laboratory 3: bridge circuits, superposition, thevenin...

Alpha Laboratories

ECSE-2010 Summer 2018 Name _____________________

Written by J. Braunstein Modified by S. Sawyer Summer 2019: 5/15/2019

Rensselaer Polytechnic Institute Troy, New York, USA

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LABORATORY 2: Bridge circuits, Superposition, Thevenin Circuits,

and Amplifier Circuits

Note: If your partner is no longer in the class, please talk to the instructor.

Material covered:

• Bridge circuits

• Voltage dividers

• Superposition

• Thevenin Circuits

• Amplifier Circuits

Part A: Resistive Bridge Circuits

R1 R3

Rbridge

R2 R4

0

Vs

Wheatstone Bridge

Wheatstone Bridge:

A Wheatstone Bridge can be used to measure the value of an unknown resistor. It

is a basic type of Ohmmeter. The bridge is shown on the in the above figure. When

the bridge is ‘balanced’, no current flows through the Rbridge resistor. If that is the

case, then both the left node and right node for that resistor must have the same

voltage. Additionally, since no current is flowing through Rbridge, the left and

right paths can be treated as voltage divider circuits with two resistors in series.

Circuit analysis then gives us

SLeft VRR

RV

21

2

+= and SRight V

RR

RV

43

4

+=

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Again VLeft = VRight, so we can set these two expressions equal, perform some

algebra and obtain a relationship for the resistors when the bridge is balanced (no

current through Rbridge) as

4

3

2

1

R

R

R

R=

If one of the resistors is unknown, R4 for example, we can then use the bridge to

find that value. Holding R1 and R3 fixed, we can vary R2 until we measure zero

voltage drop (no current) across Rbridge. Once we have found that value for R2,

we apply the above expression and determine R4. Thus, we have an Ohmmeter.

A1: Wheatstone Bridge and Parametric Analysis

We will use the Wheatstone bridge to determine the resistance of an unknown

resistor. Pick up the unknown resistor on the podium (You can of course measure

the resistor directly so that you can verify your experimental results). In the

experiment, a potentiometer is the variable resistor. By adjusting the potentiometer

such that the voltage across Rbridge is zero, the value of Runknown can be

determined. In the LTSpice simulation, parametric analysis allows varying resistor

voltages.

1) Determine the symbolic expression for Runknown when Vbridge is zero

(see laboratory introduction).

2) Using values of R1 = 2.2kΩ, R2 = 4.7kΩ, Rbridge = 100kΩ, and Runknown

= ???. R3 is a 10K potentiometer. Note: Resistors were renamed by right

clicking the given name like R4 and writing “Runknown.”

3) In LTSpice, plot Vbridge vs Rpotentiometer where Rpotentiometer is a

parametric value. In the LTpice simulation, follow the procedure to perform

a parametric analysis (details below). Using the plot and a differential

voltage marker, identify the Rpotentiometer value that results in Vbridge =

0. The LTSpice schematic is shown below.

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a) Parametric analysis: The .step command performs repeated

analysis while stepping through specified values of a model

parameter, global parameter or independent source.

1. Define the component parameter by right clicking the

resistor R3 and entering “X” for the value of resistance

(as shown in the diagram below). Note: Runknown is

given the arbitrary value of 1k so the simulation can run.

2. Add a .step command using a SPICE directive (press “s”)

which specifies the steps for a parameter

Example: “.step param X 1 10k 1k” steps the parameter S

from 1 to 100k in 1k increments.

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You may change the increments to a value that will give

you more points.

3. Add .op in the SPICE directive. (Click “.op” far right on

toolbar and add .op then place anyway on circuit)

4. Run the simulation (click “Running man”) go to “DC op

pnt tab” and click “ok”

5. Run the simulation again. (click “Running man”). The

simulation pop up window should show but without

traces with resistor values as the x axis.

6. To specify the differential probes across Rbridge, click

the node to the left of Rbridge (a red probe should

appear), hold and click the right (a black probe should

appear).

7. The trace V(N00n, N00nx) should appear (where n is

some number label of node).

8. Now find the variable resistor value when VRbridge =

0V. Use the cursor function by clicking the trace label at

the top of the diagram “V(N00n, N00nx)”. You can drag

the cursor along the curve by clicking and holding where

the horizontal and vertical lines meet.

9. Include the screenshot/plot of the balanced bridge point

with clear labels in your report.

10. Use the equations in the introduction to calculate the

Runknown value from a balanced bridge circuit.

