CIE A Level Physics (9702) 2019-2021

Revision Notes

24.2.3 Use of Light-Emitting Diodes (LEDs)

Use of Light-Emitting Diodes (LEDs)

  • A light-emitting diode (LED) is a type of semiconductor diode that emits light when current flows in a specific direction
  • The different colours of an LED are determined by the type of semiconductor material used such as:
    • Aluminium Gallium Phosphide (AlGaP) – green
    • Zinc Selenide (ZnSe) – blue
  • LEDs only requires a current of 20 mA to produce a light output or a voltage of about 2 V
  • They are an efficient way of indicating the state of an output in electronic circuits
    • For example, which path the current is flowing through when switches and relays are used
  • LEDs are likely to be connected to series resistors to limit the current flow so they don’t become overloaded
  • Remember that LEDs will emit light when they are forward biased in the same direction as conventional current flow (positive to negative) and will not emit light when in the opposite direction to the current flow

Worked example

Use_of_light-emitting_diodes_(LEDs)_Worked_Example_-_LED_Question, downloadable AS & A Level Physics revision notes

Part (a)

 Step 1:           

Determine which direction the current is flowing in

      • When the op-amp is positive the current is flowing from top to bottom

Step 2:           

Determine which LED is forward biased in the same direction

      • The green LED is forward biased when the current flows from top to bottom (is in the same direction as the current) therefore this will emit light

Part (b)

Step 1:           

Determine which direction the current is flowing in

      • When the op-amp is negative the current is flowing from bottom to top

Step 2:           

Determine which LED is forward biased in the same direction

      • The red LED is forward biased when the current flows from bottom to top (is in the same direction as the current) therefore this will emit light

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