LIBRARYDiodes and LEDs.

How a diode passes current one way, why an LED needs a current-limiting resistor, and the forward voltage by color. With a live calculator.

A diode lets current flow one way and blocks it the other. An LED is a diode that emits light, and like any diode it drops a roughly fixed forward voltage, so it needs a resistor to set its current.

A one-way valve

In the forward direction a diode conducts with a small voltage drop; in reverse it blocks. A common use is reverse-polarity protection: a diode in the power path passes current only when the supply is connected the right way round, so a reversed battery cannot damage the board.

▸Deep dive· Why good designs replace the protection diode

A series diode is the simplest reverse-polarity guard, but it is not free: its forward voltage drop is always there, so it burns its drop times the load current as heat every second the board runs, and it steals that slice from a battery. That is why more advanced designs swap the diode for a P-channel MOSFET wired as an ideal diode. Its on-resistance is milliohms, so it wastes almost nothing and stays cool, at the cost of a couple more parts. Same job, far less loss. (TI, Basics of Ideal Diodes)

Texas Instruments. Basics of Ideal Diodes application note (SLVAE57B): P-channel MOSFET reverse-polarity protection.

The LED forward voltage

An LED drops a forward voltage that depends on its color, roughly 1.8 V for red and 3.0 V to 3.4 V for blue and white at a typical 20 mA (see the forward-voltage reference below, and always read the specific LED's datasheet at your current). Because that drop is roughly fixed, you set the current with a series resistor sized from the supply, the forward voltage, and your target current.

R = \frac{V_{supply} - V_f}{I}

Inputs

Supply voltageVThe rail the LED + resistor sit across (e.g. 3.3 V or 5 V).LED forward voltageVFrom the LED datasheet (Vf). Red is ~1.8–2.0 V; blue/white ~3.0–3.4 V.Target currentmAIndicator LEDs are happy at 2–10 mA; 20 mA is a typical maximum.

Result

300

Ω · series resistor

Nearest standard value (E24): 300 Ω

Power in the resistor

7.5 mW

Size an LED's current-limiting resistor. ↗

CircuitBread. The forward voltages of different LEDs (typical Vf by color at 20 mA).

Fig 1

On a One Thousand Drones board a diode in the power input path is reverse-polarity protection: it only conducts when the supply is the right way round, and its direction and its forward drop are the whole point.

Checkpoint

Quick check

A diode does what?

Why does an LED need a series resistor?

A red LED's forward voltage is closest to which value?

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Prerequisite: Ohm's law

Calculate it: the LED series resistor calculator

Next: reactive parts and filtering

One Thousand Drones engineering team · verified 2026-07

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