LED circuit with current-limiting resistor in PCB Design - Cell-by-Cell Formula Trace

Sample Data

This data represents the parameters for calculating the resistor value and power dissipation in an LED circuit powered by a 5V supply.

Cell	Value
A1	Supply Voltage (V)
B1	5
A2	LED Forward Voltage (V)
B2	2
A3	Desired Current (mA)
B3	20
A4	Resistor Value (Ohms)
B4	150
A5	Power Dissipation (mW)
B5	60

Formula Trace

Resistor Value = (Supply Voltage - LED Forward Voltage) / Desired Current Power Dissipation = Resistor Value * (Desired Current)^2

Step 1: Supply Voltage - LED Forward Voltage

Step 2: Desired Current in Amps = 20 mA / 1000

Step 3: Resistor Value = 3 / 0.02

Step 4: Power Dissipation = Resistor Value * (Desired Current)^2

Step 5: Power Dissipation in mW = 0.06 * 1000

Cell Reference Map

   A                 B
1  Supply Voltage     5
2  LED Forward Voltage 2
3  Desired Current    20
4  Resistor Value     -> Formula uses B1, B2, B3
5  Power Dissipation  -> Formula uses B4, B3

Cells B1, B2, and B3 provide input values. B4 and B5 are calculated using these inputs.

Result

   A                 B
1  Supply Voltage     5
2  LED Forward Voltage 2
3  Desired Current    20
4  Resistor Value     150
5  Power Dissipation  60

The resistor value is calculated as 150 ohms and power dissipation as 60 milliwatts.

Sheet Trace Quiz - 3 Questions

What is the voltage across the resistor in this circuit?

A 2 V

B 3 V

C 5 V

D 20 V

Key Result

Resistor value = (Supply Voltage - LED Forward Voltage) / Desired Current; Power = Resistor * Current^2

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