Light Emitting Diodes
In this unit so far, the only light source used has been the lamp. A lamp is a useful component which converts Electrical Energy into Light Energy. The lamp does have a drawback, however, as to generate light it must also give out Heat Energy. This heat Energy is waste Energy, and so the lamp is a fairly inefficient light source.
Another light source that can be used within a circuit is the Light Emitting Diode (L.E.D.). This component gives out very little heat Energy, and so is more efficient than a lamp.
The diagram below shows the symbol for an L.E.D.:-
Note - In circuit diagrams, two arrows denote light:-
1. Two arrows leaving symbol - Light given out.
2. Two arrows entering symbol - Light taken in.
Light Emitting Diodes also control the direction of current within a circuit. LEDs will only conduct when connected in a certain direction within the circuit (for an explanation of why this is, please see the Higher course).
The large triangle of the symbol can be used as a pointer arrow:-
1. Towards the negative terminal - LED lights.
2. Away from the negative terminal - LED does not light.
The diagram below shows an LED connected correctly within a circuit:-
In the above circuit, the LED is connected in series with a Resistor. LEDs require only very low currents to function, and are easily damaged, so the function of the Resistor is to reduce the current flow to a safe level.
The size of the Resistor depends on the supply voltage and the requirements of the LED. The method below allows the calculation of the correct Resistor:-
The above diagram shows a simple LED circuit. If the LED functions at the given ratings, what is the required value of the Resistor?
Vs = VR + VLED
VR = Vs - VLED
VR = 12 - 2
VR = 10 V
IR = ILED = 10 mA = 0.01 A
R = VR / IR
R = 10 / 0.01
R = 1000 Ω
R = 1 kΩ