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 LEDs color. Hence, LEDs are essentially monochromatic (one color).

How Does an LED Work? LED wavelength and energy are related by Plancks Constant h : E = h c / Note that long wavelength ( photons are less energetic. When an LED gets hot, the color tends to change toward the red (low energy) end of the spectrum.

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).

Why Use an LED? Lamps burn out --- an LED may never burn out if used conservatively. Small lamps may be too dim --- small LEDs can be painfully bright. Lamps may use excessive current or get hot in an application --- LEDs and their resistors rarely generate much heat.

Why Use an LED? Lamps may have poor optics --- LEDs can have excellent optics. Lamp color may not be pleasing --- LED colors are pure. For a DC locomotive headlight, the lamp intensity varies a great deal with track voltage. LED intensity appears to vary far less.

LED Colors and Voltage LED color (wavelength, in nanometers ) is essentially monochromatic (one color). The color and forward voltage are determined by the diodes materials. Generally the color is specified by a vague descriptive term and by a precise wavelength.Ex: Deep Red = 660 nm. Forward voltage increases slightly with current due to internal resistance.

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.

LED Colors and Voltage Early Bright White LEDs had an undesirable blue tint. White LEDs with a warm white color typically have a filter to absorb the excess blue photons and remove the blue tint. Golden White LEDs have orange pigment mixed into the packages epoxy lens to remove the excess blue tint.

LED Packages The package controls the optics. The packages beam angle is that angle from the axis at which the light intensity is half of the on-axis intensity. Typical 3 mm (T1) and 5 mm (T1-3/4) LEDs have beam angles around 30 degrees --- well focused beams of light result.

LED Packages Typical surface mount LEDs emit light from all sides except the bottom, resulting in very large beam angles. Consequently, since surface mount LEDs typically spread their photons over a much wider angle, the on-axis intensity is less than for the 3 mm and 5 mm packages when the total photon flux is the same.

LED Packages Surface mount LED packages may be industry standard sizes or special. Typical standard sizes are the same as for surface mount capacitors and resistors: 402, 603, 805, etc. These sizes represent the bottom dimensions: x for 603.

Things to Remember about LEDs An LED is a diode - thats 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 dont 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 LEDs 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 LEDs forward voltage is a function of its material, which determines its color.

White LEDs in Locomotives Limit the Current Remember that the typical white LED voltage drop is about 3 V at low currents.. Use Ohms Law to calculate resistance. R = V (Res) / I, V (Res) = V (applied ) - V (LED) Example: If track voltage = 8.0 Volts and desired current = 3 mA, R = 5.0 V /.003 A = 1666 ohms. (Use 1500 or 1800 ohms.)

White LEDs in Locomotives When the current to a locomotive motor is briefly interrupted, the motor will create a very high inductive voltage spike. This is NOT Back EMF. The inductive voltage is V = L dI/dt, proportional to the motor inductance (L) and the rate of change of the current (dI/dt). It can easily be volts, generally very brief.

A Digression: Understanding BACK-EMF Back EMF: the generator voltage produced by a turning motor armature, regardless of whether voltage is externally applied. E g = k g Back EMF machine constant k g x rotational speed x magnetic flux. The Back EMF OPPOSES the applied voltage and varies with ROTATIONAL SPEED ONLY ( k g and are constant )

White LEDs in Locomotives When the inductive voltage spike from a motor reverse biases an unprotected white LED, it is likely to destroy the white LED. When the inductive voltage spike forward biases a white LED, the LED will emit a very brief and bright flash. Example: Headlight in early Kato E8 running backwards will flash.

White LEDs in Locomotives Protect the LED from the motor !!! Protection can be in the form of a decoder, a lighting circuit, or a diode/capacitor circuit. That diode can be another LED (for example, the backup LED protecting the headlight and vice versa.) If you cant find two diodes in a DC locomotive, your white LED is doomed.

White LEDs in Locomotives DCC Decoders appear to have all the reverse voltage protection needed. They seem better at driving LEDs than lamps. Locomotives delivered with white LEDs generally have all the protection needed IF NEITHER LED IS REMOVED. Locomotives with separate LED boards (ex: Kato 77A) are probably not protected.

White LEDs in Locomotives To prevent the flashing, connect a small ceramic capacitor across the LED at the LEDs base. Capacitors look like short circuits to high frequencies and pulses. Ceramic capacitors rated at 3.3 microfarads and 6.3 volts work very well as anti-flash capacitors and are small enough to fit between the LEDs leads.

White LEDs in Locomotives Anti-flash capacitors tend to short out the LED voltage on Aristocraft / Crest systems, which apply 15 kHz pulses to the rails. Adding a diode and capacitor in front of the current limiting resistor can store the Aristocrafts pulse energy, and provide nearly constant intensity headlights at all non-zero throttle settings.

Aristocraft Circuit C1 = Anti-Flash Capacitor D1 = Protective Diode R1 = Current Limiting Resistor C2 + D2 = Peak-Hold Circuit

LEDs for Car Lighting Using high-efficiency LEDs, currents can be VERY LOW (less than 1 mA per LED in N Scale, a few mA in HO Scale). Multiple LEDs can distribute lighting. Anti-flicker circuits using reasonable capacitors can back up low current LED lighting circuits.

LEDs for Car Lighting White LEDs are excellent for tail signs and markers with colored lenses (Ex: Tomar). Colored LEDs emit pure colors and they can be very small. They are excellent for tail lights and marker lights. Special effects like Mars Light tail lights can be achieved using LEDs with appropriate electronics.

Mounting LEDs Pick a shape to fit the application. Can sometimes modify the lens of a 3 mm LED to fit the mounting hole. Can sometimes mount a surface mount LED completely inside the mounting hole. Can superglue a surface mount LED to the back surface of an MV lens after removing the backing from the lens.

Mounting LEDs Recommended wire for surface mount LEDs: Belden Type 8058 #36 solderable magnet wire (strip using hot solder). Recommended adhesive and sealer: Pacers Formula 560 Canopy Glue --- cures clear and remains pliable. Recommended paint for blocking unwanted light: Pactra Racing Finish BLACK.