Label：LED, Electroluminescence, Polarity, Maximum Rated Current
Nov 3, 202016004
In the late 19th century, Thomas Edison invented the first commercially viable light bulb. The working principle of a light bulb is that a large amount of current flows through the filament, which is basically a wire, the filament heats up, starts to glow, and then shines.
The efficiency of this process is very low, less than 5% of the energy enters the bulb and turns into light, and the rest becomes heat. The more efficient light source is light-emitting diode or LED .
LED basically consists of two special semiconductors, they are glued together, when you provide a large enough voltage power supply, they will emit light, this process is called "electroluminescence."
Some heat is generated during the process, but overall the process is more efficient, and you can get a lot of light from a very small part. On average, they have been used continuously for more than 10 years, so you can see why they are popular.
Today, we have LED flashlights, LED street lamps, billboards, and even LED bulbs. You can buy them wherever electronic products are sold. I recommend buying some resistors together.
Regarding LEDs, you need to know three important things: polarity, forward voltage, and maximum rated current.
Let's start with the polarity, which basically means which pin you connect the LED to in your circuit . All LEDs have two pins, an "anode" and a "cathode". "Anode" (positive) and "cathode" (negative) are sometimes abbreviated as A and C. "Anode" is usually the input current, and "cathode" is usually the outflow current. For a standard 5mm LED, there are two simple ways to find out the polarity.
The anode will have longer leads and the cathode will have shorter leads. If you look closely, you will see that one side is larger and downward. The larger side is the "cathode" and the smaller side is the "anode".
So in this example I connected the "positive" power supply to the "anode" and the "negative" power supply to the "cathode", and the LED lights up as expected. If the polarity of your circuit is wrong, don't worry, for low voltage projects, the LED will prevent the current from flowing backward, it just doesn't turn on.
Now let's talk about the forward voltage LED. All LEDs need a certain voltage, the current needs to flow in the right direction, the current flows, and they start to emit light.
The particular white LED I am using has a forward voltage and needs about 3 volts. When the power supply is set to 0 volts, the LED remains off. The power supply is set to 1.5 volts, which is still not enough to turn on the LED.
But as we adjusted to 3 volts, the LED reached full brightness. Each LED has a forward voltage, and each is a little different. Once the LED lights up, there will be a relatively constant voltage drop. Next, let's talk about the maximum current limit to the LED.
In this example, I used a special function, using the power supply to limit the current, the current through the LED to a maximum of 30mA, which is almost the same as the LED can withstand. But what will happen if I ignore the restrictions?
I plan to set the power supply to 7.5 volts. If you significantly exceed the forward voltage of the LED and excessive current flows through the LED, it will burn itself. In fact, the LED may explode, and the exploded large pieces of plastic and metal fly towards your face.
A 9 volt battery is enough to destroy an LED in an instant, so something is needed to limit the amount of current to a safe value. The current setting of each LED is different.
For example, this high-power LED module can easily handle 100mA, but in general, the standard 5mm LED used at home is 20mA.