UNIVERSITY OF TURKISH AERONAUTICAL ASSOCIATION
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
EEE203 ELECTRONIC CIRCUITS LABORATORY I
FALL 2014/2015
LAB 2: RESISTORS ASSOCIATION AND THE WHEATSTONE BRIDGE
Preliminary Reading
There are different forms of connecting resistors in a circuit:
 Series connected resistors.
 Parallel connected resistors.
 Series-Parallel connected resistors.
1.
Resistors Connected in Series
A circuit composed solely of components connected in series is known as a series circuit. In a series
circuit, the current through each of the components is the same, and the voltage across the circuit is
the sum of the voltages across each component. The total resistance (the equivalent resistance) of
resistors in series is equal to the sum of their individual resistances:
Figure 1
2.
Resistors Connected in Parallel
A circuit composed solely of components connected in parallel is known as a parallel circuit. In a
parallel circuit, the voltage across each of the components is the same, but the current is different.
The total current is the sum of the currents through each component. The total resistance (the
equivalent resistance) of resistors in parallel is calculated as follows:
Figure 2
3.
Mixed Connected Resistors (Series-Parallel)
In practice, the circuits are not as simple as the ones mentioned above, because as a general rule
resistors are fitted in series-parallel groups, that is, one or several resistors are found in series with a
parallel circuit. These circuits are called mixed circuits. To solve these circuits, the series and parallel
assemblies which constitute it are solved independently, transforming the parallel resistors by blocks
into an equivalent form and associating them with the rest of the circuit.
Figure 3
4.
The Wheatstone Bridge
The set constituted by four resistors R1, R2, R3 and Rg as it is shown in Figure 4 is called the
Wheatstone Bridge.
Figure 4
The points B and D are joined by a voltmeter, which will measure the potential difference between
the two points. Supposing the voltmeter points to 0 volts, it indicates that the voltage between the
points A and B is equal to the voltage between the points A and D, consequently, the following
equations are carried out:
VB – VA = VD – VA
VC – VB = VC – VD
VB − VA = I2 • R2
I2 • R2 = I3 • R3
VD – VA = I3 • R3
VC – VB = I1 • R1
I1 • R1 = I4 • Rg
VC – VD = I4 • Rg
We also know that, as the potential difference between the points B and D is zero, then:
I1 = I2
I3 = I4
Dividing the two members of these equations reciprocally;
Or
R1 • R 3 = R 2 • R g
that is, when the voltmeter points to zero volts, the product of two opposite resistors is equal to the
product of the other two. Then, if from the four resistors, three are known, the fourth one is known
as well.
1.
The Wheatstone Bridge
In a typical Wheatstone bridge, four resistors are positioned in a circuit designed in such a way that
the current from a power supply splits, flows through the sequence of resistors, then recombines
into a single conductor. Three of these resistors have known values, one of which is variable, or
adjustable. The value of the fourth resistor is not known. By studying and manipulating the paths the
current can take through the Wheatstone bridge grid, that fourth, unknown resistance can be
identified.
To carry out this practice we will use the circuit printed as Circuit #5, shown in Figure 3.
Figure 3
We will use the Wheatstone Bridge to calculate the value of the resistors Ra. Measure the value of
resistors Ra with the ohmmeter. Write down the resistor value.
As we have seen in the theory, when the voltmeter indicates zero, a relationship among the four
resistors of the bridge is produced. We will try to make the current equal to zero and take advantage
from the relation among the four resistances.
We apply voltage to the board with the switch of the base unit. With the potentiometer of +9 volts
we can change the voltage applied to the Wheatstone bridge.
If the voltmeter does not point to 0 volts, varying the potentiometer of the circuit Circuit #5 Ra, we
can make the current and voltage equal to zero. In that moment the relationship among the four
resistors can be applied.
Q1: What is the measured value for the potentiometer resistance, Ra? ____________
Q2: What is the calculated value of resistor Ra? Show your calculations.
Q3: Repeat all the steps for a voltage of 7-V. Find Ra. Does the calculated resistance Ra depend on
the voltage we apply to the circuit?
Q4: If we place three resistances in parallel of values 1500, 2700 and 1000 ohms. What is the value of
the equivalent resistance of the group (in ohms) ?
Q5: What is the equivalent resistance of the circuit ( + series , || parallel )?
A.- R1+(R2||R3||R4)
B.- R1+(R2+(R3||R4))
C.- R1+(R3||(R2+R4))
D.- R1||(R2+R4)||R3