How to Calculate the Correct Resistor for LEDs Light Emitting Diodes

Summary

This video tutorial explains how to calculate the correct resistor needed to safely light up an LED using Ohm's law. It walks through the process of determining the voltage drop across the resistor by subtracting the LED's voltage from the power supply, then dividing by the current. The video also covers resistor selection based on wattage and provides examples of working with LEDs in series and parallel circuits. Additionally, it explains how to identify the anode and cathode of an LED and discusses practical tips to avoid damaging components.

Takeaways

• 🔌 LEDs come in various shapes, sizes, and colors, each requiring different voltages to function.
• 🧮 To calculate the required resistor for an LED, you can use Ohm's Law: Voltage (V) = Current (I) × Resistance (R).
• 🔋 The resistor calculation requires knowing the power supply voltage and the LED voltage. Subtract the LED voltage from the supply voltage to find the voltage drop across the resistor.
• 📏 Current is typically measured in milliamps (mA), but for the calculation, it should be converted to amps (e.g., 20mA = 0.02A).
• 💡 For an LED with a 5V supply and 3.2V requirement, the remaining 1.8V needs to be dropped across the resistor. Dividing this by 0.02A gives a 90-ohm resistor.
• 💭 If the exact resistor value doesn't exist, round up to the next standard resistor value to prevent the LED from burning out (e.g., use 100-ohms instead of 90-ohms).
• 🔄 In a series circuit, the total voltage is the sum of all LEDs' voltages. For two 3.2V LEDs in series, you'll need at least 6.4V to power them.
• 🔍 LEDs have a longer lead for the positive (anode) side and a shorter lead for the negative (cathode) side. The flat side of the LED also indicates the cathode.
• 🛑 For parallel circuits, the current increases, but the voltage remains the same. Two 20mA LEDs in parallel would require 40mA of current.
• 🔥 Ensure that the wattage rating of the resistor can handle the power dissipation, calculated as Voltage (V) × Current (I). Using too low a wattage will cause the resistor to overheat and fail.

Q & A

What factors determine the resistor required for an LED?

The resistor required for an LED is determined by the LED's voltage drop, the power supply voltage, and the current (measured in amps or milliamps) required by the LED.

How do you calculate the resistance value using Ohm's law?

To calculate the resistance, subtract the LED's voltage from the power supply voltage. Then, divide the result by the current (in amps). The formula is R = (Power Supply Voltage - LED Voltage) / Current.

What is Ohm's law and how is it represented in this script?

Ohm's law states that Voltage = Current × Resistance. In the script, it is represented as a triangle where V is voltage, I is current (in amps), and R is resistance. It can be used to calculate any of these values if the other two are known.

Why is it important to convert milliamps to amps when calculating resistance?

It's important because Ohm's law uses current in amps. Milliamps must be converted to amps by dividing by 1000. For example, 20 milliamps is 0.02 amps.

What resistor value should be used if the exact calculated value does not exist?

If the exact resistor value does not exist, it is generally recommended to choose a resistor with a slightly higher value. This will reduce the current slightly, which can help prolong the life of the LED.

What happens if you use a resistor value lower than required?

Using a lower resistor value than required allows more current to flow through the LED, making it brighter but potentially reducing its lifespan due to increased stress.

How do you determine the anode and cathode of an LED?

The anode (positive) is the longer lead of the LED, and the cathode (negative) is the shorter lead. Inside the LED, the larger structure is the cathode, and the smaller one is the anode. Additionally, the flat side of the LED casing corresponds to the cathode.

What changes when two LEDs are connected in series in a circuit?

When two LEDs are connected in series, their voltage drops are added together. However, the current remains the same as it would for a single LED.

How do you calculate the resistor needed for two LEDs in series?

First, add the voltage drops of both LEDs. Subtract this combined voltage from the power supply voltage. Then, divide the result by the current (in amps) to calculate the required resistance.

What is the difference in current when LEDs are connected in series vs. parallel?

In a series circuit, the current remains the same for all components. In a parallel circuit, the total current is the sum of the currents through each branch.

What should you consider regarding the resistor's wattage when designing circuits with LEDs?

You must ensure that the resistor can handle the power dissipation. To calculate the power, multiply the voltage across the resistor by the current flowing through it. Ensure the resistor's wattage rating is higher than this value to prevent overheating.

How do you calculate the power dissipation across a resistor?

Power dissipation is calculated by multiplying the voltage drop across the resistor by the current flowing through it (P = V × I). Ensure that the resistor's wattage rating can handle this dissipation.