Introduction to Circuits Laboratory (Class)
One (1) Credit Hour
Required for EE students
Complement to EE220, “Introduction to Circuits”
Name: ______________________
Fall 2010
Lab #4 Wheatstone Bridge
References to “Electric Circuits” 8th Edition, by Nilsson / Riedel: Section 3.6Materials: Various small-value resistors, test leads (preferably two sets), power supply, and multimeter (D.M.M.); alternatively, the ELVIS workstation has two built-in power supplies and a digital
meter.
Objectives


Be able to measure current and voltage in a Wheatstone Bridge (circuit)
Understand and explain how to ‘Balance’ a Wheatstone Bridge
Purpose
The lab should reinforce the theory behind balancing a Wheatstone bridge network. The
student should be comfortable and confident (by now) in taking voltage and current measurements
with the D.M.M. and the ELVIS II station. Measurements will again be compared to calculations.
Introduction
Many types of bridge circuits are used in electronics. The Wheatstone Bridge configuration
we are studying can become a functional ohmmeter as well as a balanced circuit. The Wheatstone
Bridge has two input terminals, where the battery or supply terminals are connected to each of the
‘arms’ or branches (ratio branches). These two branches are the key to understanding how the
Wheatstone Bridge is balanced. The bridge is balanced when there is an equal division of voltages
across the bridge output. This output is a current path across the ratio arms, similar to a bridge or
connector between two series-parallel paths. It does not matter what the ratios are for the bridge to
be in a balanced state, because resistances in parallel—two resistors in series, in this case—have the
same voltage across both branches. Some ratios may develop a greater sensitivity to balance than
others, depending upon the total current and total resistance of the ratio arms (each serial path).
The main point is that when the bridge is balanced, there should be no current flow ‘across’
the bridge—or from one ratio bridge to the other. In most instances, a Galvanometer is used in the
bridge to detect current flow. A Galvanometer is an electromechanical meter with the needle
pointing straight up or centered, and its electrical design is to be a two-way ‘amp-meter’ or
bidirectional ammeter. In the lab, if you do not have a Galvanometer, it is possible to use a D.M.M.
as it will indicate current flow backwards by showing the minus-sign “ – “ along with the scalar
amount.
Here is a simple Wheatstone Bridge circuit, below. There are two parallel paths consisting of
two resistors in each path. The top and bottom connections to the source are the ‘input’, while the
output is the two points A and B. There must be some resistance between A and B for unbalanced
Introduction to Circuits Laboratory Class
current to flow through this void, so when we put a proper meter or measuring device between A
and B, we are placing a resistance or path there.
Here below is the same circuit with the meter in place. If every resistor in the circuit were
exactly 1000 ohms, the bridge should be balanced and the D.M.M. should have no current flowing
through it. In this circuit, the D.M.M. should be set to read “A” or Amps. In you circuits for this lab,
be careful that you do not ‘over-current’ the meter. A severely imbalanced bridge could have a fair
amount of current flowing across the bridge. Another method would be to put the D.M.M. in “V” or
Voltage mode. The meter will have extremely high resistance so we do not have to worry about
burning an internal fuse. And, the readings will be in Volts D.C., so zero volts would mean no current
flow and the bridge is balanced. But, you do not know the actual current.
A third method would be placing a particular value of resistance across the points A and B, and
measuring the Voltage. Then use Ohm’s Law to calculate current. The resistor 𝑅𝑥 is a ‘sensing’
resistor.
Introduction to Circuits Laboratory Class
Procedures and Measurements
Part 1
Connect the following circuit on a prototyping board. Do not turn the power on until
the instructor or assistant has verified your connections.
The instructor or assistant will provide you with four resistors for 𝑅1 , 𝑅2 , 𝑅3 𝑎𝑛𝑑 𝑅4 .
You will need to calculate the source voltage 𝑉1 in order to not over-power the
resistor’s wattages. For example, if the resistors have a rating of ¼ watt, then calculate
for the resistor(s) that would use the ‘most’ power at 1/5 of a watt. Also, set the
D.M.M. to read ‘Volts’.
You should record the values from the resistor’s color-coding in the ‘Nominal’ column,
and measure the actual value and record in the ‘Measured’ column. Calculate the
Voltages for each resistor. Power on and record the actual voltage drops. You can
either use two meters—one in place in the bridge, and one for measuring other
places—or you may make your measurements without the ‘bridge’ meter in place,
using that meter, then putting it back in. Figure the percent differences…
𝑅
𝑅1
𝑅2
𝑅3
𝑅4
Nominal Ω
Measured Ω
Calculated V
𝑉𝑅1 =
𝑉𝑅2 =
𝑉𝑅3 =
𝑉𝑅4 =
Measured V
% Diff.
From the data above, what would you calculate the bridge voltage 𝑉𝐴 𝐵 to be? (Refer
to what you learned about voltage dividers.)
𝑉𝐴 𝐵 = _________
Now measure the bridge voltage.
𝑉𝐴 𝐵 (𝑚𝑒𝑎𝑠) = _________
Introduction to Circuits Laboratory Class
Part 2
Use the same circuit configuration, but change out the resistor values. The instructor
or assistant will provide values of resistances such that 𝑅1 , 𝑅2 , 𝑅3 are the same value.
𝑅4 will be initially something less (resistance) than the first three. Then, 𝑅4 will be
swapped out repeatedly.
Make voltage measurements for all values of 𝑅4 to complete the table below. Again,
calculate a ‘good’ or safe voltage to use for 𝑉1 . The column for 𝑅4 is the difference
between it and the other three. Calculate that difference and put the values in this
column. You may not find the exact resistance to substitute, so find the closest
resistance (there are standard resistance values, and the lab may not even have all of
these) and record what you use in the next column “Closest…” The next column is the
voltage reading, and the last column is to show which direction the current would be
flowing.
𝑅4 ±
𝑉𝐴 𝐵 =
Closest Ω
Current
𝑜𝑡ℎ𝑒𝑟𝑠
actual
Direction
resistor
-20%
-15%
-10%
-5%
-1%
0%
+1%
+5%
+10%
+15%
+20%
After filling in this table, do you see a pattern or trend in 𝑉𝐴 𝐵 and the current
direction?
Introduction to Circuits Laboratory Class
Part 3
Use the same circuit configuration, but change out the resistor values. The instructor
or assistant will provide values of resistances such that 𝑅1 𝑎𝑛𝑑 𝑅3 are the same value.
𝑅2 should be covered in tape to conceal the value. Then, swap out 𝑅4 repeatedly by
using calculations and measurements in order to ‘find’ the value of 𝑅2 . In this
instance, use your meter in “A” or amps mode, or try the third configuration
mentioned in the introduction—using a ‘sense’ resistor and the meter still in “V”
mode.
By theory, finding 𝑅4 should provide a reasonably accurate answer to 𝑅? . You may get
lucky and not need as many rows provided in the table.
𝑅4 Ω
values
tried
𝑉𝐴 𝐵
Or
𝐼𝐴 𝐵
Current
Direction
From the data in the table, what is your guess for 𝑅? ______________ Ω
Introduction to Circuits Laboratory Class
Questions:
Are both of these circuits balanced? Explain any differences or similarities.
Introduction to Circuits Laboratory Class