LIGHT EMITTING DIODES (LEDs)
LIGHT EMITTING DIODES (LEDs) Like an ordinary silicon diode, a light emitting diode will only allow current to flow through it when its anode is more positive than its cathode. LEDs contain gallium compounds which glow when a few milliamps flow through the diode junction. They are encapsulated in a clear plastic package with a lens shaped top. The colour of the light emitted depends upon the gallium compound used. Most suppliers will offer red, green, yellow and blue LEDs.
Light Emitting Diode The cathode is the lead nearest the ‘notch’ on the body. CIRCUIT DESIGN LEDs are used forward biased and the circuit voltage must be high enough to force a current through them. The forward volt drop (Vf ) across a LED is about 2V and sufficient light is emitted when a current of about 10mA (0.01A) flows through it. Suppose we wish to operate a LED on a 9V supply. The value of a suitable current limiting resistor can be calculated as follows:
0.01A VR
+9V Voltage across R
R Current through R
VF
R
= 7 0.01
=
9-2
=
7V
=
0.01A
=
700Ω
0V Current limiting resistor The value of the current flowing through the LED is not critical. A 680Ω current limiting resistor would be suitable in this case.
Flashing LEDs and colour changing LEDs
Light Emitting Diode The cathode is the lead nearest the ‘notch’ on the body. CIRCUIT DESIGN LEDs are used forward biased and the circuit voltage must be high enough to force a current through them. The forward volt drop (Vf ) across a LED is about 2V and sufficient light is emitted when a current of about 10mA (0.01A) flows through it. Suppose we wish to operate a LED on a 9V supply. The value of a suitable current limiting resistor can be calculated as follows:
0.01A VR
+9V Voltage across R
R Current through R
VF
R
= 7 0.01
=
9-2
=
7V
=
0.01A
=
700Ω
0V Current limiting resistor The value of the current flowing through the LED is not critical. A 680Ω current limiting resistor would be suitable in this case.
Flashing LEDs and colour changing LEDs
