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LED Lighting Basics (Revised 29-Nov-2010)
General
LEDs are actually diodes that emit light. They are semi-conductor devices and are not
related to incandescent bulbs at all. Since they are diodes, for all practical purposes, they
only conduct current in one direction. To conduct current, the LED anode must be connected
to a positive voltage source and the cathode to a negative voltage source. When the
applied voltage reaches the voltage threshold for the LED, current will flow and the LED will
light. The voltage required for the LEDs used in model railroading varies between 1.2 and
about 1.5 volts.
Since decoders do not have voltage adjustable outputs, it is best to think of LEDs as current
devices. Decoder voltage output is usually 12-14 volts or 5 volts depending on the decoder.
12-14 volts is the most common.
Identifying LED polarity
For surface mount LEDs with 6” leads, the long stripped lead is the anode. If you cut the
leads, the only way to determine polarity is by testing the LED to see which polarity lights it.
We sell an LED tester that is quite handy for that purpose.
Figure 2 – Surface Mount LED
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For 3 mm and 5 mm LEDs you can visually identify the Anode and Cathode. Also, until the
leads are cut, the Anode is the longer lead.
Figure 1 – 3 mm and 5 mm LEDs
Resistor Values for LEDs
Wire all LED anodes to the +12-14 common (blue wire). Connect each LED cathode to a negative
function output through a resistor.
The current value for LEDs is calculated using Ohms Law. I = V/R where I is current, V is voltage and
R is resistance. The target current value for 3 and 5 mm LEDs is 20 mA. For miniature surface mount
LEDs, the target current is 12-14 mA
3 mm and 5 mm LEDs
For 3 mm LEDs the normal resistor is a 560 Ohm ½ watt resistor. This runs the LED at 20-25 mA
(milliamps) which gives normal brightness. Use a higher value resistor for less light output. 20
mA is a good target value for 3 and 5 mm LEDs
Surface Mount LEDs
For the miniature surface mount LEDs, use a 1K resistor or 1.2K Ohm ¼ or ½ watt resistor. Use
one resistor for each negative function output. A 1K resistor runs the LED at 12-14 mA which is
quite bright. We use 1K ¼ watt resistors in our models. For less brightness, use a larger value
resistor. 20 mA is the absolute maximum current for the surface mount LEDs. It is a good idea to
stay below the maximum current.
Testing LEDs
The polarity of a surface mount LED with 6” leads can be determined using an LED tester (shown in
Figure 3). Insert a 3 mm LED into the desired current position in the LED tester. Using a 3 mm LED to
plug into the current contacts on the tester, gives you a visual indication when current is flowing.
Connect one end of the test lead clip to the 3 mm LED anode (+) and one end of the other test lead
clip to the cathode (-).
Clip the other end of the leads to the two stripped portions of the miniature LED leads. If the LED
leads are still factory stripped, connect the positive clip lead to the anode (long stripped wire) and the
negative end to the cathode. If you cannot determine which lead is the longer stripped lead, then you
will have to discover the polarity by processes of elimination. Try one polarity and then reverse the
polarity if the LED does not light.
By using different current settings you can also determine the desired current value for the LED.
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.
Figure 3 – LED Test Setup
When installing Miniature LEDs into a locomotive, it is sometimes difficult to trace the wires to the
LED. As a result, you do not know which LED leads are for a particular LED. Using the LED tester on a
set of LED leads will illuminate the LED so you can determine which LED lights for the particular lead
pair you are checking. The tester will light the LED even if a 1K resistor is in the circuit. It will not be
as bright but it will light.
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