Connecting LED Lighting

Hey there, Arcee here, this week we will discuss connecting LED lighting. With the darker days approaching, it is not only for show, but it also gives your model an even more realistic look.

LED lighting has become an inseparable part of our lives; we see LED lamps everywhere. However, unlike the lamps in our homes, there are several factors to consider when connecting standalone LEDs.

Fortunately, this is all much easier if you remember a few basic rules, which I will, of course, share with you here. Are you ready to dive into the wonderful world of LED lighting?

Different Types of LEDs

LEDs come in many shapes and sizes, possibly the most well-known are the 3mm and 5mm LEDs. They are used in many models. But did you know there are thousands of other sizes too? Such as SMD LEDs which are super small and even large 10mm Jumbo LEDs. You can also find LEDs in all the colors of the rainbow, whether you're looking for plain bright white or a more unique purple/pink color. Luckily, they all work the same way, so don't be thrown off by their shapes and sizes.

Basic Information about LEDs

Most LEDs have technical specifications such as nominal voltage and maximum current listed. This might also be the most confusing part if you're new to this world. Many LEDs have an operating voltage of 2-3.5V and a maximum current of 20mA. Technically, LEDs are current-driven, not voltage-driven. It's crucial that the max current value is never exceeded. The voltage of the LED can be easily adjusted by using a resistor. Below, you will find a table with the most common voltages and currents of standard LEDs; you can usually find the actual specifications from the manufacturer or supplier, which may vary slightly. As mentioned above, the shape/type of the LED doesn't matter much for the data. It's mainly the color that determines the specifications.

LED Color Voltage Current
White / Warm White 3.2 V 20 mA
Blue / Purple / Pink 3.2 V 20 mA
Red / Yellow / Green 2.0 V 20 mA

Connecting the LED

If you connect an LED directly to, for example, a 7.2V battery, it will light up very briefly and then break immediately. We obviously don’t want that, so it’s important to use a resistor. Resistors come in hundreds of variants, but fortunately, with a few standard resistors, you can connect almost everything.

Firstly, it’s important to know that LEDs are polarity sensitive. This means that you cannot simply reverse the positive and negative like you can with an incandescent lamp. You can recognize the positive side of the LED by the longer leg. A simple mnemonic to remember is that the positive side is always longer than the negative side. If you accidentally connect the LED the wrong way round, it won’t break, but no light will come out. Placing the resistor is thankfully straightforward – it doesn't matter if it’s in the positive or negative wire of the LED. However, we advise choosing the negative wire. This ensures that you can work with different light intensities using two resistors in the future, and almost all sound and light modules work with a negatively switched contact.

Now we can look at which resistor we need. The official formula might look a bit intimidating, but we can simplify it a lot.

(Battery Voltage - LED Voltage) / LED Current in A = Resistor

Let’s take an example of connecting one white LED to a 7.2V battery.

(7.2 – 3.3 = 3.9 V) / 0.02 A = 195 Ohm

However, if we always rely on the official calculation, we need many different resistors because 195 ohms is not exactly a standard value. Now, this is fortunately the minimum resistance that we must use. We can always use a heavier resistor, such as the standard 270 Ohm.

The Secret of Easily Connecting LEDs

Actually, we first showed you the official method so you understand what really happens. But if you prefer things straightforward, that's even better! LEDs have a property we can exploit to make the connection even simpler. They are super bright! If we use a higher resistance than the calculated value, we lose a fraction of this brightness, but the decrease in light output is anything but linear. If we reduce the current through the LED by 20%, the decrease in light is often only 1-5%, which is almost not noticeable with the naked eye, especially if we connect all LEDs in our model this way.

If you like it simple, then use the following table for connecting the LEDs.

Voltage Resistance LED Color
5 V 150 Ohm All
7.2 V 270 Ohm All
12 V 560 Ohm All
24 V 1 K Ohm All

Even easier is to make use of pre-wired LEDs

But what if I want to use less wiring in my model?

Initially, we covered connecting a single LED, but if you’re more experienced or would prefer not to have the excess wiring in your model, you could also connect LEDs in series or parallel. When doing this, we unfortunately cannot avoid calculating the respective resistances. It's also important to realize that if you wind up with an odd number of LEDs, achieving the same light strength can be quite a challenge. Therefore, try to always have even groups, especially if they are placed next to each other because even if the difference in light strength is not significant, it becomes much more noticeable when they are side by side.

Connecting LEDs in Series

This method is most commonly used because you can run the wiring from one LED to another. In this instance, we must add up the LED voltages, and the total voltage can never exceed the battery voltage of the model. With a battery voltage of 7.2V, you can have a maximum of 3 red/yellow/green LEDs in series (6V) or 2 white/blue/purple (6V). It is generally not recommended to put multiple colors in series, especially if the LEDs themselves have different voltages.

Let's make a sample calculation:

(Battery Voltage - LED Voltage) / LED Current in A = Resistor

7.2 V – 6V = 1.2V / 0.02 = 60 Ohm

Now we come out very low with the resistance; we can always use a higher value resistor. The loss of brightness here is minimal. In the above example, a 150 Ohm resistor would be more than adequate. If you want to connect multiple LEDs in series, then you can always create multiple groups of these LEDs by connecting them in parallel to the power supply.

This way you create groups of LEDs, and you can quickly connect large numbers for, for example, the contour lighting of your truck. If you decide to connect an odd number of LEDs, e.g., if you want to mount 8 LEDs on the side of your trailer, then it becomes a matter of trial and error to ensure all LEDs shine as brightly.

Theoretically, we can calculate this, but the time it takes does not weigh up to simply trying various resistors until you achieve the desired brightness.

Connecting LEDs in Parallel

Connecting LEDs in parallel is done less often than in series, mainly due to certain limitations: the most limiting factor being that the resistor can quickly become too warm. When connecting LEDs in parallel, we must add up the currents instead of the voltage. It sounds like we can connect an unlimited number of LEDs, and theoretically, that's true. However, we should not forget that the resistor also needs to handle this current. Let's take 5 LEDs in parallel as an example.

(Battery Voltage - LED Voltage) / LED Current in A = Resistor
7.2 V – 2V = 5.2V / 0.10 = 52 Ohm

A 150 ohm resistor would be fine, right? However, most resistors can only handle 0.125W or 0.25W power dissipation. In the example above, we will quickly notice that the resistor cannot handle the desired power. The calculation is quite simple:

P = U * I
5.2V * 0.1 A = 0.52 W.

That's twice as high as the standard 0.25W resistor can handle, which will get super warm and will quickly break. What happens if we now choose a 150 Ohm resistor because then the current would decrease? Indeed, let's see because the formula now becomes a bit more complex.

(Battery Voltage - LED Voltage) / Resistance = Current

7.2 – 2 = 5V / 150 = 0.033A

You can see now that a 150 Ohm resistor has limited the current from 0.1A to 0.033A, which is a 66% reduction. This will lead to a 20-30% loss of light output. The question is whether this percentage of light loss plays a significant role given how bright the current generation of LEDs is.

5.2V x 0.033 a = 0.17 W.

As you can see, connecting LED lighting in parallel is a lot more complex than connecting it per LED or in series. This is the same reason why most people don't use it much.

And there was light in the darkness!

Great to see you've read through these Tips & Tricks again, you now know exactly what's involved in connecting LED lighting in your model! If you like things simple, pre-wired LEDs are a handy tool; if you want to connect your model perfectly, then connecting in series is certainly an option that can save a lot of wiring, benefiting the tidiness in your model.

See you at the next Tips & Tricks?

Best regards, Arcee!