Wiring up low-voltage lighting

Let's start with some basics. Firstly the more common types of lighting used for models:-




The bulbs on the left are available in several colours and for 3 volt, 6 volt or 12 volt power supplies - more on that later. We will see that wiring up bulbs like these is the simplest approach to lighting and has very little room for error. The downside is that they require quite a lot of power and get warm. The major UK model train suppliers also have miniature light bulbs available, but these are bigger than the Grain of Wheat and take even more power.

Whilst not shown in the picture, Grain of Rice bulbs are also available. They are the same as the Wheat ones, only smaller so more suited to 2mm modelling rather than 4mm.

Next right in the photo are a couple of coloured LEDs. These are available in almost all of the colours of the rainbow these days and are extremely flexible in how they can be used. They require little power for sometimes very bright light, but more care needs to be taken in wiring them up - more of that later as well.

Finally, my photo shows some ready assembled LED lighting units. These often come with clear wiring instructions and in the case of those shown, an appropriate resistor needed when wiring to a 12 volt power supply. Even though they come with instructions, there are some extra things worth considering when wiring them up, so we will discuss those when we come to using them.

Next I will comment on suitable power supplies.

Power supplies.

I have seen and read descriptions of numerous ways of providing power for lighting. My recommendation would always be for a simple and very safe solution.

The simplest for just a few, occasionally used building lights made from LED sources could easily be powered from a pair of small batteries.

However, for larger numbers of lights, my preference is to work with 12 volts DC. A 12 watt power supply would easily satisfy the requirements of 100 LED lights or about 15 grain of wheat bulbs. This type of supply is available as a plug top unit:-

http://www.rapidonline.com/Electrical-Power/Power-Supplies/Plug-In-PSUs/5W-ndash-12W-Mini-plugtop-switch-mode-power-supply/80857

They are very cheap for what they are (even compared to the cost of batteries), and essentially, VERY safe, both from a mains point of view and also from a wiring point of view as they are virtually indestructable by any external fault - such as a wiring short circuit.

I will move on to some initial wiring pictures next and then suggest a simple starter project with a small building.

So to the fundamentals of an electrical circuit. We learn from our physics that electrons can be made to move around a conductor to form an electrical current provided they are given an appropriate stimulus - such as that produced by the chemicals in a battery. So our simplest lighting circuit is this:-



The LED illuminates because of the electrons passing through a special semiconducting junction made from silicon inside the casing of the LED. The two batteries provide a voltage of about 3 volts, but without anything other than the connecting wires shown, the amount of power taken from them is entirely dependent on the characteristics of the LED - in fact some modern high luminance devices would just blow up when connected like this. 

(By the way these red and black connecting wires are very useful for experimenting. They can be bought in packs of several colours and have a clip at each end for easy attachment to electrical components)

To see what is going on, we can attach a multimeter into the circuit, set for measuring current:-




It reads 0.1 Amps (or 100mA). That is rather a lot for an LED and if left for long, would probably result in destruction of the LED through overheating. To control the power we need to add resistance in the circuit. We can do this easily by introducing a resistor. These are available in lots and lots of different values, however when working with LEDs we probably only need to consider just a few different values.

I have included a 22 ohm resistor in the circuit between the positive red wire and the LED:-




The resistor is that white (out of focus) block at the end of the red lead. You can see that there is little change in light output from the LED, but now when we look at the meter:-




we can see that the current has dropped to just 0.03 Amps - a third of what it was before. This is much better for the life of the LED and takes less power from our battery. So we learn that for LED based lighting we need a suitable power supply, a suitable resistor and the LED itself. We will experiment with even lower amounts of power later.

One final point about LEDs:-




yes - the LED is off, but the wires are still connected. Can you see what has changed? I have reversed the connections to the battery. The LED only emits light when current flows in one particular direction, unlike a bulb which will work with current flowing in either direction. This is an important thing to remember and I will write more about it later when I show a circuit using a 12 volt power supply instead of the battery.

