Transcript Passive Transducers - Antonine Education

Passive Transducers
Electricity Tutorial 6
Thermistor and LDR
• A resistive (passive) transducer is a device
that senses a change to cause a change in
resistance.
• Passive transducers do NOT generate
electricity.
• The thermistor and LDR are both common
examples of passive transducers.
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Different Passive Transducers
Moisture detector
LDR
Device
Light Dependent Resistor
Thermistor
Strain gauge
Moisture detector
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Action
Resistance falls with
increasing light level
Resistance falls with
increased temperature
Resistance changes with
force
Resistance falls when wet
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Thermistors
Where used
Light operated switches
Electronic thermometers
Sensor in an electronic
balance
Damp meter
3
LDR
Cadmium
sulphide strip
Clear top
Resistance (W)
Symbol for LDR
Light level (lux)
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Bead
thermistor
+to
Resistance (W)
Thermistor
Positive temperature coefficient
Negative temperature coefficient
Rod thermistor
-to
Temperature
(oC)
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Characteristic of Thermistor
•The resistance scale goes up
in powers of 10.
•It is a logarithmic scale.
•This allows us to have a huge
range of resistance in a
reasonable amount of space.
•You will see more about
logarithmic graphs at A2
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Escaping from Prison (Real World)
Fingers wants to escape from prison.
But there is a big wall keeping him on
the Inside.
He can jump, but not very high.
Fingers
Inside
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Outside
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How do Thermistors Work?
Semi-conductors do not work in the
same way as metallic conductors.
Most electrons remain bound to the
atoms in a range of levels called the
valence band. There is a higher energy
level shared between atoms called the
conduction band. Between these two
is the forbidden gap.
It’s a bit like the prison wall that keeps
Fingers on the Inside, and separated
him from the Outside.
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How do Thermistors Work?
Electrons can get sufficient energy
to jump the gap and go into the
conduction band.
They do this because of the
quantum idea of probability clouds.
It’s a bit like Fingers suddenly having
just enough energy to leap the
prison wall.
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How do Thermistors Work?
In real crystals there are defects.
These might be impurities or
missing atoms. They can cause extra
levels to be present in the forbidden
gap. These are the defect levels.
Electrons can rest there some time.
Doping semi-conductors can
improve their conduction
characteristics.
It’s like Fingers having a platform to
rest on half way up the prison wall.
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