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4) Build the physical circuit using a 10kΩ Potentiometer, as shown in the

circuit below. Note, one leg of the potentiometer is floating. Turn the

potentiometer such that the measured Vbridge = 0. Once you find that value,

use an Ohmmeter to measure the resistance of the potentiometer (they are on

the center table). Be careful not to turn your potentiometer and make sure

you disconnect the circuit so you don’t measure the other resistors. Compare

your result to part 3).

R12.2k

Runknown

Rbridge

100k

R24.7k

Rpotentiometer

0

5V

Compare the LTSpice simulated value to the value obtained from your physical

circuit.

PSpice – Differential vs Nodal Measurements:

In the simulations we have done with LTSpice, we have used nodal

measurements which provides the voltage at a node relative to the

designated ground. In order to determine the voltage across a component, we

then found the voltage difference between the two nodes associated with that

component. In practice, measuring the voltage across a component where

neither of the nodes is connected to ground can be problematic. To safely

make those kind of measurements, we use differential probes. Fortunately

for us, the Discovery Board only makes differential measurements.

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Part B: Analog Discovery Board Variable Sources and Superposition

Discovery Board – Variable Sources:

In the last laboratory, we used the fixed 5V supply. This source is

constant. If we want to vary the source voltage, we need to use the

function generator channels instead. There are two channels available,

labelled W1 (yellow wire) and W2 (yellow striped wire) on the

Discovery Board.

To access the software, when you bring up the Waveforms main menu

a. Select WaveGen, the second item under the Welcome settings.

b. We will want to use both Channels at various times during the course.

(When we use only one Channel, you can turn off the other one if you

want more space on your Desktop.) To enable both Channels, click on

“Channels” pull down menu. Select both Channel 1 (AWG1) and

Channel 2 (AWG2) such that there are check marks by both. Your

window will probably refresh.

c. We will use DC sources for now. Select the straight line from the

column of waveform shapes (it should be the first icon).

d. Go to the Offset pull down menu and set the DC voltage level.

e. To output the voltage on the W1 (AWG1) wire, you need to select

make sure the Channel is both Enabled and running. In the upper right

of the window, make sure “Enabled” is checked. Click Run.

f. Repeat steps c.-f. for AWG2

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B1: Two Sources/Superposition

Construct the following circuit. You will need to use both source channels (AWG1

and AWG2) on the Discovery Board to build the circuit. Note: The diagram below

was created in PSpice but please duplicate this in LTSpice.

V1 V2

R1

1k

R22.2k

R3

2.2kV+

0

V-

1) Analytically, obtain an expression for the voltage across R3 in terms of the

voltages V1 and V2. You should use superposition in your analysis (for

practice). You are looking for an expression of the form

VR3 = a(V1)+b(V2)

where a and b are coefficients determined by your circuit analysis.

2) Build the circuit using the AWG wires (yellow and striped yellow) for the

sources. Set V1 to 2 [V] and plot the voltage across R3 as a function of V1

for 0<V1<4Volts (pick a few values for V1 in that range and measure R3).

3) Repeat with V2 set to -2 [V], again plotting the voltage across R3 as a

function of V1 for 0<V1<4Volts.

4) For both plots, compare your results with a plot of your expression from part

1).

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Part C: Amplifier Circuits

Overall notes:

TL072CP chip (dual op-amp):

The data sheet for the chip can be found online from any number of sites. One is

provided below (it is long and contains several chips)

http://www.ti.com/lit/ds/symlink/tl071.pdf

A copy of the pin connections is shown below

There are two op-amps on the chip, indicated by the ‘1’ and the ‘2’ pin labels. For

example, 1IN+ is the V+ and 1IN- is the V- of the first op-amp, with 1OUT being

the Vout. Power connections are +Vcc at pin 8 and –Vcc at pin 4.

http://www.ti.com/lit/ds/symlink/tl071.pdf

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In LTSpice, you can use the “UniversalOpamp2” component or “opamp”

component. The “opamp” component does not have power levels and is assumed

ideal. It is useful for simplified drawings, but your simulations will not be the same

as the experiments. As such, please use the “UniversalOpamp2” component, with

LTSpice details shown below.

A summary of the connections for LTSpice “UniversalOpamp2” component:

• input, - (left), inverting input

• input, + (left), non-inverting input

• -(bottom), V-: Negative power, 9 V

• +(top), V+, Positive power, 9V

• Right node: Vout, output voltage

The DC power sources will be the 9 Volt batteries that you have in your kit. Note

the orientation of the batteries when you connect the leads.

Again, for LTSpice simulations, the circuits on the following pages indicate how to

power a uA741 op-amp. The input and output connections depend on the circuit.