Next step is to look at how we might use a mains powered supply.

Maybe time to join in, if you want to prove you can wire up model lighting?

Firstly take a look at your LED:-




With a bit of luck it has one lead longer than the other. The longer lead needs to be connected to the positive side of the supplied power for the LED to illuminate.

When experimenting like this, it can be useful to use a length of connector block (we used to call them chocolate blocks!)




I have attached the LED to two of the terminals - making sure that I knew which was the longer lead. I have connected a 1 K ohm resistor from the longer lead to a third terminal. I have then connected a rectifier diode to this terminal. I thoroughly recommend the use of such a diode in all LED lighting circuits. We will see later, that we only need one of them for a whole string of LED lights.

Now to connect some power. As Jeff raised the question of using old no-longer used supplies, I have found a Nokia Phone Charger which says it is 10V DC:_




I have connected the red lead to the diode and the black lead to the short leg of the LED. Conveniently, the wires inside the Nokia cable are coloured red and black, but they might not have been, and if we don't have a meter available, we would not know which wire was positive. Here is where the diode comes in. If we had connected the supply the wrong way around:-




all that happens is the LED does not illuminate. Nothing gets blown up, and there is no smoke!!

This will also work if the power supply is AC. In that case it wouldn't matter which way round the connections were made to the supply, as the diode would ensure that only the correct polarity was fed to the LED.

The resistor value is quite arbritrary. If you want a brighter light, lower the value of the resistor. Personally I find that working off 12 volts, the use of 1K ohm is perfectly satisfactory. This limits the current to about 10 milliamps, which would mean that 100 LEDs could be powered by a single 1 Amp 12 volt supply (often referred to as a 12 watt supply).

Just a warning about  using supplies like this phone charger. They will probably not have the same level of output protection that one of the supplies I showed earlier has. Which means that shorting out the connections could spell end of supply, and maybe even some smoke! Also, only use them for applications like simple LED lighting. They are very unlikely to be suitable for any kind of electronic circuitry, and I certainly wouldn't use one to power the track.

I would recommend that beginners purchase a purpose designed supply.

Lighting a small building coming next.

Here is a suitable building to make a start with:-




They are made with a convenient hole underneath, but I have noticed that bright light shines through the casting as it is very slightly translucent!

Anyway, to prepare a LED:-




I have chosen a white LED for this application as by the time I have added card inserts for rooms and floors, and perhaps added curtains and even some furniture, much of the light will have dispersed. I cut short the long lead, soldered a 1k Ohm resistor to it, and then added my diode. I have used a 1N4148 diode. These are very useful for single LEDs or a half dozen or so. They take 150mA and will protect against a reverse voltage as high as 75V. And they can be bought for as little as a penny each.

I have used red and green wires to help with the correct connection of power, but as we have already seen, with the diode in the circuit, no harm will be done if the power connection is mistakenly reversed.

To stop the bare wires touching each other, I have then added a length of heatshrink sleeving:-




So then all that remains is to fit the light inside the building and apply power:-




I usually aim to have the LED up high in the building hidden by pieces of card so that there is no direct visibility of the LED through the windows.

Here is a second project:-




This time I am using a yellow LED which has a built in resistor appropriate for operating from 12 volts. When buying these types of LEDs they are usually listed as 12v LEDs. So all I need is the diode, again a 1N4148. This time I am using very small wires left over from a DCC decoder.

The installation is for a Scalescenes garage:-




I have fitted the LED into the rafters and run the wires down them, along the inside of the roof, and down a rear wall where they will not be seen from normal viewing angles.




Here is the effect I am after looking up through the main front doors. And if we turn the sun out a minute (by closing the curtains!):-




I am only using the old phone charger 10v supply, but even so, I find that these LEDs with prefitted resistors are often quite dim. Perfectly okay for what I wanted here, but wouldn't be bright enough for a multiroom building like the first one.

