Resistor Online IGCSE Phyics Class

Resistor is a device that limits the flow of the current.  The most commonly used film resistors can be divided into types according to the materials used: carbon-film, metal-alloy and metal-oxide

Discuss the use of 

  1. Varistor
  2. LDR / Photoresistor
  3. Thermistor



  1. Varistor stands for “variable resistor” and is an electronic component.
  2. It is used to protect sensitive electronic devices and circuits from voltage spikes and transient overvoltage conditions.
  3. Their resistance decreases as the voltage across them increases
  4.  When a voltage spike occurs, varistors rapidly change their resistance, becoming highly conductive and diverting excess current away from the protected circuit.


  1. A photoresistor, also known as a light-dependent resistor (LDR) or photocell, is an electronic component that changes its resistance based on the intensity of light falling on it.
  2.  When exposed to light, the photoresistor’s resistance decreases, allowing more current to flow through it.
  3. In darkness or low light conditions, the resistance of the photoresistor increases, restricting the flow of current.
  4. They are commonly used in light-sensitive applications, such as automatic lighting systems, photographic light meters, and ambient light sensors in electronic devices


  1. A thermistor is an electronic component that changes its resistance with a change in temperature.
  2. It is made from a material that exhibits a significant change in resistance over a specific temperature range.
  3. NTC Thermistor – When the thermistor detects high temperature, the resistance decreases
  4. PTC Thermistor – When the thermistor detects high temperature, resistance increases

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