LED Series Current-Limiting Resistor
LED Series Current-Limiting Resistor
An LED’s current, not its voltage, is what must be controlled — exceed its rated current even briefly and the LED can be damaged or destroyed. A series resistor sized against the supply voltage and the LED’s forward voltage drop is the simplest way to limit that current to a safe value.This is one of the first calculations anyone learns in basic electronics, but it remains genuinely important in real designs — an LED wired directly across a supply with no series resistor will draw essentially unlimited current and fail almost immediately.
The series resistor is R = (V_s − V_f)/I, the voltage dropped across the resistor divided by the desired LED current. where V_s is the supply voltage, V_f is the LED’s forward voltage drop, and I is the desired LED operating current.
Inputs
V_s := 5 V=5 V
V_f := 2 V=2 V
I_led := 20 mA=0.02 A
Whatever voltage is not dropped across the LED itself must be dropped across the series resistor, so dividing that leftover voltage by the desired current gives the required resistor value.
R_led := (V_s - V_f)/I_led=150 Ω
Results
A result of 150 Ω is a very typical LED series resistor value for a 5 V supply. Choosing the next larger standard resistor value (rather than rounding down) is good practice, since it errs on the side of slightly under-driving the LED rather than risking exceeding its rated current if the forward voltage assumption is slightly off.
Assumptions & limitations
- LED forward voltage treated as a fixed value (it actually varies somewhat with current).
- Steady DC supply, not pulsed or PWM-dimmed operation.
- Standard resistor tolerance and power rating not selected here.