An LED is a semiconductor diode that emits light when current flows through it. We can use it as an indicator, for lighting, or as an emitter in optical communications.
Don’t be fooled by its simplicity. An LED is, first and foremost, a diode. And as such, if we don’t respect its rules of the game (current and voltage), we will burn it out in a fraction of a second.
In this post, we will learn how to properly size its circuit and distinguish between the different types that exist.
Basic Operation
An LED is a semiconductor diode capable of emitting light when a current passes through it in the forward direction. The color of the light does not depend on the plastic casing, but on the internal semiconductor material (gallium arsenide, gallium nitride, etc.) and the energy released by the electrons when crossing the barrier.
Like the 1N4007 diode, the LED has polarity:
- Anode (+): the long lead.
- Cathode (-): the short lead (and generally coincides with a flat side on the package).
Unlike an incandescent bulb, if you connect an LED backwards, it won’t light up. And if you exceed the reverse voltage (typically more than 5 V), you will destroy it.
Current Control
The current through an LED increases significantly with a small change in its forward voltage. This is why we should not simply connect it to a voltage source.
An LED is not powered by voltage; it is powered by current. It is a non-linear component.
- If you give it less voltage than it needs, it doesn’t conduct (off).
- If you give it just a bit more voltage than it needs, the current skyrockets exponentially and the LED burns out.
Therefore, we always need a current-limiting element in series: a resistor.
For a simple indicator, we use a series resistor. For lighting or higher power applications, a constant current driver is usually preferable.
Resistor Calculation
We need three pieces of data:
: supply voltage. : forward voltage of the LED at the operating current. : desired current.
The resistor will be:
If we want to connect a blue LED with
We choose the next higher standard value, 220 Ω. We must also check the maximum current and voltage drop of the pin driving the LED.
The resistor will dissipate approximately:
Forward Voltage and Color
The forward voltage depends on the semiconductor material, the current, and the temperature. Red LEDs typically have a lower voltage than blue or white ones, but color only serves as an initial estimate.
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Types of LEDs
Leaded LEDs
The 3 mm and 5 mm models are mainly used as indicators. Many handle maximum currents in the tens of milliamps range, though they often provide sufficient brightness with less.
RGB LEDs
These are essentially three LEDs (red, green, blue) packaged together. They have 4 leads.
- Common Cathode: the 3 colored leads go to positive pins and share the ground (GND). This is the easiest to use.
- Common Anode: they share the positive supply (Vcc) and we activate the colors by connecting the leads to GND (inverse logic).
SMD LEDs
These are mounted directly onto the surface of the PCB. The package code usually describes its dimensions, but it does not determine the power, color, or pinout on its own.
High Power LEDs
This is a different ball game. These LEDs draw 350 mA, 700 mA, or more.
Two things are important:
- They require a heat sink (aluminum) without exception, or they will burn out in seconds.
- No current-limiting resistor is used (it would get too hot). Constant current drivers are used instead.
