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Home » GATE Study Material » Electrical Engineering » Electrical Elements » LEDs

LEDs

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LEDs

Light Emitting Diodes (LEDs)

        A light emitting diode - a.k.a. an LED - is precisely what the name implies.

  • It emits light.  Usually the light is very pure - being pretty much monchromatic - and it comes in a restricted range of colors.  The most common LED color for the light is red.

  • It is a diode.  That means that current only flows through an LED in one direction.  If you try to make current flow in the reverse direction, no current will flow, and you won't get any light either because you need current flowing in the LED to get any light.



Goals for this lesson unit:

        Here are the goals for this lesson unit.

  • Given a circuit with a light emitting diode,

    • Be able to use a simplified model of an LED to predict when current flows through the diode, and when it does not - and when it lights,

    • Be able to use information about current flowing to predict other behavior in a circuit.


FACTS ABOUT LEDS

        The circuit symbol for an LED is pretty much the same as for a regular diode - since it is, in fact, a diode.  In many ways, you can treat an LED as though it is a diode, but you do need to remember one way that it differs from a typical silicon diode.

  • In a typical silicon diode, the forward voltage drop will be about .6 or .7 volts.  In other words, when current is flowing forward through the diode, there will be .6 or .7 volts across the diode.

  • In an LED the forward drop is more like 1.7 or 1.8 volts.  The drop may also depend upon the color of the LED, but this value is typical for a red LED.

        The circuit symbol for an LED looks much like the symbol for a regular diode.  There's usually an additional little arrow to indicate the light that comes from the diode.  Here's the symbol.

        Now, let's take a look at the LED in a simple circuit.  Let's imagine that you have the following situation.

  • You have a signal that can change.  In one state it is 0 volts - a binary zero, and in the other state it is 5 volts - a binary one.

  • When the signal is a one, you want an LED to light, and when the signal is a zero you want the LED off.

        Here's a circuit that will do what you want when you choose the components correctly.  The 5v source is what you have when the signal is a binary one.  At that point, current should flow through the resistor and the diode should light.

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