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