bridge circuit design homework

8/2/2019 Bridge Circuit Design Homework

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© 2011 Robert Prins, Keith Holland 1

ENGR 313

Bridge Circuit Design Homework

The lead wires you soldered to the board last week will be connected to a power supply that

will furnish the bridge excitation voltage V in. The nominal resistance of the gage is 120

.

In the simplest form, gage completion resistors R1, R2, and R3 would each be specified as 120

in order to match the gage nominal resistance. In real life, resistors will vary from their nomi-

nal value; this can cause the bridge to be unbalanced. Although the output voltage V out should

be zero in the case of zero strain, an unbalanced bridge has a non-zero output for zero strain.

In order to compensate for this, bridges may be equipped with balancing capability. This

means that they can be tuned to give zero output for zero strain. The bridge you design and

build will have tuning capability; the next page describes how to do this.

Design of a Bridge Circuit with Balancing Capability

As part of the next lab you will build a Wheatsone bridge circuit to measure the resistance

change of a strain gage. Ideally, a bridge circuit containing a strain gage (or gages) will provide

an output voltage of zero when the gages are not in tension or compression. In reality, this is

not the case; however, a circuit can be designed that allows the user to adjust the output volt-

age to zero under zero strain conditions.

V out V in

R gage

R3 R2

R1

A B

V ref

The bridge you will build in lab will be similar to the schematic shown below.




If the resistor R1 could be adjusted, the output of the bridge could be set to zero for the initial

strain state (zero in our case). Rather than adjust R1, we will add a potentiometer and two re-

sistor in place of R1, as shown in the figure below (R1 is the potentiometer, R2 and R3 are stan-

dard 5% tolerance resistors). The strain gage is R5 in the figure below.

R gage

A potentiometer can be thought of as a variable resistor. Potentiometers are often used to

allow for fine tuning (or “trimming”) of a circuit. The pictures below show a typical potenti-

ometer available in our lab, and a schematic of a potentiometer. In operation, a screwdriver is

used to turn the dial; as the dial position changes, the resistance across the output pins is var-

ied. You should be familiar with this idea from your use of string potentiometers in Dynamics

lab. The potentiometers available in the lab require approximately 1/2 of a revolution to

cover their entire resistance range.

The potentiometer above is typi-

cal of the potentiometers avail-

able in the lab. Use an ohmme-

ter to verify which terminal is

which.

B CA

The potentiometer schematic above shows the

two end terminals (B and C) and the wiper ter-

minal (A). As the wiper is rotated clockwise theresistance between terminals A and B increases

while the resistance between terminals A and C

decreases. The resistance between terminals B

and C remains constant.

Wiper




1) Select appropriate nominal values of the potentiometer (R1) and the corresponding resis-

tors (R2 and R3) that will allow you to tune the resistance of the adjustable leg of the bridge

to 120 +/- at least 20 Keep in mind that you will want to minimize tuning range

(while still observing the specified range requirement) or the adjustment will become sen-

sitive to small adjustments of the potentiometer.

State your selected potentiometer (R1) and resistor values (R2 and R3) . Choosefrom the list of available parts below

Determine the range of resistances your resistor/potentiometer combination is ca-

pable of producing. Show your work.

Available Resistors ():

100, 110, 120, 130, 150, 160, 180, 200, 220, 240, 270, 300, 330, 360, 390, 430, 470, 510, 560,

620, 680, 750, 820, 910, 1000

Available Potentiometers ():

0-200, 0-500, 0-1000, 0-5000, 0-20000, 0-50000, 0-100000, 0-200000, 0-500000

The second part of the bridge design is to sketch a layout of how your components will be

placed on the circuit board. Keep in mind that the circuit board holes can only accommodate

one component’s lead wire. Multiple lead wires from adjacent holes can be soldered together

to form a junction (see soldering guide).

Your strain gage assembly from the previous lab should include three lead wires. This configu-

ration improves gage accuracy by compensating for lead wire resistance which would other-

wise be seen as additional strain gage resistance (see Vishay application note TT-612 for dis-

cussion). Consult the figure below for the wire connections required to connect the three wirebridge circuit.

V in

V out

Req

R4 R6

R gage

R L1

R L2

R L3

In the figure above, Req is the equivalent resistance of the potentiometer circuit designed

above. R4, and R6 are the gage completion resistors. RL1, RL2, and RL3 represent the resistance

in the lead wires that are soldered to the strain gage.




2) Use the figure below to determine how you will assemble your circuit board. Resistor

lengths are such that their leads should be spaced at least 4 holes apart. The potentiometer

leads are about 2 holes apart, but can be tweaked to accommodate slightly different spacing.

The output voltage, V out , should be connected to lead wires to accommodate data collection.

You should already have V in lead wires soldered in place from the previous lab. As with the

previous lab, the solder connections will be made on the opposite side of the board. Show your intended location of the following components or connections:

3 bridge completion resistors

Potentiometer

3 strain gage wires

2 output voltage wires

V in

V in

bridge circuit design homework

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