A couple of pictures of the installation I carried out for tunnel lighting on the underground module http://yourmodelrailway.net/view_topic.php?id=6077&forum_id=92&page=18




As there were several LEDs in a neat row, I used copper tape for each connection. The LED leads come through holes in the top of the tunnel and are soldered to the tape. The longer lead is first soldered to a resistor which is then sleeved in heatshrink before soldering to the tape.

Here is where the power connection is made:-




And this is a view inside:-




So, if you have been paying attention, you will have seen what was missing in this installation ?????

A series diode to protect against reverse voltage?

[user=95]Sol[/user] wrote:

A series diode to protect against reverse voltage?

Yes, Sol correctly got there first - is anyone else with us?

I thought a LED was a Light Emitting Diode - so would already protect itself from reverse polarity ?

I'm confused.  Which is precisely why I am following this topic.  

By the end of it I aim to have unconfused myself with Geoff's noble assistance

[user=465]Stubby47[/user] wrote:

I thought a LED was a Light Emitting Diode - so would already protect itself from reverse polarity ?

Unfortunately not the case! Yes it only produces light when current flows in one direction and not the other, but the nature of the "diode" junction is much reduced compared to the usual capability of a diode. Most LEDs will only withstand a maximum of 5 volts in reverse before the internal junction breaks down. The 1N4148 diode used will handle reverse voltages up to 75v before it breaks down - hence its inclusion protects the LED.

[user=422]Geoff R[/user] wrote:

[user=465]Stubby47[/user] wrote:

I thought a LED was a Light Emitting Diode - so would already protect itself from reverse polarity ?

Unfortunately not the case! Yes it only produces light when current flows in one direction and not the other, but the nature of the "diode" junction is much reduced compared to the usual capability of a diode. Most LEDs will only withstand a maximum of 5 volts in reverse before the internal junction breaks down. The 1N4148 diode used will handle reverse voltages up to 75v before it breaks down - hence its inclusion protects the LED.

Ahhhhh, that explains it then.  Thanks :thumbs

Does it matter which way around the diode is fitted??  i.e. is there a set direction or can the power be feed to either end?

[user=3]Gwent Rail[/user] wrote:

Does it matter which way around the diode is fitted??  i.e. is there a set direction or can the power be feed to either end?

It is essential, Jeff, to fit the diode the correct way around. This photo shows it best:-




The flow of current through the diode is almost always marked by a band at the "exit" of the current (or water in Perry's hydraulic analogy).

So the positive connection is made to the other end as seen by the red wire in the above photo.

If the diode were fitted the other way around, and the power wires connected correctly, no current would flow so the LED would not light up. Trouble is, if one then assumed the power wires were wrong (overlooking the wrongly fitted diode), and reversed the power leads, the diode would then conduct and provide a reverse voltage across the LED - the very thing the diode is meant to protect against. The LED would still not light up, but it might well blow up!!!

Thanks Geoff. Having bought a few 1N4148 diodes yesterday, I've checked them and there is indeed a black band on the very end of each one.
I'll make sure that the band is nearest to the LED when soldering up. thumbs

No more comments in here please. use the comments section for this April project.

A suggestion for providing power to a large number of 12v model lamps (not LEDs).

Rick, and I guess others, is trying to power 12v station lamps. They are good models and can be sourced in large numbers at reasonable prices. However, they do draw much more current than LEDs. Probably around 65mA each. However, a good quality switched mode power supply can be purchased for about the same cost as a 100 off pack of said station lamps. Here is one:-

http://www.rapidonline.com/sku/Electrical-Power/Power-Supplies/Desktop-PSUs/90W-Desktop-switch-mode-PSU/79713/85-2981

It will provide 7Amps, and is fully protected. 100 of those 12 station lamps will require 6.5Amps, so a full pack could be powered from this one supply. Okay it doubles the cost of the overall set up, but perhaps well worth the investment.

I use a similar supply for my 12v around the layout, it is a 5Amp version and I use it for all lighting, semaphore signal control, as well as other electronic items and my Heljan Turntable, As I said, can be well worth the money.