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2.2 Analogue electronics. Light-emitting diode (LED)

A light-emitting diode (LED) is a special kind of diode that glows when electricity passes through it. Most LEDs are made from a semi-conducting material called gallium arsenide phosphide.

LEDs can be bought in a range of colours. They can also be bought in forms that will switch between two colours (bi-colour), three colours (tri-colour) or emit infra-red light.

In common with all diodes, the LED will only allow current to pass in one direction. The cathode is normally indicated by a flat side on the casing and the anode is normally indicated by a slightly longer leg. The current required to power an LED is usually around 20 mA. Usually it's connected in series with a resistor to limit the current. More current would fuse it. The forward bias voltage can be between 1.6 V and 3.5 V depending on the colour (2 V for red colour).

A white LED light     

 Exercise 32. Calculate the value of the resistance in this circuit. Use the Ohm's law.

 Exercise 33. If we change the battery with a 5V one, and the resistor has 47Ω. Is the LED in the circuit save? Why?

 Exercise 34. Build the circuit, first with the Crocodile, after with the kit. 


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