LED Basics Jim Hinds RICHMOND CONTROLS

What is an LED? An LED (Light Emitting Diode ) is a semiconductor DIODE that has been optimized to create and emit photons of light. A semiconductor diode is an interface between two slightly different compounds selected to pass current only in one direction.

How Does an LED Work? Electrons crossing the diode junction in the forward direction must give up energy. Emitting photons of light is one option. Analogy: Water going over a waterfall. The energy lost by each electron is a function of the diode materials, and that determines the LED’s color. Hence, LEDs are essentially monochromatic (one color).

How Does an LED Work? Photon energy is expressed in electron-volts (eV ) , the energy an electron acquires when experiencing a voltage difference of one volt. Visible photons are in the range between 1.8 eV and 2.5 eV. LED intensity (photons per second) is a linear function of current (electrons per second).

LED Colors and Voltage Infrared: ~920 nm, 1.3 V Deep Red: ~660 nm, 1.7 V * Amber: ~590 nm, 2.0 V * Yellow-Green: ~565 nm, 2.2 V * Kelly Green: ~525 nm, 2.5 V Aqua Green: ~505 nm, 2.8 V Blue: ~470 nm, 3.0 V “White”: many wavelengths, 3.0 V * The colors most often encountered.

LED Colors and Voltage “White” LEDs generally use blue LEDs to excite a phosphor coating on the LED chip. The phosphor mix is chosen to re-emit several photon colors when excited, resulting in light that appears to be “white”. Excess blue tint is from insufficient absorption of blue photons by the phosphor.

Things to Remember about LEDs An LED is a diode - that’s what the “D” is. Thus, current normally only flows through an LED in one direction. Generally LEDs cannot withstand very high reverse voltages. “White” LEDs are particularly poor at withstanding reverse voltages.

Things to Remember about LEDs You don’t drive LEDs the same way you drive lamps. Circuits meant to drive lamps are often not good for driving LEDs. Generally you MUST limit an LED’s current with a resistor. LEDs can be bright , needing little current.

Things to Remember about LEDs To drive an LED, sufficient voltage must be available, and then you must control the applied current. (To drive a lamp, sufficient current must be available, and then you must control the applied voltage.) An LED’s forward voltage is a function of its material, which determines its color.