LED circuit with current-limiting resistor in PCB Design - Real Business Scenario

Scenario Mode

👤 Your Role: You are an electronics technician working on a new product prototype.

📋 Request: Your manager wants a clear report showing how to design an LED circuit with a current-limiting resistor to ensure the LED operates safely.

📊 Data: You have data on LED forward voltage (Vf), desired current (I), and supply voltage (Vs).

🎯 Deliverable: Create a report with calculations for the resistor value, a simple schematic diagram, and a summary dashboard showing the resistor value and expected LED current.

Sample Data

LED Color	Forward Voltage (Vf)	Desired Current (mA)	Supply Voltage (Vs)
Red	2.0	20	5
Green	2.2	20	5
Blue	3.0	20	5
White	3.2	20	5

1

Step 1: Calculate the resistor value for each LED color using the formula R = (Vs - Vf) / I, where I is in Amps.

For Red LED: R = (5V - 2.0V) / 0.02A = 150 ohms For Green LED: R = (5V - 2.2V) / 0.02A = 140 ohms For Blue LED: R = (5V - 3.0V) / 0.02A = 100 ohms For White LED: R = (5V - 3.2V) / 0.02A = 90 ohms

Expected Result

Resistor values: Red=150Ω, Green=140Ω, Blue=100Ω, White=90Ω

2

Step 2: Create a simple schematic diagram showing the supply voltage connected to the resistor, then to the LED, and finally to ground.

Draw: Vs (+5V) -> Resistor (calculated value) -> LED (color) -> Ground

Expected Result

A clear schematic with correct resistor values for each LED color

3

Step 3: Build a dashboard table summarizing LED color, resistor value, and expected current.

Table columns: LED Color, Resistor Value (Ω), Expected Current (mA) Values from step 1 calculations

Expected Result

Dashboard table: Red | 150Ω | 20mA Green | 140Ω | 20mA Blue | 100Ω | 20mA White | 90Ω | 20mA

4

Step 4: Explain why the resistor is important to limit current and protect the LED.

Text explanation: The resistor limits current to prevent LED damage by dropping excess voltage and controlling current flow.

Expected Result

Clear understanding that resistor prevents LED burnout by controlling current

Final Result

Resistor

↓

Resistor

✓Each LED color requires a different resistor value due to different forward voltages.

✓Using the correct resistor ensures the LED receives the desired 20mA current.

✓Without the resistor, the LED could draw too much current and burn out.

✓The simple circuit with resistor and LED connected to 5V supply is effective and safe.

Bonus Challenge

Design a circuit with two LEDs of different colors connected in series with one resistor. Calculate the resistor value and expected current.

Show Hint

Add the forward voltages of both LEDs, then use R = (Vs - (Vf1 + Vf2)) / I to find the resistor.

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