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
Poteniometer
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',
R'' to 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 is a circuit that can be used to measure resistance. In industrial applications
accuracies are in the order of 0.1%. A Wheatstone Bridge is created with 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 terminal resistance, and design and construct an equivalent wye configuration.
1. 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
2. Measure and calculate the resistance between the terminals of your delta configuration.
Compare both values and explain the differences.
3. Use the delta /wye conversion formulas given above to design an equivalent wye
configuration.
4. Build the wye configuration. You can use two resistors in series for each of the calculated
resistors to get a closer value. Verify that it has the same behavior than the delta
configuration.
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.
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ECE 211 Electrical Circuits Lab I
LAB #5
Fall 2013
1. Calculate and measure Rin.
2. Calculate R6, R7 and R9 in order to make configuration A and B equivalent.
3. 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. Measure the value of Ry. Compute Ry. Compare this value with the
computed resistance obtained using the Wheatstone bridge.
2. Set R1 to 2K. Compute Rx again. Does it have the same value than 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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