LED circuit with current-limiting resistor in PCB Design - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the purpose of the current-limiting resistor in an LED circuit

Why is a current-limiting resistor necessary when connecting an LED to a power source?

A It prevents the LED from drawing too much current and burning out.

B It increases the voltage supplied to the LED to make it brighter.

C It stores electrical energy to power the LED when the source is off.

D It changes the LED's color by altering the current flow.

💡 Hint

Think about what happens if too much current flows through a small light bulb.

The resistor limits the current to a safe level so the LED does not get damaged by excessive current.

❓ dax_lod_result

intermediate

Calculating the resistor value for a 5V LED circuit

You have a 5V power supply and an LED with a forward voltage of 2V and a desired current of 20mA. What is the correct resistor value to use?

PCB Design

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

A 150 Ohms

B 300 Ohms

C 100 Ohms

D 50 Ohms

💡 Hint

Use Ohm's law: R = (V_supply - V_LED) / I

Calculate: (5V - 2V) / 0.02A = 3V / 0.02A = 150 Ohms.

❓ visualization

advanced

Identify the correct LED circuit diagram with a current-limiting resistor

Which of the following circuit diagrams correctly shows an LED connected with a current-limiting resistor to a power source?

A Power source connected to resistor, resistor connected to ground, LED connected in parallel to resistor.

B Power source connected directly to LED, LED connected to resistor, resistor connected to ground.

C Power source connected to resistor, resistor connected to LED, LED connected to ground.

D Power source connected to LED, LED connected directly to ground without resistor.

💡 Hint

The resistor must be in series with the LED to limit current properly.

The resistor should be placed before the LED in series to control current flow safely.

🔧 Formula Fix

advanced

Troubleshooting an LED that does not light up

An LED connected with a resistor to a 9V battery does not light up. Which of the following is the most likely cause?

A The resistor is connected in parallel with the LED.

B The battery voltage is too high for the LED.

C The resistor value is too high, limiting current below LED threshold.

D The LED is connected in reverse polarity.

💡 Hint

LEDs only allow current in one direction.

If the LED is reversed, it blocks current and will not light up.

🎯 Scenario

expert

Designing a multi-LED circuit with individual current-limiting resistors

You want to connect 3 LEDs to a 12V power supply. Each LED has a forward voltage of 2.2V and requires 25mA. Which design is best to ensure each LED receives the correct current?

A Connect all LEDs in series with one resistor calculated for 25mA total.

B Connect all LEDs in parallel, each with its own resistor calculated for 25mA.

C Connect all LEDs in parallel with one resistor calculated for 75mA total.

D Connect all LEDs in series without any resistor.

💡 Hint

Consider how voltage and current behave in series vs parallel circuits.

In parallel, each LED needs its own resistor to control current individually. In series, voltages add but current is the same, so one resistor won't protect each LED properly.

Practice

1. What is the main purpose of adding a current-limiting resistor in an LED circuit?

easy

A. To reduce the voltage supply to the LED

B. To increase the brightness of the LED

C. To change the color of the LED

D. To protect the LED by controlling the current flowing through it

Solution

1. Step 1: Understand LED behavior

   LEDs need a limited current to avoid damage because too much current can burn them out.

2. Step 2: Role of resistor

   The resistor limits the current to a safe level, protecting the LED from excess current.

3. Final Answer:

   To protect the LED by controlling the current flowing through it -> Option D

4. Quick Check:

   Current limiting resistor = LED protection [OK]

Hint: Resistor limits current to protect LED [OK]

Common Mistakes:

• Thinking resistor changes LED color
• Assuming resistor increases brightness
• Believing resistor reduces voltage only

2. Which formula correctly calculates the resistor value needed for an LED circuit?

easy

A. Resistor = (Supply Voltage - LED Voltage) / Desired Current

B. Resistor = Supply Voltage / LED Voltage

C. Resistor = LED Voltage / Supply Voltage

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

Solution

1. Step 1: Recall Ohm's Law for resistor calculation

   The resistor value is calculated by dividing the voltage drop across the resistor by the desired current.

2. Step 2: Apply to LED circuit

   Voltage drop across resistor = Supply Voltage - LED Voltage, so Resistor = (Supply Voltage - LED Voltage) / Desired Current.

3. Final Answer:

   Resistor = (Supply Voltage - LED Voltage) / Desired Current -> Option A

4. Quick Check:

   Voltage difference divided by current = resistor [OK]

Hint: Subtract LED voltage from supply, then divide by current [OK]

Common Mistakes:

• Dividing supply voltage by LED voltage
• Reversing numerator and denominator
• Ignoring voltage drop across LED

3. Given a 9V supply, an LED voltage of 2V, and desired current of 20mA, what resistor value should be used?

medium

A. 3500 Ω

B. 110 Ω

C. 350 Ω

D. 7 Ω

Solution

1. Step 1: Calculate voltage across resistor

   Voltage across resistor = 9V - 2V = 7V.

2. Step 2: Calculate resistor value using Ohm's Law

   Resistor = Voltage / Current = 7V / 0.02A = 350 Ω.

3. Final Answer:

   350 Ω -> Option C

4. Quick Check:

   7V / 0.02A = 350 Ω [OK]

Hint: Subtract voltages, divide by current in amps [OK]

Common Mistakes:

• Using current in mA without converting to A
• Subtracting voltages incorrectly
• Choosing wrong resistor units

4. A designer calculated resistor as 220 Ω but used 22 Ω instead. What is the likely effect on the LED?

medium

A. LED will be dimmer than expected

B. LED will burn out due to excessive current

C. LED will not light up at all

D. LED color will change

Solution

1. Step 1: Understand resistor role in current control

   A smaller resistor allows more current to flow through the LED.

2. Step 2: Effect of using 22 Ω instead of 220 Ω

   Using 22 Ω instead of 220 Ω increases current roughly tenfold, risking LED damage.

3. Final Answer:

   LED will burn out due to excessive current -> Option B

4. Quick Check:

   Lower resistor = higher current = LED damage [OK]

Hint: Too low resistor causes too much current [OK]

Common Mistakes:

• Thinking LED will be dimmer
• Assuming LED won't light
• Believing resistor changes LED color

5. You have a 5V supply, a red LED with forward voltage 1.8V, and want 15mA current. Which standard resistor value is best to use?

hard

A. 220 Ω

B. 330 Ω

C. 270 Ω

D. 150 Ω

Solution

1. Step 1: Calculate ideal resistor value

   Voltage across resistor = 5V - 1.8V = 3.2V. Resistor = 3.2V / 0.015A = 213.33 Ω.

2. Step 2: Choose closest standard resistor value

   Standard resistor values near 213 Ω are 220 Ω and 270 Ω. Choosing 220 Ω is closer to ideal and provides slightly higher current but still safe.

3. Final Answer:

   220 Ω -> Option A

4. Quick Check:

   Closest standard resistor near ideal value is best choice [OK]

Hint: Pick nearest standard resistor value close to ideal [OK]

Common Mistakes:

• Picking resistor lower than calculated
• Ignoring standard resistor values
• Not converting current to amps