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SMD LEDs
SMD LED polarity
exhibition stand kit
Definition
LED stands for Light Emitting Diode. Put simply, a diode is a two terminal component which only allows current to flow in one direction. An LED is an electrical component which, when properly installed in an electrical circuit, emits light.

Pre sales testing
Railwayscenics sell a wide range of LED products and we want you to receive these items in good working order. To achieve this, all through hole LED products sold are tested before being packaged. This normally means that we connect the LEDs to a regulated 12v supply. This supply has two sets of wires connected to it. The first is a 12v supply and the other is a 12v supply fitted with a resistor to reduce the voltage to something that will light all LEDs. Any colour changing or flashing LEDs are allowed to run through their inbuilt sequence.

LEDs that we sell
Railwayscenics sell many different types of LEDs. They are
SMD LED - Is an abbreviation for Surface Mounted Device Light Emitting Diode. An SMD LED is a type of LED that uses surface mount technology to mount an LED chip on to a printed circuit board. As electronics have shrunk in size, SMDs have become more popular than their traditional through hole counterpart due to greater packaging efficiency.
Through Hole - These are standard type LEDs which as the name suggests are fitted from the back, through a hole. These LEDs come in many shapes, sizes and colours, but all have a long anode and cathode leg for wiring.

Each type of LED may also contain some or all of the below types and effects.

Flashing
Flashing LEDs are used as attention seeking indicators without requiring external electronics. They look the same as standard LEDs but they contain a built in circuit that causes the LED to regularly flash with a typical period of one second. In diffused lens LEDs, this circuit is visible as a small black dot. Most flashing LEDs emit light of one colour, but more sophisticated devices can flash between multiple colours and even fade through a colour sequence using RGB colour mixing.
Flickering LEDs
These LEDs look the same as normal LEDs but instead of having a circuit that regularly flashes the light, they use a different circuit to produce a flickering effect.
Bi colour
Bi colour LEDs incorporate two different LED emitters in one case. They consist of two dies connected to the same two leads. Current flowing in one direction emits one colour, and current in the opposite direction emits the other colour. All you have to do to get the two different colours is to reverse the voltage through the same wiring.
Tri colour
The name is a bit misleading as the LEDs usually still only contain two colours but use a common anode or cathode. This gives the LED three wire connections instead of the usual two. They are easier to wire to get both colours operating as they are basically two different circuits and two different LEDs in one case.
RGB
RGB LEDs come with red, green, and blue emitters all in the same case, in general using a four-wire connection with one common lead (anode or cathode). These LEDs can have either common positive or common negative leads. Some only have three leads and the third colour is obtained by joining two of the other colours.
LED Strips
These innovative light strips consist of high-powered SMD LEDs of different size and colours mounted on a super-thin flexible circuit board with an adhesive coating on the back. These strips have many uses from lighting models or to whole layouts. They generally do not need any resistors fitting as they are included on the circuit board, and they operate of something like 12v. Strips can usually be cut into shorter lengths as long as they include a complete circuit, usually in threes.
LED Colours
LEDs can be available in a wide range of colours. The colours of an LED are created by altering the material that surrounds the diode, and also the wavelength of the LED. Most colours are available, but we sell different types in blue, cool white, green, orange, pink, red, warm white, yellow and warm white.
LED Specifications
Reading a table of technical data for LEDs
Most good suppliers of LEDs will show a table of technical data for each of the different LEDs sold on the product information page. These tables contain a good deal of useful information in a compact form but they can be difficult to understand if you are not familiar with the abbreviations used.

These are the important properties for LEDs:

Maximum forward current: IF max: Forward just means with the LED connected correctly.
Typical forward voltage: VF typ: This is VL in the LED resistor calculation, about 2V, or 4V for blue and white LEDs.
Luminous intensity: Brightness at the specified current, e.g. 32mcd @ 10mA (mcd = millicandela).
Viewing angle: 60deg for standard LEDs, others emit a narrower beam of about 30deg.
Wavelength: The peak wavelength of light emitted, it determines the colour of the LED, e.g. red 660nm, blue 430nm (nm = nanometre).

