Components and their Characteristics

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Components

Thermistor- Resistance decreases as temperature increases

(http://communityofrobots.com/sites/default/files/images/u1/ldrsymbol.jpg)LDR- Resistance decreases as light intensity increases

Variable Resistor- Used to change resistance in a circuit

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Light Dependent Resistors

  • A light dependent resistor (LDR) is an input transducer, converting light energy to a change in electrical properties. Its resistance decreases as light intensity increases.
  • As photons of light hit a cadmium sulfide track, they give bound electrons enough energy to jump into the conduction band

(http://physicsnet.co.uk/wp-content/uploads/2010/06/LDR-graph.jpg)

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Thermistors 1

  • Negative temperature coefficient (NTC) thermistors are input transducers that have a decreasing resistance as temperature is increased.
  • As the surrounding temperature increases, the electrons in the metal oxide of the thermistor gain energy. This increases the number of charge carriers, decreasing resistance.

(http://physicsnet.co.uk/wp-content/uploads/2010/06/thermistor-graph.jpg)

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IV Graphs

(http://jcsu.jesus.cam.ac.uk/~rpc25/notes/physics/electricity/charac1.gif)A resistor at constant temperature

(http://www.cyberphysics.co.uk/Q&A/KS5/electricity/characteristic/Q4.png)A filament lamp

(http://halestrom.net/darksleep/blog/011_copper_diodes/IV_diode.svg)Diode

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Thermistors 2

  • A thermistor is a component that changes resistance when the temperature changes
  • NTC (Negative Temperature Coefficient) thermistors increase in resistance when the temperature decreases
  • PTC thermistors exist but they are not discussed on this course
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Determining the characteristics of an NTC Thermist

1) Place the thermistor into hot water

2) Measure the temperature

3) Measure the resistance using a multi-meter

4) Repeat the experiment for a range of temperatures

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Thermistors 3

  • An NTC thermistor is a component that ecreases in resistance as the temperature increases
  • These can be placed into cicruits to form switches and sensors
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Using Thermistors

  • Thermistors can be used in sensor circuits to set off alarms or to turn devices on and off at certain temperatures
  • Different thermistors have rate of change of resistance in different temperature ranges and the current one needs to be selected to suit the conditions
  • Examples of their uses include fans on cooling units in electrical circuits.
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