LEDs are current-driven devices with a very steep I-V curve. Without a current-limiting resistor, even a slight increase in voltage above the LED's forward voltage (Vf) can cause excessive current to flow, quickly destroying the LED. The resistor limits the current according to Ohm's Law: I = (V_source − V_f) / R. For most standard 5mm LEDs, the safe operating current is between 5–20 mA.

Forward voltage (Vf) is the minimum voltage required for an LED to conduct current and emit light. It varies by color due to the different semiconductor materials and bandgap energies used. Red LEDs typically have Vf ≈ 2.0V (gallium arsenide), green ≈ 2.2V (gallium phosphide), while blue and white LEDs require 3.0–3.4V (gallium nitride). This is why a 3V coin cell can directly power a red LED but struggles with a blue one.

Use the formula: R = (V_source − V_f) / I_desired. For example, with a 5V supply and a red LED (Vf=2.0V) targeting 15mA: R = (5 − 2) / 0.015 = 200Ω. The nearest standard value is 220Ω. Always round up to the next standard resistor value to stay within safe current limits. This simulator lets you experiment with different values to see the effect in real time.

The LED will not conduct and remains off. Diodes (including LEDs) act as one-way valves with a threshold. Below Vf, the depletion region in the semiconductor junction prevents current flow. In this simulator, you'll see the current drop to zero and the LED symbol go dark when V_source ≤ V_f. Try setting the voltage to 2V with a blue LED (Vf=3.0V) to observe this cutoff behavior.

Most standard 5mm through-hole LEDs are rated for a maximum continuous current of 20–30 mA. Exceeding this causes accelerated degradation and eventual burnout. High-brightness or power LEDs can handle higher currents (100mA–1A+) but require proper heatsinking. In this simulator, a warning appears when current exceeds 25mA. For longest LED lifespan, aim for 10–15mA operating current.

Series: Add the forward voltages. For two red LEDs in series (2×2.0V=4.0V) on a 9V supply with 15mA target: R = (9−4)/0.015 = 333Ω. The same current flows through both LEDs.
Parallel: Each LED needs its own resistor because Vf varies slightly between units. Without individual resistors, the LED with the lowest Vf hogs current and may burn out. This simulator focuses on the single LED + resistor series circuit — the essential building block.