TRANSDUCERS

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TRANSDUCERS

• What is a transducer ?
• A transducer is an electronic device that
  converts energy from one form to another.
• Common examples include microphones,
  loudspeakers, thermometers, position and pressure
  sensors. Although not generally thought of as
  transducers, photocells, LEDs (light-emitting
  diodes), and even common light bulbs are
  transducers.
• Efficiency is an important consideration in any
  transducer. Transducer efficiency is defined as
  the ratio of the power output in the desired form
  to the total power input. Mathematically, if P
  represents the total power input and Q represents
  the power output in the desired form, then the
  efficiency E, as a ratio between 0 and 1, is
  given by
• E Q/P

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The Wheatstone Bridge

• a circuit used to precisely measure resistance
• Used in conjunction with transducers to measure
  physical quantities such as temperature, strain,
  and pressure

The Wheatstone bridge can be used in a balanced
or an unbalanced condition. The condition of
operation depends on the type of application.
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The Wheatstone Bridge Circuit
Consists of 4 resistors, a dc voltage connected
across the top and bottom points of the
diamond. The output voltage is taken across the
left and right points of the diamond between
points A and B
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The Balanced Wheatstone Bridge
The voltages across R1 and R2 are equal (V1 V2)
and the voltage across R3 and R4 are equal (V3
V4)
The voltage ratios can therefore be written as
V1 / V3 V2 / V4
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Use this formula to find the value of resistor R1
in terms of the other resistor values when the
bridge is balanced. You can also find the value
of any other resistor is a similar way.
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Using the Balanced Wheatstone Bridge to Find an
Unknown Resistance
Assume R1 in the figure below has an unknown
value Rx
Resistors R2 and R4 have fixed values so their
ratio R2 / R4 also has a fixed value.
Since Rx can be any value, R3 must be adjusted to
make R1/R3 R2/R4 in order to create
a balanced condition.
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Therefore, R3 becomes a variable resistor which
we call RV.
When a resistance Rx is placed in a bridge, RV is
adjusted until the bridge is balanced as
indicated by a zero output voltage.
Then the unknown resistance is found as
The value of RV at balance multiplied by the
scale factor R2/R4 is the actual resistance of
RX If R2/R4 0.5, then RX 0.5RV
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The Unbalanced Wheatstone Bridge
The Wheatstone bridge is unbalanced when the
output voltage (VOUT) between terminals A and
B is not equal to zero.
Used to measure several types of physical
quantities such as temperature, mechanical
strain, or pressure, by connecting a transducer
to one leg of the bridge.
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The resistance of the transducer changes
proportionally to the changes in the parameter
that it is measuring.
If the bridge is balanced at a known point, then
the amount of deviation from the balanced
condition, as indicated by the output voltage,
indicates the amount of change in the parameter
being measured.
Therefore, the value of the parameter being
measured can be determined by the amount that the
bridge is unbalanced.
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A Bridge Circuit for Measuring Temperature
Here we can use a thermistor, which is a
temperature sensitive resistor, where the
resistance changes in a predictable way as the
temperature changes.
A change in the temperature causes a change in
the thermistor resistance, which causes a
corresponding change in the output voltage of the
bridge as it becomes unbalanced.
The output voltage is proportional to the
temperature therefore, either a voltmeter
connected across the output can be calibrated to
show the temperature or the output voltage can be
amplified and converted to digital form to drive
a readout display of the measured temperature.
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Such a bridge is designed so that it is balanced
at a reference temperature and becomes unbalanced
at a measured temperature.
For this particular case, the change in output
voltage (?VOUT) can be shown to be related to the
change in Rtherm by the following formula
NOTE Formula only applies to case where all
resistances in the bridge are equal when the
bridge is balanced.