Components in Series & Parallel Circuits

Thermistors

• A thermistor is a resistor whose resistance changes with temperature
• The standard schoolroom thermistor is a negative-temperature-coefficient (NTC) thermistor:
  • resistance is high in cold conditions
  • resistance is low in hot conditions
• The relationship is non-linear; a graph of resistance against temperature falls steeply at low temperatures and then levels off at high ones
• Uses include thermostats for ovens, fridges and central-heating systems; fire alarms; and temperature sensors in electronics

Light-dependent resistors (LDRs)

• A light-dependent resistor (LDR) is a resistor whose resistance changes with the light intensity falling on it:
  • resistance is high in dark conditions
  • resistance is low in bright conditions
• The relationship is again non-linear; a graph of resistance against light intensity falls steeply as the light first grows brighter and then levels off
• Uses include automatic street lamps that switch on at dusk, dawn-detection circuits in alarm clocks, and the light meters in cameras

Lamps and light-emitting diodes (LEDs)

• A filament lamp lights up when current flows through it, because the filament heats up enough to glow white-hot. Lamps are non-ohmic (their resistance climbs as they get hot) but they conduct current in either direction
• A light-emitting diode (LED) is a special kind of diode that emits light when current flows through it in forward bias
  • Like any diode, an LED conducts in one direction only; mounted backwards in a circuit, it simply does not light up
  • LEDs are widely used as indicator lights because they need very little current to be visibly on, last for years and produce very little heat
• An LED's standard symbol is a diode triangle with two small arrows pointing outwards, meaning light leaves the device. The LDR's symbol carries two small arrows pointing inwards, meaning light is arriving at the resistor