Current-limiting resistors

Once an LED turns on it has almost no resistance, so connecting it straight across a battery lets a runaway current flow — and it burns out in a flash. The fix is a series resistor: it sits in line with the LED, soaks up the leftover voltage, and caps the current at a safe level.

Sizing the resistor is pure Ohm’s Law in three steps:

• Find the leftover voltage: Vresistor = Vsupply − Vled
• Pick a safe LED current, usually ~20 mA (0.02 A)
• Solve: R = Vresistor / Iled

For a 5 V supply, a 2 V LED drop and a chosen 20 mA current, R=(5−2)/0.02=150 Ω. 150 Ω is already a standard resistor value. Choosing 220 Ω instead reduces the estimated current to about 13.6 mA and is a common conservative choice, provided the component ratings allow it.