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An example of an op-amp reaching saturation is shown below. The input is a

sinusoidal. If the op-amp was ideal, the output would also be a sinusoid with a

scaled amplitude. However, saturation occurs and the output voltage cannot exceed

(positive or negative) the source voltages.

C1: Amplifier Circuits

Build the comparator circuit shown above. V+ and V- will be your inputs and Vout

will be the output. In Analog Discovery experiments, use the TL072CP chip (or

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equivalent. You only need one amplifier for this part). In the LTSpice simulations

use the “UniversalOpamp2” component.

1) We will use W1 and W2 for out amplifier inputs. The Voltmeter channels

inputs will act as the RLarge.

a. Connect W1 (yellow wire) to the V+ op-amp input and ground

(orange striped wire) to the V- op-amp input.

b. Ground the V- op-amp input.

c. To compare input voltage to output voltage, use the Voltmeter to

measure the output voltage (refer to Lab 1).

d. Using the Discovery board, set the W1 output voltage to DC mode

and check the output voltage of the op-amp for the following input

voltages

e. Comment on your results and expectations when Vin = 0 V.

Vin [V] Vout [V]

2

1

0

-1

-2

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2) In LTSpice, build the comparator circuit using a UniveralOpamp2 op-amp.

You will need to add a load resistor at the output node since LTSpice does

not allow nodes to float (be unconnected). A 1E6Ω load is fine (use

exponential notation since M in LTspice is 1E-3). Compare the output

voltages between LTSpice and Analog Discovery. You should see some

differences, what causes these differences?

Vin [V] Vout [V]

2

1

0

-1

-2

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3) Remove the ground connection at V- and use AWG2 (W2, yellow striped

wire) to provide a 1.5V input at the V- opamp input. Effectively, your circuit

will behave as if there was a 1V source at the negative input, as shown

above.

If you didn’t use AWG2 for the 1.5V input, what type of circuit can you use

to produce the 1.5V? (Consider the 5V Discovery board connection from

Lab 1.)

a. Repeat the output voltage measurements again

Vin [V] Vout [V]

2

1

0

-1

-2

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4) Again, compare your Analog Discovery experiment to the LTSpice

simulation.

Inverting Op-amp, Non-inverting op-amp

Build and test the following circuits. (Again, this schematic was created in PSpice.

Recreate in LTSpice. Remember the power connections. They have been removed

to simplify the drawing but they must still be included in the circuit).

5) Inverting Op-amp with a gain of -2 U2

uA741

+3

-2

V+

7V

-4

OUT6

OS11

OS25

VoutVin

0

R2

R1

a. When considering the saturation voltage, what is the maximum Vin

such that Vout = -2 Vin? Choose appropriate resistors.

b. Build the circuit in LTSpice and verify that simulation is constant

with experiment (within the limits of the respective saturation

voltages).

Vin [V] Vout [V]

2

1

0

-1

-2

Vin [V] Vout [V]

5

3

1

0

-1

-3

-5

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Part D: Alpha Squad Laboratories Applications

You will use the information gathered from the data sheet in the last lab to begin to

build the analog sensor signal transduction circuit! Then you’ll take it to the next

step which is to simulate and build a circuit that makes a decision.

D1: Application of Voltage Dividers – Bridge Circuit

1) Simulate the bridge circuit and voltage output for dusk on a clear day

compared to lux values during a dust storm using the determined LDR

resistance found from the data sheet in Lab 1. (You may use parametric

analysis like you did in part A or simply use the two values of

resistances.)

2) Determine the voltages for dusk on a clear day with and without a dust

storm.

3) Build the circuit with the Analog Discovery board using a potentiometer

instead of a LDR. Compare your experimental results to the LTSpice

results. You must consult Beta Lab group(s) and compare your results

with that of an actual LDR. Write down the name of the Beta Lab

group(s) you have consulted.

4) Questions:

a. After finding two different voltages from your bridge circuit, what

circuit would you use to make a decision about the presence of a

dust storm at dusk?

b. How does your design effect the ease of making that decision?

D2: Another Op Amp Decision Making Circuit

1) What op amp circuit enables the ability to provide a specific output when

the input is between two threshold voltages?

2) Design this circuit in LTSpice making reasonable assumptions for

voltages that define a clear and cloudy day without a dust storm.

3) Build this circuit and compare experimental results to your LTSpice

results.

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1) EXTRA CREDIT: Simulate and build yet another op amp circuit that gives

a specific output for a range of inputs. Describe exactly how this circuit

works. Compare this op amp circuit with the one you chose in D2. (i.e.

does it use more parts, is it easier or harder to change threshold values, does

it consumer more or less power, is it susceptible to errors…)

laboratory 3: bridge circuits, superposition, thevenin...

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