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 / l. Note that
long wavelength ( l ) 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 electronvolts (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 w f = Back EMF = machine
constant kg x rotational speed w x
magnetic flux f.
• The Back EMF OPPOSES the applied
voltage and varies with ROTATIONAL
SPEED ONLY ( kg and f 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 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:
Pacer’s Formula 560 Canopy Glue --- cures
clear and remains pliable.
• Recommended paint for blocking unwanted
light: Pactra Racing Finish BLACK.