Thermistor- Resistance decreases as temperature increases
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
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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.
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IV Graphs
A resistor at constant temperature
A filament lamp
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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