A diode is a one-way valve for current: it conducts freely in one direction (forward) and blocks in the other (reverse). An LED — light-emitting diode — is a diode that emits light when it conducts. Both are non-linear, which is what makes them interesting and what makes them easy to destroy.
Forward voltage
A diode only begins to conduct once the forward voltage across it exceeds a threshold, its forward voltage drop (Vf). Below that it passes almost no current; above it, the current rises very steeply for only a small increase in voltage. Typical values:
- Silicon signal/rectifier diode: Vf ≈ 0.7 V
- Schottky diode: Vf ≈ 0.2–0.4 V
- Red LED: Vf ≈ 1.8–2.2 V
- Green / blue / white LEDs: Vf ≈ 3.0–3.4 V
Because the current-versus-voltage curve is so steep past Vf, you cannot control an LED by setting a voltage — a tiny voltage change swings the current wildly. You control it by limiting the current instead.
Why an LED needs a resistor
Connect an LED straight across a supply above its Vf and it will try to draw effectively unlimited current, overheat, and burn out. A series resistor absorbs the difference between the supply and the LED's forward voltage, fixing the current at a safe value.
Sizing the resistor
The supply voltage splits between the resistor and the LED. The resistor drops (Vs − Vf), and Ohm's Law sets the current through that drop. Rearranged to solve for the resistor:
R = (Vs − Vf) / I
Drive a red LED (Vf ≈ 2 V) from a 5 V supply at a target of 13.6 mA:
R = (5 V − 2 V) / 0.0136 A ≈ 220 Ω
A standard 220 Ω resistor gives about (5 − 2)/220 ≈ 13.6 mA, a healthy brightness for most indicator LEDs. Want it dimmer? Increase R. Note that the resistor dissipates P = I × (Vs − Vf) ≈ 0.0136 × 3 ≈ 41 mW here, well within a 1/4 W part.