objective of lecture - virginia techliab/analogue electronics/week 7/loop... · objective of...
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Objective of Lecture Provide step-by-step instructions for loop analysis,
which is a method to calculate voltage drops and mesh currents that flow around loops in a circuit.
Chapter 3.2
Basic Engineering Circuit Analysis by
J.D. Irwin and R.M. Nelms
Loop Analysis Technique to find voltage drops around a loop using
the currents that flow within the loop, Kirchhoff’s Voltage Law, and Ohm’s Law
First result is the calculation of the loop currents
Which can be used to calculate the current flowing through each component
Second result is a calculation of the voltages across the components
Which can be used to calculate the voltage at the nodes.
Definition of a Loop Mesh – the smallest grouping around a subset of
components in a circuit that are all connected to one another
Multiple loops are defined so that every component in the circuit belongs to one or more loops.
Step 3 Label the voltage across each component in the circuit
i1
i2
+ V1
_
Vin
+ V3
_
+ V5
_
+ V6
_
+ V2 - + V4 -
Step 4 Use Kirchhoff’s Voltage Law
i1
i2
+ V1
_
Vin
+ V3
_
+ V5
_
+ V6
_
+ V2 - + V4 -
0
0
543
6321
VVV
VVVVVin
Step 5 Use Ohm’s Law to relate the voltage drops across each
component to the sum of the currents flowing through them.
Follow the sign convention on the resistor’s voltage.
RIIV baR
Step 5
i1
i2
+ V1
_
Vin
+ V3
_
+ V5
_
+ V6
_
+ V2 - + V4 -
616
525
424
3213
212
111
RiV
RiV
RiV
RiiV
RiV
RiV
Step 6 Solve for the loop currents, i1 and i2
These currents are related to the currents found during the nodal analysis.
213
542
62171
iiI
IIi
IIIIi
Substituting in Numbers
kiV
kiV
kiV
kiiV
kiV
kiV
1
3
6
5
8
4
16
25
24
213
12
11
0
012
543
6321
VVV
VVVVV
Substituting the results from Ohm’s Law into the KVL equations
0365
0158412
2221
12111
kikikii
kikiikikiV
Chugging through the Math
One or more of the loop currents may have a negative sign.
Mesh Currents (mA)
i1 740
i2 264
Chugging through the Math Voltage across
resistors (V)
VR1 = -i1R2 -2.96
VR2 = i2 R2 5.92
VR3 =(i1 – i2) R3 2.39
VR4 = i2 R4 1.59
VR5 = (V4 – V5) 0.804
VR6 = (V5 – 0V) 0.740
The magnitude of any voltage across a resistor must be less than the sum of all of the voltage sources in the circuit
In this case, no voltage across a resistor can be greater than 12V.
Chugging through More Math
Currents (mA)
IR1 = i1 740
IR2 = i1 740
IR3 = i1- i2 476
IR4 = i2 264
IR5 = i2 264
IR6 = i1 740
I Vin = i1 740
The currents through each component in the circuit.
Check None of the loop currents should be larger than the
current that flows through the equivalent resistor in series with the 12V supply.
ARVI
kR
kkkkkkR
eqeq
eq
eq
m74012
2.16
136584
Summary Steps in Loop Analysis
1. Identify all of the loops in the circuit 2. Label the currents flowing in each loop 3. Label the voltage across each component in the circuit 4. Write the voltage loop equations using Kirchhoff ’s
Voltage Law. 5. Use Ohm’s Law to relate the voltage drops across each
component to the sum of the currents flowing through them.
6. Solve for the loop currents 7. Once the loop currents are known, calculate the voltage
across all of the components.