1. Passive Transducers
Electricity Tutorial 6

2. 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.
Tuesday, 20 November 2018
Leeds City College

3. Different Passive Transducers
Moisture detector
LDR
Thermistors
Device
Action
Where used
Light Dependent Resistor
Resistance falls with increasing light level
Light operated switches
Thermistor
Resistance falls with increased temperature
Electronic thermometers
Strain gauge
Resistance changes with force
Sensor in an electronic balance
Moisture detector
Resistance falls when wet
Damp meter
Tuesday, 20 November 2018
Leeds City College

4. LDR Tuesday, 20 November 2018 Leeds City College
Cadmium sulphide strip
Clear top
Symbol for LDR
Light level (lux)
Resistance (W)
Tuesday, 20 November 2018
Leeds City College

5. Thermistor Tuesday, 20 November 2018 Leeds City College
Resistance (W)
Temperature (oC)
Positive temperature coefficient
Negative temperature coefficient
-to
+to
Bead thermistor
Rod thermistor
Tuesday, 20 November 2018
Leeds City College

6. 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
Tuesday, 20 November 2018
Leeds City College

7. 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
Outside
Tuesday, 20 November 2018
Leeds City College

8. 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.
Tuesday, 20 November 2018
Leeds City College

9. 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.
Tuesday, 20 November 2018
Leeds City College

10. 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.
Tuesday, 20 November 2018
Leeds City College