💡 Circuit Design & Hardware Prototyping
LED Series Resistor Calculator
Calculate current-limiting series resistors ($R = \frac{V_s - V_f}{I_f}$), nearest standard E12/E24 resistor values, and safe power dissipation wattage ratings.
🔴 Red (2.0V)
🟡 Yellow (2.2V)
🟢 Green (3.2V)
🔵 Blue (3.3V)
⚪ White (3.3V)
🟣 UV (3.4V)
Circuit Voltage & Current
V
USB = 5V, Car / Battery = 12V, Arduino = 3.3V / 5V
V
mA
Exact Calculated Resistor Value
150 Ω
Resistor Wattage
60 mW
Safe Rating
1/4 Watt (0.25W)
Total LED Drop
2.00 V
💡 Circuit Engineering Formula:
$$R = \frac{V_s - (n \times V_f)}{I_f} = \frac{5.0 - 2.0}{0.020\text{ A}} = 150\text{ }\Omega$$
Ohm's Law for Light Emitting Diodes
The series resistor absorbs excess supply voltage while maintaining a constant forward current:
- Formula: $$R = \frac{V_{\text{source}} - V_{\text{LED}}}{I_{\text{LED}}}$$
- Power Dissipation: $$P_{\text{resistor}} = (V_{\text{source}} - V_{\text{LED}}) \times I_{\text{LED}} = I^2 R$$
- Wattage Safety Rule: Choose a physical resistor rated for at least $2\times$ the calculated power dissipation (e.g. use a $1/4\text{W}$ resistor for loads $< 125\text{mW}$).