In this instructable we're talking about LED - Light Emitting Diodes and how we can connect them if we have multiple units. This is one lesson I wish I knew from the start because back when I started tinkering with electronic circuits I built a few projects with LEDs which let’s say could have been made better if I knew the stuff I’m gonna present in this instructable.
Step 1: Watch the Video!
The video describes the different connection methods with their advantages and disadvantages, things that I have learned while making my own mistakes. It contains schematic examples and everything is explained with simple words so everyone can understand. The are subtle things which are present in the video that we're left out from this instructable for the sake of simplicity.
Step 2: Order the Parts Needed
If you plan on building projects with LEDs, you might need some LEDs :-) You might already have some but if you need some more you can get a pretty good deal by ordering assortment kits from ebay or aliexpress. These assortments kits may come in different size and shapes, you may find them with SMD LEDs or Through Hole LEDs but one thing is for sure, they will contain different colored LEDs, sometimes even in different sizes.
I have an assortment of SMD LED in two sizes 0603, 0805 and five colors: red, green, blue, yellow, white because are the sizes and colors I commonly use in my projects. I have another assortment kit for through holes LEDs in 3mm and 5mm sizes also in five colors: red, green, blue, yellow, white.
Here are some links to where you can find these assortments kits:
- Amazon: SMD LED Kit, Through Hole LED Kit.
- Aliexpress: SMD LED Kit, Through Hole LED Kit.
- Ebay: SMD LED Kit, Through Hole LED Kit.
While you are ordering the LEDs you might also be interested in a couple of tools like a soldering iron or a screwdriver set. I highly recommended the TS100 soldering iron and the Xiaomi Wiha Screwdriver Kit. These two items have excellent price/quality ratio :
Step 3: Forward Voltage and Forward Current
Let’s start by talking about forward voltage and forward current, these are two basic parameters that you need to know. So an LED is a diode that emits light under certain conditions. It will have a forward voltage which is the level at which the diode starts conducting. Let’s take for example a green LED which typically has a forward voltage of about 1.9V. The LED will start conducting when the voltage across its anode and cathode is at least 1.9V. So if you connect a 1.5V voltage source to this LED it will not conduct, it will not turn on. If you apply for example 3V it will start conducting, the LED will turn on.
Once it starts conducting the LED will let the current pass but if there is nothing to limit the current, it will continue increasing, above the rated forward current, the junction inside the LED will basically melt, damaging the LED. So that’s why we have a parameter called forward current. The manufacturer of the LED will characterize the part and they will mention this forward current at which it is safe to run the LED without destroying it. A typical value for LED forward current might be 20mA. Sometimes if you want to be extra cautious you can design the circuit with a safe margin It’s good practice to leave a 25% margin and go below the full rated current if the application allows it. Especially for a small indicator LED you will not need to go full rated current.
Step 4: Limitting the Current With a Resistor
So in order to limit the current, and avoid going over the specified forward current we can add a resistor in our circuit. To calculate the resistor we need to use the two parameters mentioned before: forward voltage and forward current as well as the supply voltage for our circuit.
We know the LED will drop 1.9V while the resistor will have to drop the remaining 1.1V up to our supply voltage. Knowing the forward current of the LED is 20mA, we can calculate the value of the resistor using ohms law. R = U / I and in our case U is 1.1V divided by 20mA, resulting 55 ohms.
Step 5: Connecting LEDs in Parallel
As you can see it’s pretty simple to limit the current and drive a single LED, but what happens if we need more LEDs?
We could connect them in parallel and use the same resistor to limit the current but we run into some problems. First due to variations in the manufacturing process, each LED will have a slightly different forward voltage, this means it will draw a slightly different current which could results in uneven illumination or even failure of the LED if it’s driven over the specified current.
In modern applications like for example display backlight it is very important to have even illumination across all LEDs. So it's better to avoid connecting LEDs in parallel.
Step 6: Connecting LEDs in Series
The best way to connect multiple LEDs to have the same current driving each LED would be in series. You would connect the 5 LEDs that we had earlier in series and drive them with a constant voltage source, same as before limiting the current with a resistor. And you would think problem solved but the problem is only partially solved because of another property of diodes that also applies to LEDs as well.
The forward voltage will decrease with an increase in temperature. If the forward voltage drops, the forward current will increase, heating the LED even more. And there is another property that we need to be aware off the exponential curve between forward voltage and forward current. This tells us that with a small change in forward voltage we get a big change in forward current and this could potentially lead to LED failure due to thermal runaway.
So the way we solve this problem is by using a constant current source. These days there are many integrated circuits especially designed for driving LEDs with a constant current because the advantages are clear. One might argue that if you have LEDs in series, if one of them fails, the whole string will be inactive and that is true from an electrical point but in practice, LEDs have become so reliable that if the circuit is properly designed there is very little chance of an LED failing over the quoted lifespan
So, that was all, I hope this instructable was useful if so please leave a comment or hit the like or dislike button to send me some feedback.