LED Resistor & PCB Footprint Calculator

Electronics Engineering Tool for calculating LED Series Resistors and selecting the correct SMD package.

Calculate Resistor Value & Power

V
V
Typical: Red=2.0V, Green=2.2V, Blue/White=3.3V
mA
Typical for indicator LEDs: 10-20mA
Exact Resistance Required
150
Ω
Nearest Standard E24 Resistor
150 Ω
Power Dissipated
60 mW
Suggested Footprint
0603
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Interfacing LEDs: High-Side vs. Low-Side Switching

When driving an LED from a microcontroller (like an Arduino or ESP32) or a transistor, you must choose a circuit topology. There are two primary ways to interface an LED:

High-Side Switching (Sourcing)

The microcontroller pin or P-Channel MOSFET acts as the switch connecting the LED to the positive supply voltage ($V_s$). The circuit path goes: MCU Pin -> Resistor -> LED -> Ground.

  • Logic: Pin HIGH = LED ON. Pin LOW = LED OFF.
  • Pros: Intuitive logic for programmers.
  • Cons: Microcontrollers are typically worse at "sourcing" current than "sinking" it. Total combined sourcing current is heavily limited.

Low-Side Switching (Sinking)

The LED is connected permanently to $V_s$. The microcontroller pin or N-Channel MOSFET connects the circuit to ground. The circuit path goes: $V_s$ -> Resistor -> LED -> MCU Pin.

  • Logic: Pin LOW = LED ON. Pin HIGH = LED OFF.
  • Pros: MCUs can usually "sink" more current safely. This is the preferred method for driving heavier loads. N-channel MOSFETs (for low-side switching) have lower resistance and are cheaper than P-channel equivalents.

Why PCB Footprint Power Ratings Matter

In PCB design, the resistance value is only half the battle. You must calculate the power dissipated by the resistor as heat: P = I² * R or P = V_drop * I.

If your resistor dissipates 200mW of heat, but you select a tiny 0402 SMD package (rated for 62.5mW), the resistor will rapidly overheat, desolder itself, or burn the PCB traces. Our calculator explicitly suggests standard SMD footprints (0402, 0603, 0805, 1206) to ensure your design is thermally safe. It is always best practice to select a package rated for at least double your calculated power dissipation to ensure long-term reliability.