LAB
2
Parallel Circuits, Kirchhoff’s Current Law, and Current Divider Rule
Name:________________
I. Purpose.
1. Review the concept of voltage subscripts and how to measure the voltage as designated by its
subscript.
2. Introduce the concept of elements that are connected in parallel.
3. Introduce how to construct a DC parallel circuit.
4. Introduce voltage across parallel elements.
5. Introduce how to calculate total resistance of resistive elements that are connected in parallel.
6. Introduce how to apply Kirchhoff’s Current Law in analysis of a DC parallel circuit.
7. Introduce how to apply the Current Divider Rule in analysis of a DC parallel circuit.
II. Equipment.
Digital Multimeter (DMM)
Dual DC Power Supply
Protoboard and Test Leads
560 Ohm resistor
1k Ohm resistor
220 Ohm resistor
III. Preparation.
Review procedures for measuring resistance, voltage, and current.
IV.
Lab Procedure.
You must read and complete each step.
Step One: Construct a DC parallel circuit.
□
On a protoboard construct the DC parallel circuit in figure 1.
“a”
T
IT
“b”
“c”
“d”
T
T
T
I1
I2
I3
Es = 10 Volts
1000 Ohm
Resistor
560 Ohm
Resistor
T
“h”
220 Ohm
Resistor
T
T
T
“g”
“f”
“e”
Figure 1
1 of 4
A Practical Exercise
Step Two: Verify voltages of parallel elements and Kirchhoff’s current law.
Elements or branches are said to be in parallel connection when they have exactly two nodes in
common, and therefore voltages across all parallel elements in a circuit will be the same. Kirchhoff’s
current law states that the summation of currents entering a node is equal to the summation of currents
leaving that node.
□
Measure the voltages and currents indicated in the table below.
Remember the current that you wish to measure must pass through the DMM. It may be difficult to
visualize how to measure the current through each resistor. Figure 2 depicts how to measure current
through the 560 ohm resistor. Use this figure as a guide for your other current measurements.
“a”
“b”
“c”
T
T
T
DMM
T
I1
560 Ohm
Resistor
T
“g”
Figure 2
Measured voltage values
Measured current values
Vah
IT
Vbg
I1
Vcf
I2
Vde
I3
Are your measured DC voltage values the same? _____________
Why? ______________________________________________________________________________
____________________________________________________________________________________
2 of 4
A Practical Exercise
□
In figure 3 indicate the direction of the DC currents in all branches and the polarity of the DC
voltages across the voltage source and resistors.
“a”
“b”
“c”
“d”
T
T
T
T
T
T
T
T
“g”
“f”
“h”
“e”
Figure 3
□
Verify Kirchhoff’s Current Law
Σ Ientering node = Σ Ileaving node
Σ ___________= Σ______________
Do your measured DC current values verify KCL? _________________________________
Step Three: Resistors in parallel.
□
Calculate the equivalent resistance, RT, as seen by the source.
RT = ________
Is RT smaller or larger than the smallest resistor in the circuit?
smaller or larger
If the same resistors were connected in series, would they have a greater equivalent resistance than the
equivalent resistance of this parallel circuit? ___________________________
□
Measure RT, as seen by the source.
RT(measured) = ________
3 of 4
A Practical Exercise
Step Four: Current Divider Rule.
The current divider rule can be used to determine how current entering a node is split between the
various parallel resistors connected to the node. IX = (RT / RX) IT; IX is the current through RX, RT is the
total resistance of parallel resistors, and IT is the total source current entering the parallel circuit.
□
Verify your measured branch currents using your measured total source current, IT, your
calculated total equivalent resistance, RT, and the given resistor values.
Calculated current values
I1
I2
I3
From this practical exercise can we conclude that most of the current in a parallel circuit will follow the
path of least or most resistance?
least or most
4 of 4