LEDs (Light Emitting Diodes) have dominated artificial lighting after displacing many alternatives over the years.  Even though electricity can easily damage them, their properties make them a component nearly every hardware developer can easily use.  This project will cover:

• Essential LED properties and LED representations in software
• Part 2: White LEDs for illumination
• Part 3: LED Non-ideal behaviors and failure prevention

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Technology Comparison

It’s helpful to first appreciate differences between LEDs and their close counterparts: incandescent lamps, and diodes that are not LEDs

LED versus Incandescent

A filament is effectively a resistor made of tungsten or molybdenum wire.  Incandescent sources rely on intense joule heating of a metal filament until it becomes hot enough to glow.

To some extent, a filament’s glow is a byproduct of its high temperature.  By comparison, LEDs use a more direct conversion of electricity to light.  Even the least-efficient LEDs may have more than double the efficacy of an incandescent source.

LED versus Non-Light Emitting Diodes

Conventional diodes (i.e. silicon rectifiers) and LEDs have similarities, such as:

• Conducting current in one direction and resisting current in the reverse direction
• Being polarized with a distinct anode and cathode pin
• Using semiconductor effects rather than intense heat for their operations

Yet, an LED’s semiconducting junction has a direct bandgap that can directly produce light from electric charges.  Silicon rectifiers typically have an indirect bandgap, where the odds of light emission at any given time is low.

LED Digital Representations

Schematics often represent diodes as a collection of at least 3 visual objects:

• A value such as a part number, color, package type, or (in simulators) the model name
• A reference designator (i.e. instance number), beginning in “D” or “LED”
• A symbol with an arrowhead aimed in the LED’s forward direction

The arrowhead indicates the direction of conventional charge flow (CCF) that can illuminate the LED.  Engineers now know that real-life electron flow (EF) happens in the opposite direction, so it is important to know which convention is being used.  In CCF terms, the anode must be at a higher voltage than the cathode for the LED to illuminate.

An LED has 2 terminals (pins) labelled anode and cathode.  The bar at the end of the arrowhead is located at the cathode.  Originally, the diode symbol represented the pointed metal wire and crystal grain of a point-contact rectifier.  Not every diode in the 21^st century is a point-contact rectifier, but the symbol applies to many diodes, and 2 smaller arrows will indicate if the diode is light-emitting.

Kits like the Arduino, Raspberry Pi, and MSP launchpads include built-in LEDs that the user’s own software can control.  The software programmer must know whether the software controls the LED anode or cathode, and not just assume outputting a “1” means “turn the light on.”

LED Current-Limiting Practices

LEDs may need current-limiting protection.  Otherwise: they will fail early, irreversibly change color, or fail immediately.  The simplest form of protection is a resistor in series with one or more LEDs (each arranged in series).  Advanced designs may use an LED driver chip to accomplish the same.

In either case, the LED conducts a forward current (I_D).  This causes a forward voltage drop (V_D) across the LED’s anode and cathode.  The equation below can approximate the relationship between I_D and V_D , however parameters I_S and η are unspecified in LED datasheets.  The LED manufacturer will most often express the I_D – V_D relationship as a chart instead…

I_D=I_s\left(e^{\left(\frac{qV_D}{\eta k_BT}\right)}-1\right)

Where:

• I_D is the estimated forward current (unit: A)
• I_S is the saturation current (expect about 10^-12 A to 10^-18 A for an LED)
• q is the elemental charge (approximately: 1.602 x 10^-19 C)
• V_D is the diode forward voltage drop (unit: V)
• η is the ideality factor (expect 2 < η < 8 for white LEDs) (unitless)
• k_B is the Boltzmann constant (1.380649 x 10^-23 J·K^-1)
• T is the absolute temperature of the diode junction (unit: °K)

LED Color vs. Forward Voltage

Even if the LED package is already tinted a specific color, the color of the emitted light is controlled by the LEDs semiconductor and its temperature.  Generally, the shorter wavelengths are associated with larger forward voltages…

As of the early 21^st century, many white LEDs are actually blue LEDs with additional materials that emit yellow light.  These blue-yellow emissions mix in the human eye to fool human vision into perceiving white light.

White LEDs will be covered in part 2 of this series.

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