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. Tuesday, 20 November 2018 Leeds City College

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

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

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

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

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

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

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

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