.8 0.8 0.8

4.5 4.7 4.5 4.5 2.1 2.0 2.0 0.9 0.9 0.9

5.0 5.2 5.0 5.0 2.3 2.2 2.2 1.1 1.0 1.0

Table 1

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Post-Lab Tasks:

1. Answer the following Questions?

a. What is the advantage of using LTSPICE in circuit analysis?

We can create check and analyze electric circuit.

A user can create their own device models, import

Downloaded models from many electronic component

Manufacturers.

It provides graphical representation of circuits.

We can check the results through simulation easily.

b. What would happen if a wire having no resistance at all (0 Ω) was connected directly across
the terminals of a 6 volt battery? How much current would result, according to Ohm’s
Law?

6V
+
-

When there is no resistance connected in circuit all the current Will flow through the circuit
without any resistance this is also Known as short circuit sometimes it causes the powe
source to Be destroyed.

i.e.

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If Voltage = 4V

Resistance = 0Ω

. Then, I = V/R

= 5/0

= 0 (undefined)

Hence, large amount of current will flow.

c. How would you place a DC current source with downward direction on LTSPICE
schematic?

First we select the DC current source from the components and

Then we will place at desired place after that we can change its

Direction by shortcut key “Ctrl + R”

d. When you simulate the circuit (Figure 2.6) in LTSPICE, the magnitude of current through
all elements is same; however, negative sign appears with current through voltage source.
What is the reason?

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When negative sign appears with current it means that our

Circuit polarity is opposite. And when we change the polarity of

Voltage source value of current will become positive and vice

Versa.

Critical Analysis / Conclusion

In the experiment I verify the ohm’s law. In this law the current increases

and the voltage drops and also increases the resistance which is kept

constant. And the current decreases as the resistance increases provided

the voltage across resistors remain same. Mathematically,

V~i
V=iR

I also learn how to use LTSPICE software. It uses mathematical models to

describe circuit elements and allows DC and time transient analysis of

nonlinear circuits. By using this software we can create and analyze our

circuits before creating them physically.

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