Download Wheatstone_Bridge

Wheatstone Bridge
Schematic of a Wheatstone Bridge
VBA = 0V and IR3 = IR4
if R1 = R2 and R3 = R4.
Typically, the resistors are selected such
that R1 = R2 = R3 and the resistance of R4
determines the value of VBA and difference
between IR3 and IR4.
Applications
 Resistance measurement technique
 Can be used to determine the resistance of an unknown
component extremely precisely if the value is close to the
value of the three known resistors.
 Used in sensing applications
 Very small changes in the properties of a known
component can be correlated to changes in temperature,
humidity, adsorbed molecules on the component, etc.
Alternative Layout
 VBA is equal to the voltage at
node B minus the voltage at
node A.
VBA = VB - VA
 When the bridge is balanced
 i.e., when R4 = 4.7 kW in the
circuit shown to the left
VBA = 0 V
Note: There is no component between node B and node A, the lines extended
between these nodes are there to demonstrate that the probes of your DMM are
placed at these two points: red at B and black at A to match the polarity of VBA.
Circuit Construction
 +9 V supply and ground
available on the ANDY
board.
 Three 4.7 kW resistors
 One 10 kW trim pot
As a reminder, you should not wire the connection
between the +9 V supply and ground (shown in red)
as it is made internal in the printed circuit board.
Series Combination
• R1 and R3 are in series

The voltages across R1 and
R3 can be calculated using
the equation for voltage
division
 R2 and R4 are in series
 The voltages across R2 and
R4 can be calculated using
the equation for voltage
division.
Equivalent Resistances
Parallel Combination
• The series combination of R1 + R3 is in parallel with the series
combination of R2 + R4
 The current that flows through R1 and R3 is equal to IR3, which was
shown in the schematic on Slide 2.
 The current that flows through calculated R2 and R4 is equal to IR4,
which was shown in the schematic on Slide 2.
 IR3 and IR4 can be calculated using the equation for current division.
Wheatstone Bridge
Place the Parts
1.
2.
VDC
R, which you place 3
times.
The numbering of
the resistor increases
sequentially as you
place each resistor.
3.
R_VAR, the variable
resistor, which is
used to simulate the
trim pot.
Wire the components together
Click on the pencil icon on the
toolbar. Then, left click on the ends
of the two components that you
want to connect with a wire. Right
click to end the wiring operation.
Place PARAM
You need one extra part – PARAM
– to be able to run simulations
where the value of the component
is changed during the simulation.
Place this part to one side of the
circuit so that it doesn’t obscure
any of the other components or
labels.
Create a Variable Name
Double click on the word PARAMETERS, which will
cause PARAMETERS: to be highlighted in read and
the PartName param pop-up window to open.
Select NAME1
and assign it the
value Rx, the
name of the
variable that will
be ramped from
0W to 10 kW
during this
simulation.
Click Save Attr.
Assign a Value to the Variable
Then, select VALUE1 and assign it the value 10 kW.
This will be the value used by PSpice when calculating the Bias Point Detail.
Click Save Attr and then click OK to close the pop-up window.
Assign Variable Name to R_VAR
Now you have to assign the parameter to the variable resistor. To do this,
double click on the symbol for R_VAR, which will cause the symbol to be
highlighted in red and will open the PartName: r_var pop-up window.
Click VALUE and assign Rx as its value. Then click Save Attr.
Set
You must also change SET from 0.5 to 1. If you do not do this step, the value of the
variable resistor will be 50% of the value assigned by the parameter. I.e., the value of
the R_VAR will be ramped between 0 – 5 kW, and fixed at 5 kW during the calculations
for the Bias Point Detail.
A common mistake is to forget to change SET so verify that it is equal to 1
if you find that your results differ from the expected results.
Then, click Save Attr and finally click OK to close the pop-up window.
Select the Type of Simulation
Change the other
component values:
1. VDC = 9V
2. R1 = R2 = R3 = 4.7k
Once you have finished laying out the schematic, you should save the schematic.
Now, you are ready to run the simulation. First, you have to select the type of
simulation. Either click on the Analysis Setup icon or go to Analysis/Setup from the
upper toolbar.
Make sure that Bias Point Detail is enabled. Click on the words DC Sweep, which will
enable this simulation and cause a pop-up window to open.
Select Global Parameter under Swept Var. Type.
DC Sweep
The name of the parameter is Rx.
The start value is 0 W.
The end value is 10 kW.
The increment should be
small to yield a smooth
curve on the plots that will
be displayed; 100 W is
reasonable.
NEVER put a space between the number and the prefix in PSpice.
Run the Simulation
The yellow icon is RUN,
which may be at a
different location on the
upper toolbar of your
installation of PSpice.
Mine is located next to
the Analysis Setup icon
on the left.
Error Message
If you have followed my directions exactly, you will have error messages displayed
in the window that is launched when the simulation is run.
Note that the error message is that several of the nodes are floating. The cause is
either (a) the wire connection to the components did not register and you need to
go back to the components and make a node show up at the connection of the end
of each part and the wire or (b) there is no reference point (ground) in the circuit.
Add a Ground
Use Voltage Markers
Place a voltage marker between R1 and
R3 and a second marker between R2
and R4 to automatically generate a plot
of the node voltages.
Node Voltages as a Function of Rx
To Plot VBA
Click on Trace/Add Trace
(the shortcut is Insert).
Entering Mathematical Expressions
VBA is the difference
between the node
voltage between R2
and R4 and the node
voltage between R1
and R3. These are the
two voltages that were
plotted automatically
by the placement of
the voltage markers.
This the expression
that must be entered in
the Trace Expression
box.
Trace Expression
Read the names from the bottom of the plot.
Subtract the voltage that contains R3 from the
one that contains R4.
Then, click OK. A third curve will be displayed
on the plot at this time
Note that your voltage labels may have a :1 instead of a
:2. This depends on how many times you rotated the
resistors before placing them on the schematic.
Add a Plot of the Currents
Click Plot/Add Plot to Window. An empty plot will be inserted above the one that was
automatically generated.
Note that this additional plot (as well as the trace expression) that you added for VBA will disappear if you
rerun the simulation.
Select the Currents to be Plotted
Left click inside the blank plot area.
Then, click on Trace/Add Trace.
In the pop-up window that opens,
type in I(R3), I(R4) into the trace
expression box. Then, click OK.
Final Set of Plots
To increase the width, change the color or other attribute of each curve, right click on
the curve and select Properties.
Note that the currents displayed are negative. This is not correct.
Current Markers
If current markers were placed at the ends of R3 and R_Var,
then the plot of the currents would be positive.
Note that the labels at the lower left of the plots are –I(R3) and –I(R4).
Preferred Image
Printed a black and white version of the plot using File/Print or capture an image from
your display after selecting File/Print Preview and include this plot in your report
template. This will insure that the size of the report template is reasonably small when
you upload it on Scholar. The time stamp and the file location should be included.
Results from Bias Point Detail
Use the Analysis/Display Results on Schematic option if you would like to see the d.c.
values of the voltages and currents from the Bias Point Detail calculation. The
calculate is performed when Rx is equal to its maximum value.
The schematic with the labels does not have to be included in the report.