Reading Time: 9 minutes

What Is an LED?

The simple answer is that an LED is a type of light. It’s also a type of diode. The acronym LED stands for Light Emitting Diode.

Just as a diode is a semiconductor, an LED uses semiconductor properties to allow it to illuminate – unlike an incandescent light which uses a filament that heats up to extreme temperatures to illuminate which is very inefficient.

LEDs are small, efficient, and cheap which make them very appealing to use in electronic products compared to other lighting devices of yesteryear.

Having a small footprint means that there’s less space and less material to use in creating products – being efficient means there’s less waste in energy in the form of heat for devices, which is always something highly sought after – and being cheap is, well, what everyone wants for a price of something.

---

What Is a Diode?

A diode is a two-terminal semiconductor device that allows the flow of current in one direction. It carries current when its voltage is positive and no current when its voltage is negative.

A diode is considered a PN junction – meaning that it’s partly-made of a positive-type semiconductor material (P), and is partly-made of a negative-type semiconductor material (N). When oriented in its forward-bias direction, it allows the flow of current through it. When oriented in its reverse-bias direction, it blocks the flow of current.

---

How Can a Diode Produce Light?

An LED, like a diode, is made of two materials that are placed close together. One material is usually a metal, such as aluminum – the second material is usually a combination or compound of materials, such as gallium arsenide (GaAs).

The makeup of these materials allow them to emit and absorb electrons when an electric potential is applied across the LED’s terminals. When properly biased, electricity is allowed to flow through the LED and the process of emitting and absorbing electrons between the two materials produces light.

---

Efficiency of LEDs

Remember earlier that it was mentioned that a diode produces light by having two materials in its makeup that allow it to emit and absorb electrons to produce light? This process makes the LED highly efficient.

Unlike incandescent light bulbs that use resistive tungsten wire that produce extreme heat for lighting, the LED does not use heat to produce light. An LED efficiently uses electricity to create more photons to produce light, therefore reducing a significant amount of energy loss.

---

Details of an LED Using Its Datasheet

Now we’ll take a look at a specific LED to understand a little bit about it and what its capabilities are. For the following, I chose a 5mm (millimeter) orange diffused LED manufactured by QT Brightek (QTB). We’ll obtain all the information that we’ll cover over this LED through its datasheet, which you can download for yourself below.

QBL8XX60D_Orange-LED Datasheet

Download Datasheet – QBL8XX60D_Orange-LED

LED Current

Above is a table from the datasheet for our orange LED. The highlighted section above is for the forward current (I_F), which tells us how much current the orange LED is able to handle continuously. We can see that for this particular LED that we can give it 20mA or less.

We can see from the second table for the LED’s maximum ratings that its peak forward current (I_FP) is 100 milliamps (mA) at a one-tenth (1/10) duty-cycle or a pulse width for 0.1 milliseconds (ms) – meaning that this LED can handle short bursts of current up to 100mA, but not for very long.

LED Voltage

Looking at the table from the datasheet above, we see the highlighted section for the forward voltage (V_F) of the orange LED. We see that the typical forward voltage for the LED is 2 volts and its maximum forward voltage is 2.6 volts. Knowing these numbers helps us to decide how much voltage our circuit will need to supply enough voltage to the LED.

Circuit Example Problem Using Datasheet Values

Let’s say that we are using the orange LED for this simple switch circuit that contains a 9V battery for the power supply, a switch, a resistor, and the LED. We know from the datasheet that our orange LED needs at least 2 volts supplied to it to turn on and from the LED current information we looked at above that the LED can handle 20mA of current continuously. All we need to do is figure out the right resistor value to provide this 20mA. We can do this by simply using Ohm’s Law:

\begin{equation}
V = IR
\end{equation}

\begin{equation}
R = \frac{V}{I}
\end{equation}

\begin{equation}
R = \frac{9V}{20mA}
\end{equation}

\begin{equation}
R = \frac{9V}{0.020A}
\end{equation}

\begin{equation}
\boxed{R = 450\Omega}
\end{equation}

So, for our simple switch circuit, using an orange LED requires us to have a resistor value of at least 450Ω or more. Sometimes resistor kits don’t have the exact values that you calculate, so you may need to use a different value. Use a resistor value larger than what you calculated. So, in this case we can use a 470Ω (yellow–violet–brown) resistor or even a 510Ω (green–brown–brown) resistor would work just fine.

LED Wavelength and Color

The color of an LED, also known as its emission wavelength, depends on the materials used to makeup the LED. The two specifications for wavelength that are used to indicate an LED’s color are:

• λ_P or the peak wavelength
• λ_D or the dominant wavelength (color seen by human eye)

Looking at the datasheet information for the orange LED below, we can see that we’re given the dominant wavelength (λ_D) with a value of 605. The dominant wavelength is the color actually seen by the human eye. Looking at the chromticity diagram above, if we look around the value of 600, we can see that this wavelength value is around the color orange.