The following two properties can be ignored for most circuits:

Maximum forward voltage, VF max: This can be ignored if you have a suitable resistor in series.
Maximum reverse voltage, VR max: This can be ignored for LEDs connected the correct way round.

Wiring LEDs without a resistor
The LEDs that we sell that do not require a resistor are rated at a maximum of 14 volts, and this should not be exceeded or damage may result. Correct voltages are given in the product descriptions. These LEDs can be wired directly into a correct voltage DC supply.

Wiring LEDs using a resistor
Resistors are relatively inexpensive and basically restrict the current passing through the LED, as the LED has no current limit itself. If you put an LED on a battery with no resistor it would be extremely bright for a fraction of a second before it blows.

LEDs are also polarity sensitive. That means that they will only work once wired correctly. If an LED fails to light, try reversing the wiring. All LEDs have a marked anode, which is the positive and a cathode which is the negative.

Each LED should have its correct resistor wired to the positive terminal. Only use one resistor per LED. Each of our LED listings includes references to the correct minimum size of resistors that can be used with different voltages. You can go slightly larger than the recommended value, but never go lower.

It is not necessary to supply a separate feed to each resistor and LED.

It does not matter which way the resistor is fitted as they are not voltage sensitive.

Each LED and resistor combination should use its correct input voltage as detailed on our website.

Calculating resistor values
LEDs have just two really important parameters for this calculation:
Forward voltage (Vf), typically around 2.5V
Rated current (If), typically around 20mA
Often, you will see the maximum current specified, with the rated brightness at a lower current. For example:
Maximum current: 25mA
Typical brightness: 1000mcd (at 20mA)
It is the figure in brackets you need.
The current drawn by an LED varies wildly over a small range of voltages, as it is a semiconductor. By using a series resistor, we "damp out" this sensitivity to voltage.

The calculation
We want our series resistor to drop the excess voltage (supply minus the LED forward voltage) and pass the LEDs rated current (horribly over-simplified, but it works). Simple Ohms law stuff - here is an example for the 2.5V, 20mA resistor above, running off a 12V supply:

Resistance = (Voltage / Current) = ((12V - 2.5V) / 0.02A) = 475 ohms

Choose the closest available standard resistor value: here, 470 ohms. Do not go much smaller than the value you calculate, but you can go significantly larger to be safe, although that will cost you a bit of brightness.

Other stuff about LEDs
The other important thing to notice in the LED product descriptions is the viewing angle. A wide angle means the light will not travel far, but will instead spread out over a large area. However a narrow viewing angle means light will be more concentrated onto a smaller area, like a street lamp or car head light.

Wire used to wire LEDs
As LEDs have a low power consumption, there is no need to use a large size wire. The 7/0.2 wire that we sell is rated at up to 1.4 amps and will be more than adequate to power up to 40 or 50 LEDs all lit at the same time on most model railway layouts. If your layout is really large and you have long wire runs you may need to use the next size up wire as you may need to factor in voltage drop. Now if like me you split the LEDs up into different areas, then you can have hundreds of LEDs lit of a small power supply and small wires.

To work out the power consumption of all of your LEDs you need to multiply the figure given as the IF max in the specifications by the number of LEDs in the run. Then divide that number by 1000 to give amps. For example, if one LED uses 30mAh and you are using 40 then multiply 30 by 40 to get 1200mAh. To get the amperes, divide the figure by 1,000, which will give you 1.2 amperes.
Should I use my DCC power bus to power LEDs
At Railwayscenics, we always recommend that the DCC power bus is used just to power and drive the locomotives. Lighting is really a separate entity, and should therefore be kept separate. All you need is to run another power bus under the layout using its own power supply to power all your building and lighting effects. Should one then go wrong, the other will always work.

It may be necessary to run more than one circuit to enable different areas of the layout lighting to be switched on and off at different times to other areas.