![]() ![]() ![]() In use, an LED must be wired in series with a current-limiting device such as a resistor. Typical optical output power figures - in millicandelas - of five basic types of 5mm round red, yellow, and green LEDs. Note in the ‘red’ LED column that the Ultrabright and Hyperbright devices (which use water-clear lenses) are 143 and 500 times brighter, respectively, than a standard red LED.įIGURE 4. The table in Figure 4 presents typical optical output-power and viewing-angle figures for the five types of 5mm round LED. The brightness level is usually specified in milli-candelas (mcd), with the LED passing an operating current of 20mA. LEDs are available in five different ‘brightness’ categories, which are usually known as Standard, High Brightness, Super Bright, Ultrabright, and Hyperbright. Some LEDs give a diffused output in which the light intensity falls off gradually beyond the viewing angle and is thus clearly discernable over a wide angular range others (particularly ‘Hyperbright’ types) have a sharply focused output in which the light intensity falls off very sharply beyond the specified viewing angle. One important but confusingly-named LED parameter is its ‘viewing angle,’ at the extremes of which the LED’s optical output intensity falls to half of its maximum axial value. Special fittings are readily available for fixing most sizes of LED to front panels, etc. The LED case has a polarity-identifying ‘flat’ molded into the side of its base adjacent to the cathode lead, which is usually shorter than the anode lead when untrimmed. Typical physical details of ‘round’ LEDs and methods of recognizing their polarity. Round LEDs use a clear or colored plastic case with a lens molded into its dome, and are designed to be viewed end-on, looking towards the dome, as indicated in the diagram.įIGURE 3. LEDs are available in a variety of styles, the most popular being the ‘round’ type that has the basic shape shown in Figure 3 and which is readily available in standard diameter sizes of 3mm, 5mm, 8mm, or 10mm. If an LED is reverse-biased, it starts to pass significant current at a fairly low voltage value (typically 3V to 5V) and eventually avalanches (zeners) at higher voltages. Typical forward voltage values of standard LEDs at a current-limited value of 20mA. ![]() Roughly 2V are developed across them when passing a useful forward current Figure 2 lists the typical forward volt drops (Vf) of different colored standard 5mm diameter LEDs at forward currents of 20mA.įIGURE 2. LEDs are pn junction diodes, usually made from gallium arsenide (GaAs) or aluminum-gallium arsenide (AlGaAs) types of semiconductor materials, and emit light when stimulated by a forward current. Standard LED symbol, together with its terminal notations. ![]() LED BASICS INTRODUCTIONįigure 1 shows the standard symbol that is used to represent an LED in this article, together with its basic anode (a) and cathode (k) terminal notations.įIGURE 1. A variety of such circuits are shown in this article. LEDs have typical power-to-light energy conversion efficiencies some 10 to 100 times greater than a simple tungsten filament lamp and have very fast response times (less than 0.1µS, compared to 10s or 100s of milliseconds for a tungsten lamp), and are thus widely used as visual indicators and as simple ‘flashing light’ units. The most widely used of all optoelectronic devices is the simple LED (light emitting diode), which emits a fairly narrow bandwidth of visible (usually red, orange, yellow, or green) or invisible (infrared) light when its internal diode junction is stimulated by a forward electric current. ![]()
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