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Light Emitting Diodes
by Tom Patterson

This web page was last updated on  March 8th, 2012.


Definition: a diode is a type of two-terminal electronic component with nonlinear resistance and conductance (i.e., a nonlinear current–voltage characteristic), distinguishing it from components such as two-terminal linear resistors which obey Ohm's law

A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals.

Electrical Characteristics:

Resistance is very low in the forward, or conducting, direction, and very high (to a limit) in the reverse, or non-conducting direction.

Note 1: In normal model railroading applications, the forward current must be limited by a resistor.

Note 2: In the forward direction, between 0.3 and 0.7 volts will be dropped on a semiconductor diode.

Note 3: Ensure that the voltage rating of the diode is sufficient, typically greater than 12 volts.

Physical Characteristics:

In the forward direction, current will flow from the anode to the cathode

 Light Emitting Diodes (LEDs)

Definition:  A semiconductor light source.

Electrical Characteristics: Similar to a semiconductor diode, except that light is emitted.  Another class of LEDs has three leads, and essentially contains two LEDs attached at their cathodes.



A current limiting resistor must be placed in series with the LED to prevent burn out. Most LEDs have current specification. Divide this current value into the nominal voltage that you expect (say, 12 vdc) get obtain an approximate resistance value.

LEDs also have a voltage drop in the conducting direction, which is much higher than that for a diode. A voltage drop of 3.5 volts can be expected. Therefore, it is best to experiment with a variety of resistance value to obtain the lighting intensity desired. See Application Notes below.

Physical Characteristics:

LEDs are also polarity sensitive.  The long lead is positive (anode), (DCC blue wire, or “common”) and the “hat rim” is cut off on the negative (kathode) lead side.  As a matter of standard, always attach the resistor to the negative lead.

For the “micro” LEDs (ditch light size), the longer wire is positive (DCC blue wire, or common).

Application Notes:

Soldering – Generally speaking, the heat from a soldering iron will not damage a diode, as long as the heat is modest and applied briefly, or unless current is flowing in the diode while heat is applied.

Multiple LEDs in Parallel – It is not good practice to attach LEDs in parallel, and then in series to a common resistor, particularly if the LEDs are not identical. Differing internal resistances will cause imbalanced current flows, which may cause an LED to burn out.

Resistance Values – In many cases, the LED package will contain current and voltage specifications, and suggest resistance values.

If not, an example is provided  for the LEDs provided by Peter Nesbitt. They are orange in colour, but light up bright white. They drop about 3.5 volts and the current should be limited to 3 ma.  For a 12 volt source, a resistance of about 2800 ohms would be in order, although a 3 to 4 K resistor can be used to reduce the amount of light. A watt resistor would be OK.

NOTE – for other than Peter Nesbitt’s “orange” LEDs, an initial resistance of at least 1000 ohms should be used with a 12 Vdc supply.

For the “micro” LEDs (ditch light size), a resistor of at least 2 K ohms, watt is required.

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