LED Brightness

Looking at the table from the datasheet above, we see the highlighted section for the luminous intensity (I_V) with a minimum value of 70 and a typical value of 120. The value of luminous intensity is a measure of how bright the LED can get. A value of 120 is bright enough to use as an indicator light, for example.

We can see from the datasheet that the luminous intensity (I_V) is given in a unit called the millicandela (mcd). A candela is the unit of luminous intensity in the International System of Units or SI units.

LED Dimensions

Looking at the portion of the datasheet on the LED’s dimensions, shown above, we can see that our orange LED is a 5mm LED – meaning that it’s dome top has a diameter of 5 millimeters, a typical LED size. Another common dimension for LEDs is 3mm.

---

Types of LEDs

Through-hole LED

Through-hole LEDs have terminals called leads (pronounced lēdz) that are capable of being fed through printed circuit board (PCB) holes for circuit components, then soldered. Common through-hole LED sizes are 3mm and 5mm. These types of LEDs come in a variety of colors, such as Red, Orange, Yellow, Green, Blue, and White.

SMD LED (Surface Mount Light Emitting Diodes)

A surface mount LED (SMD) has conductive pads that are capable of being soldered on the surface of a PCB. SMDs are typically smaller in size than the through-hole LEDs and have a smaller profile.

Bi-color LED

Bi-color LEDs can emit two colors. They have three through-hole terminals that are configured in a way to allow them to emit one color when current passes through them in one direction and another color when current passes through them in another direction.

RGB LED (Red-Green-Blue)

RGB LEDs use three colors schemes (red-green-blue) to create an array of colors when adjusted in intensities.

Using the RGB color code (R, G, B), we can create just about any color we’d like just by adjusting the values from 0 to 255. For example, to create the color red, we’d use the RGB color code (255, 0, 0). To create the color green we’d use (0, 255, 0). To create the color blue we’d use (0, 0, 255). You can adjust these numbers to even create a lime green color (178, 255, 102), such as shown below.

---

How to Use an LED

When deciding to use an LED in your circuit, there are three things you’ll need to consider: 1) Polarity, 2) Brightness, and 3) Power.

1. Polarity: An LED, like a diode has a certain direction it must face in a circuit to allow the flow of current. An LED has an anode (+) and a cathode (-). The anode must face toward the positive side of the voltage supply and the cathode must face toward the negative side or ground of the voltage supply.

2. Brightness: To determine how bright you want your LED, it’s just a matter of choosing a resistor value for your circuit. First, you’ll have to know the operational current of the LED from its datasheet, like we saw with LED Current in our discussion above. Then, you can use Ohm’s Law to determine the minimum resistance value you’ll need for your LED, and from there you can play around with higher resistance values to determine how bright you want your LED to shine.

3. Power: If you were to connect an LED straight to a power source, such as a battery, with no current limiting resistor in the circuit path, then the LED would try to dissipate as much power as it could causing it to burst in a quick flash and burn out — destroying the LED.

The formula for power is given as:

\begin{equation}
P = IV
\end{equation}

I found my EBL 9V battery specifications at EBL’s website. This gave me the battery capacity of my battery, which is 600 mAh. So, without a current limiting resistor in the circuit the full 600mA is going through the LED.

\begin{equation}
P = IV
\end{equation}

\begin{equation}
P = (600mA)(9V)
\end{equation}

\begin{equation}
P = (0.6A)(9V)
\end{equation}

\begin{equation}
\boxed{P = 5.4W}
\end{equation}

The circuit is providing 5.4 watts of power to the LED, which it should only handle much less than that:

\begin{equation}
P = IV
\end{equation}

\begin{equation}
P = (20mA)(9V – 2V)
\end{equation}

\begin{equation}
P = (0.020A)(7V)
\end{equation}

\begin{equation}
P = 0.14W
\end{equation}

\begin{equation}
\boxed{P = 140mW}
\end{equation}

The LED should only be able to handle about 0.14 watts or 140 milliwatts of power. Note that for the value of the voltage we subtracted the voltage drop across the LED (given in datasheet – see section on LED Voltage above) from the battery voltage – meaning we have the required 2 volts across the LED for it to function and the remaining 7 volts is the voltage drop across the resistor.

With a current limiting resistor in the LED circuit, we limit the current going through the LED, therefore limiting the power the LED must dissipate – making the LED perform without destruction!

---

How to Quickly Test an LED

The Coin Cell Method

The Multimeter Method

---

Conclusion

At this point you should feel pretty confident about LEDs. You now know what an LED is, how one produces light, how to read an LED’s datasheet, you know of some different types of LEDs, and know how to perform some basic calculations for circuits using LEDs.

If you’re looking for a more hands-on approach in your learning, try this Simple Switch Circuit here that involves connecting a switch to an LED in a circuit. If you want more of a full-blown project to do involving LEDs, try this awesome Chewing Gum Box LED Night Light Project here.

I encourage you to try out the switch circuit and chewing gum box project mentioned above to help further your understanding of how LEDs work and how to manipulate them in a circuit. Remember to stay motivated and keep at it!