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SMD LED polarity
through hole leds
I hope here to describe and explain a little about the LEDs that are sold on our website. There are many types, sizes and colours of LEDs and it can be confusing so hopefully this will not confuse further. LED technology is continually advancing so the products need to do so also.
What is an LED
A light-emitting diode (LED) is a special kind of diode that glows when electricity passes through it. LEDs can be bought in a range of colours. They can also be bought in forms that will switch between two colours, three colours or emit infra-red light.

LEDs are quite different to light bulbs. Light bulbs are designed to work on a voltage, and can work on almost any sensible DC voltage if you have the correct value resistor fitted. LEDs are polarity sensitive and will only work when correctly wired. The cathode is normally indicated by a flat side on the casing and the anode is normally indicated by a slightly longer leg. On SMD LEDs there are pained marks to designate the cathode and anode. Wiring with the wrong polarity will not harm the LED, as long as the maximum voltage has not been exceeded. LEDs also give off no heat when lit and use minimal power. Used properly LEDs have life expectancies in the tens of thousands of hours. They will far outlast an incandescent bulb and usually the life of the modeller. This makes them far more efficient than older style incandescent light bulbs.
LED colours
LEDs come in many colours. The colours are created by changing the semiconducting element inside the actual LED. The two main types of LEDs presently used for lighting systems are aluminium gallium indium phosphide alloys for red, orange and yellow LEDs, and indium gallium nitride alloys for green, blue and white LEDs. Slight changes in the composition of these alloys changes the colour of the emitted light.

We currently sell a range of LEDs which are available in blue, green, orange, red, yellow, cool white, warm white and natural white. To confuse things even further each of the colours are also available in different outer packages. These outer packages can be diffused or coloured, milky white or water clear. There are also LEDs with different brightness's. The brightness is usually measured using a luminosity intensity rating which on our site is the figure after Lum. int . The higher the figure the brighter the LED.
The LEDs that we sell
Before we dispatch any LEDs, we test them to ensure that they operate and that the colour is what has been ordered. The only way to return a faulty LED is when its been working and in operation for a while and it fails.

We sell two main types of LEDs. These are SMD LEDs which generally are soldered to circuit boards, and are much smaller than other types, and through hole LEDs which are what most people associate with LEDs. These are generally called through hole LEDs as they are mounted through holes. Each of these types is available in a range of different voltages, colours and styles. Within these types they can again be split into those that DO require a resistor, and those that DO NOT. All of our LED listings will tell you whether a resistor is required, and what value resistor will give the best light. If you already have resistors, please make sure that they are of at least the recommended value or higher.
Through hole LED types and sizes
Through hole LED sizes
Through hole LEDs are available in a range of sizes. Our website currently lists 1.8mm, 2mm, 2.4mm, 3mm and 5mm LEDs. These LEDS are also available in the following types.
Standard LEDs
These LEDs are available in a range of colours and have a constant light output regardless of the type of outer package they come in.
Flashing LEDs
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 standard LEDs but instead of having a circuit that regularly flashes the light, they use a different circuit to produce a candle type flickering effect.
Bi Colour LEDs
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 LEDs
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 LEDs come with red, green, and blue emitters all in the same housing, 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. There are also RGB colour chaning LEDs that have just two connections and automatically cycle through the range of colours.
Surface Mount Device LEDs are very small and also come in different packages, sizes and types like the through hole LEDs described above. As they are so small, they have lots of uses within modelling hobbies for things like model vehicle lighting where space is limited. They are also available in a full range of colours, types and sizes just like the through hole versions.
SMD LED types
Unlike through hole LEDs which are sold by their size, SMD LEDs have a number allocated which describes the size. We sell 0603, 0805, 1206, and 3528 types. There are others, but we shall not complicate things too much and will only deal with the ones that we sell. These different types are also available in the same range of colours and types as the through hole LEDs. Our product descriptions give all the information we think is required to enable you to select the right product for the job. Being so small means that it is very difficult to solder small wires to this type of LEDs. You need good eye sight and a steady hand.
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 rear. 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 can be bought to work on 3v, 5v, 12v, 24v and 220v. Strips can usually be cut into shorter lengths as long as they include a complete circuit, usually in threes. We sell some strips that can be separated into individual LEDs.
Unlike light bulbs LEDs will only work when connected properly and will burn out if the maximum voltage is exceeded. If our product listing says you need a resistor, that is the minimum size required for a particular voltage. To keep things simple, we recommend using one resistor per LED, but it is possible to use one resistor for several LEDs.
Power Supplies for use with LEDs
It is possible to use many things to power LEDS. From batteries up to desktop PSUs. I generally use a plug-in power supply of 1 or 2 amps to power mine on a layout or display. Being plug-in devices they are safe to use as long as you do not overload a socket or extension lead.
How many LEDs can I wire together
We spend a long time writing our product descriptions so they contain all the information that is required for you to use choose the correct product for your specific application. The LED pages are no exception. Each LED listing shows the specification of the item on the page. This shows things like voltage required to light the LED, brightness, power usage and the viewing angle.
IFmax: 30mA. - This is the current required to light the LED
VFtyp: 11.5. - Shows typical voltage required
VFmax: 14V. - Maximum voltage that should be applied to the LED.
Lum. int (mcd)@ IF; (9v): 20. - Brightness of the LED
View angle: 40deg. - Best angle to view the LED light from
Wave-length: 625. - This is basically the colour

From that specification above we can see that the IFmax is 30mA or 0.030 amps, so if you had a 1amp supply you could use 33 of the same specification LEDs. Without going over the maximum rating of the power supply.
Wiring LEDs which DO NOT require a resistor
The LEDs we list with a voltage mentioned already contain a built-in resistor. These LEDs are generally rated up to a maximum of 14 volts, and this should not be exceeded or damage may result. They will work down to a voltage as low as 9 volts. If you have a supply that will exceed the voltage then it is possible to add in a series resister just to lower the voltage.
Wiring LEDs that DO require a resistor
Resistors are relatively inexpensive and basically restrict the current passing through the LED, as the LEDs 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.

Each LED should have its correct resistor wired to the either terminal, but it is preferred to use the positive connection. Only use one resistor per LED is required. All of our LEDs that require resistors have various resistor values listed and the voltages for that resistor. As long as you use a resistor with a value higher than that listed for your voltage, the LED will be fine. It is possible to slightly increase or decrease the brightness of an LED by changing its resistor.

It is not necessary to supply a separate feed to each resistor and LED, but they are best wired in parallel, rather than in series.
How can LEDs be used in Railway Modelling
I see no reason why LEDs of some sort cannot be used to light buildings and rolling stock interiors and exteriors. They can be used in street and platform lighting. They can be used in locomotives as head and tail lights. They can be used in small vehicle as front and rear lighting. The special effect ones can be used to simulate flickering fires in steam locomotives or even house fireplaces bonfires and barbeques. The bright white ones can be used to simulate a welding effect without the need for circuits to create the flashing or flickering effect. The diffused ones can be used in control panels to show points settings, or whether a circuit is switched on. The strips can be used to illuminate whole layouts rather that spot lights or fluorescent tubes. I am sure there are more uses such as powering fibre optics to get really small beads of light, but that I will leave to your imagination.
Hopefully after reading all that you know a little more about LEDs. They are better to use that the old style incandescent light bulbs in many cases as they use less power, give off no heat and will last for years. They are not that expensive to buy, and prices are becoming lower. The wiring for an LED is no more complicated that for a light bulb, so there is no real reason not to use them.

Good luck and try to use LEDs where you can. If nothing else you may help save the planets energy consumption just a tiny bit.