LED circuit with current-limiting resistor in PCB Design - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a current-limiting resistor in an LED circuit?

It protects the LED by controlling the amount of current flowing through it, preventing damage from too much current.

beginner

How do you calculate the value of a current-limiting resistor for an LED?

Use the formula: Resistor (Ohms) = (Supply Voltage - LED Forward Voltage) / Desired LED Current.

beginner

What happens if you don't use a current-limiting resistor with an LED?

The LED may draw too much current, causing it to burn out quickly or get damaged.

intermediate

Why is it important to know the LED's forward voltage when designing the circuit?

Because it tells you how much voltage the LED uses, which helps calculate the correct resistor value to limit current.

intermediate

If your supply voltage is 9V, LED forward voltage is 2V, and desired current is 20mA, what resistor value should you use?

Resistor = (9V - 2V) / 0.02A = 350 Ohms. You can use a standard 350 Ohm resistor or the nearest standard value, such as 360 Ohms.

What does a current-limiting resistor do in an LED circuit?

A Controls the current to protect the LED

B Increases the voltage to the LED

C Stores electrical energy

D Changes the LED color

The resistor controls the current flowing through the LED to prevent damage.

Which formula is used to find the resistor value for an LED circuit?

A Resistor = Desired Current / Supply Voltage

B Resistor = Supply Voltage / LED Current

C Resistor = LED Forward Voltage / Supply Voltage

D Resistor = (Supply Voltage - LED Forward Voltage) / Desired Current

The correct formula subtracts LED forward voltage from supply voltage, then divides by desired current.

What could happen if you connect an LED directly to a power supply without a resistor?

A LED will glow brighter safely

B LED will not light up

C LED may burn out quickly

D LED will change color

Without a resistor, too much current can flow, damaging the LED.

If the LED forward voltage is 2V and supply voltage is 5V, what voltage does the resistor drop?

A 3V

B 2V

C 5V

D 0V

The resistor drops the difference: 5V - 2V = 3V.

Why is it important to choose the correct resistor value in an LED circuit?

A To make the LED blink

B To control brightness and protect the LED

C To increase power consumption

D To change the LED color

The resistor controls current, affecting brightness and preventing damage.

Explain why a current-limiting resistor is necessary in an LED circuit and how to calculate its value.

Think about what happens if too much current flows through the LED.

Describe what could happen if you omit the resistor in an LED circuit and why the LED's forward voltage matters.

Consider the LED's voltage needs and current limits.