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?

• LED wavelength and energy are related by Planck’s 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 diode’s 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 package’s epoxy lens to remove the excess blue tint.

LED Packages

• The package controls the optics.

• The package’s “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: 0.060” x 0.030” for “603”.

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.

White LEDs in Locomotives

• Limit the Current

• Remember that the typical white LED voltage drop is about 3 V at low currents..

• Use Ohm’s 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 60-80 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.

• Eg = kg Back EMF machine constant kg x rotational speed x magnetic flux .

• The Back EMF OPPOSES the applied voltage and varies with ROTATIONAL SPEED ONLY ( kg 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 can’t 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 LED’s 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 LED’s 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 Aristocraft’s pulse energy, and provide nearly constant intensity headlights at all non-zero throttle settings.

Aristocraft Circuit

C1 = Anti-Flash CapacitorD1 = Protective Diode

R1 = Current Limiting ResistorC2 + 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: Pacer’s Formula 560 Canopy Glue --- cures clear and remains pliable.

• Recommended paint for blocking unwanted light: Pactra Racing Finish BLACK.