LED’s in Model Railways

The LED is rapidly becoming the new lighting wonder of Railway and other Modellers however some need resistors, some need drivers and to complicate matters some work off voltage directly.
The Technology is changing every day now the mounting and shapes are changing almost daily. In the following short and we hope simple description we try to unravel the mysteries surrounding this apparent magical source of lighting

The Mystery Revealed

What is an LED it is simply a Diode (see Glossary) that emits light when a voltage is connected in a forward direction and its called a Light Emitting Diode (hence LED).
An LED has many advantages over the incandescent bulbs of the past for instance a grain of wheat bulb at 12v will require 80ma whereas an LED may only need 20ma or less we will look at other advantages and disadvantages later.

An LED is a semiconductor device so can be classed as current operated BUT and its a big but is also quite sensitive to current and voltage of higher than designed levels. An LED has an almost infinite life if used at the correct operating specification

Most “Standard” LEDs have what is known as a Forward current (generally between 10 and 40ma and a Voltage usually between 1.8 and 4v. (This does vary so check the Specification).

This is quite daunting to the average modeller who generally does not wish to have a degree in electronics or learn complicated stuff so lets make it easy.

Ok so let us say the LED needs 2,2v and the supply is 12v so we have 9.8v to effectively get rid of. but we must limit the current to 30ma so effectively the Resistor has to drop 9.8v and limit the current to 30ma (or less)

so if we divide 9.8v by 0.030 it will give 326.6 so we need a resistor of around 326.6 ohms

The Mathematical formula is shown below. It is a point to note that this is a simplified calculation and 0.25 watt (see Glossary) are usually suitable.

The formula is really quite simple but we have provided a calculator above so you do not have to do any maths