ECE 211 Electrical Circuits Lab I
LAB #5
Fall 2013
Name ___________________
Section ______________
Delta/Wye conversions and the Wheatstone bridge
OBJECTIVE:
1. To become familiar with delta to wye conversions and its usefulness in circuit analysis
2. To understand the behavior of the Wheatstone bridge.
3. To measure resistors using the Wheatstone bridge.
COMPONENTS AND EQUIPMENT:
1.
2.
3.
4.
5.
Digital multimeter
Variable DC supply.
Leads
Resistors
Potentiometer
BASIC DELTA-Y TRANSFORMATION
The transformation is used to establish equivalence for networks with 3 terminals. Where three
elements terminate at a common node, the node is eliminated by transforming the resistances. For
equivalence, the resistances between any pair of terminals must be the same for both networks.
Equations for the transformation from Δ-load to Y-load:
The general idea is to compute the resistance Ry at a terminal node of the Y circuit with resistances R'
and R'' at adjacent nodes in the Δ circuit by
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ECE 211 Electrical Circuits Lab I
LAB #5
Fall 2013
where RΔ are all resistances in the Δ circuit. This yields the specific formulae
Wheatstone Bridge
The Wheatstone Bridge circuit measures resistance. In industrial applications, accuracies are in the order
of 0.1%. A Wheatstone Bridge is created as the following circuit with Rx as the measured resistor..
When the measured resistor is in the network, the voltage is measured across the circuit and R3 is
changed until the voltage measured is zero. The measured resistor, Rx, is computed by the following.
𝑅𝑥 =
𝑅2
𝑅
𝑅1 3
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ECE 211 Electrical Circuits Lab I
LAB #5
Fall 2013
PROCEDURE
EXPERIMENT 1
In this part of the experiment, you will construct a delta configuration of three resistors, measure and
record the terminal resistance, and design and construct an equivalent wye configuration.
1. Acquire a 3.3kΩ, 5.6kΩ, and 4.7kΩ resistor and measure each to at least three significant
figures.
2. Build the circuit of Figure 1. DO NOT APPLY ANY POWER.
Figure 1
Ra= 3.3 K Ω
Rb= 5.6 K Ω
Rc= 4.7 K Ω
3. Measure and calculate the resistance between the terminals of your delta configuration.
Compare both values and explain the differences.
4. Use the delta /wye conversion formulas given above to design an equivalent wye
configuration.
5. Build the wye configuration. You can use two resistors in series for each of the calculated
resistors to get a closer value. Don’t forget to measure the new resistors.
6. Verify that the resistance between terminals is approximately the same as the delta
configuration.
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ECE 211 Electrical Circuits Lab I
LAB #5
Fall 2013
EXPERIMENT 2
In this part of the experiment, you will substitute the original delta configuration for the wye
configuration. This will verify that the delta/wye substitution is valid for circuit.
1. Acquire a 1kΩ and 2kΩ resistor and measure each to at least three significant figures. Reuse
resistors from Experiment 1 for R1, R2, and R4.
2. Calculate and measure Rin for Configuration A.
3. Reuse resistors from Experiment 1 for R6, R7 and R9 in order to make configuration A and B
equivalent.
4. Calculate and measure the input resistance of Configuration B. Is it the same than
Configuration A?
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ECE 211 Electrical Circuits Lab I
LAB #5
Fall 2013
EXPERIMENT 3
The G in the circuit above is a voltmeter. It measures the voltage drop between A and B.
1. Set R1 and R2 to 1K. Measure and record these values. Put a potentiometer where Rk is and
pick a random resistor for Rx. Set V0 to 5 V. Change the potentiometer until the voltmeter
measures zero volts. Compute Rx using the known resistor values of the Wheatstone bridge.
Compare this with the value obtained by measuring Rx with a multimeter.
2. Set R1 to 2K. Adjust the potentiometer then compute Rx again. Does it have the same value
as before?
3. Change the value of Vo and compute the resistance again. How does the voltage affect the
measurement of the resistor?
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ECE 211 Electrical Circuits Lab I
LAB #5
Fall 2013
Deliverables
Turn in the following for the lab report:



Experiment 1:
o All necessary calculations
o A table of resistances for individual resistors containing the following entries
 Nominal Value
 Measured Value
o A table of measured resistances between each terminal pair for both the Delta and Wye
configuration
Experiment 2:
o All necessary calculations
o A table of resistances for new resistors containing the following entries
 Nominal Value
 Measured Value
o Measured resistance between terminals for both configurations
o Address whether the resistance between terminals is approximately the same for both
Experiment 3
o All necessary calculations
o A table of resistances for new resistors (including Rx) containing the following entries
 Nominal Value
 Measured Value
o The value of the potentiometer used in computing Rx for both steps 1 and 2
o Answers to all quesionts
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