Electronics Principles V11

Clive W. Humphris

DIODE APPLICATIONS: Light Emitting Diode 1.  

The Light Emitting Diode LED, is a PN junction diode that will emit light when forward biased, that is when current flows from its anode to the cathode. When selecting an LED for a particular application it should be remembered it's a visible indicator and so both brightness level, especially during daylight and the viewing angle need to be considered.

The forward voltage drop can be between 1.6V and 2V depending upon the device type. The more forward current the brighter the light output. In this application the logic inverter places 0V or 5V on the cathode that turns the LED ON or OFF. The series resistor R1 determines the forward current. Normally a flat on the base of the LED identifies the cathode or the cathode is the shortest leg.

A range of high quality bright LED's are available to the constructor with red, green and yellow with diffused lens in 2 sizes 3mm and 5mm. The optical construction ensures that they give a good visible light output, low drive current and fast response time. LED's are also suitable for pulsed operation the frequency and mark space ratio of, which will effect the visible light output as for a proportion of the time the device, will be switched off. The viewing angle is usually around 80° to 120°. The recommended forward current is 10 to 20mA, the more current the brighter the output.

Fixed 5V and 12V types are now often used. These LED's are particularly applicable to logic circuits as they include an internal resistor, designed to operate directly from five or 12Volts, greatly simplifying circuit design. Making good logic level indicators. The easiest way to confirm you have this type is to measure the forward resistance; it will always be higher than normal type owing to the value of the series resistor. If in doubt you should always check as replacing with a normal type will cause excessive current to flow, which will destroy the LED and possibly other components feeding the circuit.

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