How LEDs Work in a Series Circuit

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​LEDs (Light-Emitting Diodes) are semiconductor devices that emit light when electrons cross a p–n junction in the correct direction, releasing photons with quantized energy in the visible range of about 400–700 nm. In a series circuit, multiple LEDs are connected end-to-end in a single path, so the same current (I) flows through each device. The total voltage (V) across the circuit equals the sum of the forward voltage drops of all the LEDs, with each LED receiving the voltage it requires to operate. A resistor (R) is typically included to regulate this current and ensure stable operation.

 

A resistor is a passive component that opposes the flow of current, quantified by its resistance R (measured in Ohms, Ω). When placed in series with LEDs, the resistor limits current to prevent overdriving the LEDs. The required resistance can be calculated using Ohm’s law:

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R = Vs − ∑ VLED / I​

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• Vs = supply voltage (from the power source)

• ∑VLED = total forward voltage drop of all LEDs in series

• I = desired current through the LEDs

• R = resistance needed to limit current safely

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Semiconductor Materials in Red, White, and Blue LEDs

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• Red LEDs: Primarily made from Aluminum Gallium Indium Phosphide (AlGaInP) have a wavelength red light (~620–650 nm).

• ​Blue LEDs: Made from Indium Gallium Nitride (InGaN) on a Gallium Nitride (GaN) base have a wavelength blue light (~450–470 nm).

• ​White LEDs: Not a direct semiconductor color. Most common white LEDs are blue InGaN/GaN LEDs coated with a yellow phosphor (typically cerium-doped Yttrium Aluminum Garnet, or Ce:YAG). White LED's have broad spectrum wavelength rather than emitting light at a single wavelength. This mix appears white to the human eye, but the spectrum covers most of the visible range (~400–700 nm).

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Key Principles of LEDs

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• Forward voltage (Vf): Each LED requires a minimum voltage (typically 1.8–3.5V depending on color) to turn on and emit light. Red LEDs are ~1.8–2.2V, white/blue ~3.0–3.5V.

• Polarity: LEDs only allow current in one direction (anode to cathode). Reverse polarity = no light and possible damage.

• Current limiting: LEDs need a specific current (usually 10–20mA for standard indicator LEDs) to shine brightly without burning out. Too much current destroys them quickly.

• Current-Limiting Resistor: Essential! It prevents too much current from damaging the LEDs. Calculate it using Ohm's Law: R = (Supply Voltage – Total LED Voltage Drop) / Desired Current (e.g., 0.02A for 20mA).

• Add headroom for the current-limiting resistor The resistor needs at least 2–5V across it to effectively limit current and provide stable operation. Minimum supply voltage = Total LED drop + 2–5V headroom.

• Choose a standard power supply voltage. Pick the next common higher voltage (e.g., 12V, 24V adapters are widely available and inexpensive).