What is an LED?

1
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.

2
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).

3
How Does an LED Work?

• LED wavelength and energy are related by Plancks
  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.

4
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).

5
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.

6
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.

7
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.

8
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.

9
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.

10
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.

11
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.

12
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.

13
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.

14
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.

15
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.

16
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.

17
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.)

18
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.

19
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 ).

20
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.

21
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.

22
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.

23
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.

24
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.

25
Aristocraft Circuit

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

26
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.

27
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.

28
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.

29
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.