Typical forward voltages by colour — starting points only. Forward voltage depends on the die and the drive current; enter the value from your LED's datasheet.
CIRCUIT PARAMETERS
V
V
mA
CIRCUIT DIAGRAM
CALCULATION SHEET
Recommended Resistor
150 Ω
Next E24 value at or above the calculated 150.0 Ω (never rounded down)
Actual Current
20.0 mA
(100% of target)
Power Dissipation
0.060 W
at the actual current through the E24 value
Recommended Rating
0.125 W
≥ 2× dissipation, next listed rating
Efficiency
40.0%
share of supply power reaching the LED(s)
Warnings
Only 40% of the supply power reaches the LED(s); the resistor dissipates the rest. Consider a constant-current driver.
Calculation Steps
- 01LEDs in series: 1
- 02Total LED voltage: 2 V × 1 = 2 V
- 03Voltage across resistor: 5 V - 2 V = 3 V
- 04LED current: 20 mA = 0.02 A
- 05Required resistance: R = V / I = 3 / 0.02 = 150.00 Ω
- 06Next E24 standard value at or above minimum (never rounded down): 150 Ω
- 07Actual current with 150Ω: 3 / 150 = 0.0200 A = 20.0 mA
- 08Resistor power: P = V × I = I² × R = 3 × 0.0200 = 0.060 W
- 09Recommended power rating: 0.060 W × 2 = 0.120 W → next listed rating 0.125 W (2× design margin)
- 10Efficiency: 0.040 W / 0.100 W = 40.0%
- 11⚠️ Low efficiency - consider using constant current driver instead
REFERENCE — LED RESISTOR GUIDE
Formula:
R = (Vsupply - VLED) / ILED
Typical forward-voltage ranges (verify on the datasheet):
- • ● Red: 1.8-2.2V @ 20mA
- • ● Green: 2.0-2.4V @ 20mA
- • ● Blue: 3.0-3.4V @ 20mA
- • ● White: 3.0-3.4V @ 20mA
Power Rating:
Rating is chosen at 2× the calculated dissipation as a design margin. Common ratings: ⅛W, ¼W, ½W, 1W, 2W, 5W. Resistor tolerance (±5% for E24) and supply-voltage variation are not applied to the reported current.