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LED’s or to give them their proper name, Light Emitting Diodes, are the solid state era equivalent of the humble light bulb. Both emit light but this is where the similarities end.

LED’s are available in similar sizes to the small light bulbs traditionally used in railway modelling and much, much smaller, many of you would be surprised just how small an LED can be and still give off ample amounts of light for our needs. An LED correctly driven is several magnitudes more reliable than any light bulb.

When LED’s first appeared on the electronics scene they were only available in red and green and later yellow. For many years the holy grail was a blue LED that gave off a reasonable amount of light with a purity similar to its red, green and yellow counterparts. This was eventually achieved and the next logical step was a white LED, these have only been around for a few years and are still more difficult to obtain than any of the standard colours. For our purposes red, green and yellow immediately find a use in colour light signals. To be useful in a locomotive we need white.

One of the questions I am asked the most is “What voltage LED’s do I need”.

Let’s clear this one up right now before we go any further. The above statement is totally meaningless, an LED is not a voltage dependant component. Obviously it requires some voltage to work, more about this in a moment but what is important is the amount of current allowed to flow through the LED, it matters not a jot whether we are talking about  a supply voltage of 10 volts or 1000 volts as long as the current flow is correct. This is the reason why we put a resistor in series with LED’s.

There is an exception to the above (isn’t there always). LED’s are available that state the voltage that they are to be used on e.g. 12v red LED. These LED’s have a built in resistor calculated to be correct for the specified voltage. You can use them on lower voltages than that specified, they just won’t be as bright as they should be. If they are used on a higher voltage than that specified don’t be too surprised if the expire very quickly.

OK let’s get down to some serious stuff. This is the electronic symbol for an LED.

 

The “A” means Anode and the “K” means Cathode – Anode is positive or + , Cathode is negative or -

The following picture shows a clear body LED. This is by no means a definitive method of identifying the connections on a LED but as a rule of thumb the longer lead is the Anode and the shorter one the Cathode, there is also often a flat on the body next to the Cathode lead. If you look at the inset picture the Cathode, on the right is the larger of the two junctions within the LED.

A more definitive method is to use a multimeter set to diode test as indicated in the picture below. Most of the cheap meters around will not give a reading on the display no matter which way round you attach the leads to a LED but when you have the red lead to Anode and the black lead to Cathode the LED will usually light up as long as the meter’s internal battery is reasonably good. Normal diodes (not light emitting) will give a reading one way round and nothing the other way round if they are good.

OK so earlier we said that LED’s are not voltage dependant devices. This is almost true depending on how you look at it. They do have a minimum voltage at which the will strike up and emit light, for coloured LED’s this is approximately 2 volts, white and blue LED’s need slightly more, usually around 4 volts. It is important to know what this minimum voltage is in order to properly calculate the value for the resistor you will need to place in series with the LED for a given voltage. This is why I stated earlier that it doesn’t matter if the supply voltage is 10 volts or 1000 volts, as long as the series resistor value has been calculated properly the LED will still work. Normal light bulbs do not work like this, a 12 volt bulb on a 24 volt supply will be extremely bright for a short while before it expires. 

The table below shows some fairly typical values for some common LED’s. If your source of purchase supplies this information then it is fairly useful to have it otherwise its going to be a best guess based on typical values.

While we are on the subject of knowing the parameters of your chosen LED its worth mentioning a practice that is thankfully less common now than it used to be was to sell on manufacturers “sweep ups” to the hobby market. These are what they say – sweep ups from the floor and devices that may have failed quality control but still work. Think about it for a second, a 3mm red LED is a 3mm red LED – how do you know if its low current, super bright, bright etc. Devices that fail quality control are even worse, you may have two LED’s from the same batch that you know are standard 3mm LED’s but the intensity of the light might be different. OK a bit extreme maybe but it looks crap on a locomotive when the marker lights are a different brightness. Moral of the story – If you want things to work right, buy from a reputable source.

 Back to our table above. Here is an explanation of what the various columns mean.

Incidentally, the body of a LED does not have to be red for it to emit red light or green or any colour for that matter. It can be a totally clear package.

The columns we are really interested in are. Colour, I_F max and V_F typ.

OK everyone still with me, not gone to sleep yet. Take a look at the following diagram.

Hopefully not too scary, it’s just a LED drawn connected to a supply rail at the bottom and via a resistor to another supply rail at the top. There are no values and as such the diagram just represents a circuit, we don’t know the supply voltage, the LED could be any LED and because we do not know these parameters we do not know the value of the resistor.

So lets fill in some detail and make the whole thing a bit more meaningful.

Same diagram but now we have some parameters to play with. Gnd - is ground or 0 Volts. VS - is our supply voltage. I - is current and the arrow represents the direction it is flowing. VL - is our LED's typical forward voltage VF from the table. VS - VL  is part of the calculation we are about to make to determine the                  value of R

Scary part coming up – specially for all those of you terrified by formulae.
Lets set some values first.
Our chosen LED - Standard red V_F = 1.7   I_F max 30mA (From the table above)
Our supply voltage V_S = 12 Volts (Chosen by me because its a popular supply voltage)
The value we want to know is R

So our formula is        R = (V_S - V_L)/ I 

or spoken in english:-

Our resistor value = (Our supply voltage minus Our LED's typ forward voltage) divided by its max forward current

Or            R = (12 - 1.7) / 0.030

 Now some of you will have noticed that 30mA is suddenly written as 0.030

The reason for this is that when we work with formulae like this everything defaults to its base units, so Volts are Volts (V), Current is Amps (I) and resistance is ohms (R) or the Greek symbol omega Ω.

30mA or 30 milliamps is a fraction of an Amp. 1000mA = 1A so 30mA = 0.030A

A similar thing is going to happen when we get our answer to R.

Depending on what our supply voltage is, our answer could be 1000s of Rs but we don’t call a 1000 ohm resistor a 1000 ohm resistor, we call it a 1 K resistor – K means Kilohms or 1000s of ohms like in grams and kilograms.

Back to our formula    R = (12 - 1.7) / 0.030

Work out the bit in brackets first - 12 minus 1.7 is (Hands up)  10.3

So    R = 10.3 divided by 0.030 which is     343.33 ohms.

Resistors don't come in neat precise values like this so the nearest value that is higher would be our chosen resistor which would be 390R or 390 ohms. 

Before we finish lets do the same calculation again but this time we will change the supply voltage to 16 Volts which is approx what the outputs of a DCC decoder supply if the output is not dimmed.

R = (16 - 1.7) / 0.030        or         R = 14.3 / 0.030      which is      476.66

Our nearest resistor value would be 480R or 480 ohms.

Thats all there is to it, your LED's will now be correctly driven.