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LED and its series resistor

Diodes and rectifiers · Beginner · about 10 minutes · part of Zen’s Electronics Design Mastery

An LED is a diode that emits light. Its voltage is nearly fixed (about 2 V for red, 3 V for blue or white), so it must never be connected straight across a supply. A series resistor sets the current: R = (V_s − V_F) / I_F.

LED and its series resistor: the circuit on the simulator's bench

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How it works

Design for 20 mA: R = (Vs − VF) / I = (9 − 1.9) / 0.020 = 355 Ω → use the next standard value up.

With R = 330 Ω: I = 21.5 mA, VLED = 1.904 V

Resistor heat: I²R = 153 mW. LED: 40.9 mW. A 5 mm LED is happy at 5–20 mA and dies above about 30–60 mA.

Try it on the bench

  1. Change the LED colour to blue (3 V): with the same resistor the current drops a little.
  2. Set R to 1 kΩ: dimmer but still clearly lit, and the LED lasts longer.
  3. Set R to 22 Ω and watch the LED burn out. Undo restores it.

Parts on the bench

In the course

Zen’s Electronics Design Mastery, module “Diodes & Power Supplies”.

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