Week 3 Wheatstone Bridge - PowerPoint Presentation

Slide1

Week 3

Wheatstone Bridge

Slide2

Schematic of a Wheatstone Bridge

V

BA = 0V and IR3 = IR4 if R1 = R2 and R3 = R4.

Typically, the resistors are selected such that R

1

= R

2

=

R

3

and the resistance of R

4

determines the value of

V

BA

and difference between I

R3

and I

R4

.

Slide3

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

V

ery small changes in the properties of a known component can be correlated to changes in temperature, humidity, adsorbed molecules on the component, etc.

Slide4

Alternative Layout

V

BA 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 leftVBA = 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 V

BA

.

Slide5

Circuit Construction

+9 V supply and ground available on the ANDY board.Three 4.7 kW resistorsOne 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.

Slide6

Series Combination

R1 and R3 are in seriesThe voltages across R1 and R3 can be calculated using the equation for voltage divisionR2 and R4 are in seriesThe voltages across R2 and R4 can be calculated using the equation for voltage division.

Slide7

Equivalent Resistances

Slide8

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.

Slide9

Sweeping a Variable Resistor

Wheatstone Bridge

Slide10

Place the Parts

VDC

R

, which you place 3 times.

The numbering of the resistor increases sequentially as you place each resistor.

R_VAR

, the variable resistor, which is used to simulate the trim pot.

Slide11

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.

Slide12

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.

Slide13

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 0

W

to 10 k

W

during this simulation.

Click

Save

Attr

.

Slide14

Assign a Value to the Variable

Then, select VALUE1 and assign it the value

10 k

W

.

T

his 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.

Slide15

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

.

Slide16

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 k

W

, and fixed at 5 k

W

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.

Slide17

Select the Type of Simulation

C

hange the other component values:VDC = 9VR1 = 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.

Slide18

DC Sweep

Select

Global Parameter under Swept Var. Type.

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.

Slide19

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.

Slide20

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.

Slide21

Add a Ground

Slide22

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.

Slide23

Node Voltages as a Function of Rx

Slide24

To Plot VBA

Click on

Trace/Add Trace

(the shortcut is Insert).

Slide25

Entering Mathematical Expressions

V

BA

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.

Slide26

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 timeNote 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.

Slide27

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 V

BA

will disappear if you rerun the simulation.

Slide28

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

.

Slide29

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.

Slide30

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).

Slide31

Preferred Image

P

rinted a black and white version of the plot using

File/Prin

t 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.

Slide32

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.