Inductors

What is an Inductor?

An inductor is a coil of wire that stores energy in a magnetic field when current flows through it. While a capacitor resists changes in voltage, an inductor resists changes in current. This complementary behaviour makes inductors essential in power supplies, filters, and radio circuits.

Inductance (L)

Inductance is the measure of an inductor's ability to store magnetic energy. The SI unit is the Henry (H), named after Joseph Henry. In practice, most inductors range from microhenries (μH) in high-frequency circuits to millihenries (mH) or henries (H) in power applications.

• •1 H = 1,000 mH = 1,000,000 μH
• •Small signal inductors: 1 μH – 100 μH
• •Power inductors: 100 μH – 10 mH
• •Filter chokes: 1 mH – 10 H

How an Inductor Works

When current flows through a coil of wire, it creates a magnetic field around the coil. If the current tries to change (increase or decrease), the magnetic field changes too, which induces a voltage in the coil that opposes the change. This induced voltage is called back-EMF (electromotive force). The result is that inductors smooth out current — they slow down increases and cushion decreases.

What Affects Inductance?

• •Number of turns (N): More turns → stronger magnetic field → more inductance. Inductance scales with N².
• •Core material: Air-core has low inductance. Ferrite or iron cores concentrate the magnetic field, greatly increasing inductance.
• •Coil dimensions: A larger cross-sectional area and shorter length increase inductance.

Types of Inductors

• •Air-core inductor — Simple coil with no core material. Low inductance but good for very high frequencies (radio circuits).
• •Ferrite-core inductor — Wrapped around a ferrite core for much higher inductance. Common in power supplies and EMI filters.
• •Iron-core inductor — High inductance, used at low frequencies (power line filtering). Heavy and large.
• •Toroidal inductor — Wound on a doughnut-shaped core. Very efficient because the magnetic field is contained within the core, reducing interference.
• •SMD inductor — Tiny surface-mount package for modern circuit boards.

RL Time Constant

Just as capacitors have an RC time constant, inductors paired with resistors have an RL time constant: τ = L/R. After one time constant, the current reaches about 63 % of its final value. After 5τ, it's ~99 % (fully established). Note the difference: for RC circuits τ = R × C, but for RL circuits τ = L / R.

Inductors in Series & Parallel

Inductors combine like resistors: in series they add, in parallel the reciprocals add. This makes sense because putting coils in series increases the total number of effective turns.

• •Series: L_total = L₁ + L₂ + L₃ (inductances add)
• •Parallel: 1/L_total = 1/L₁ + 1/L₂ + 1/L₃ (total is less than smallest)

Common Uses of Inductors

• •Power supply filtering — Smooths current ripple in switch-mode and linear regulators.
• •EMI filtering — Blocks high-frequency noise from entering or leaving a circuit.
• •Energy storage in switch-mode supplies — The inductor stores and releases energy each switching cycle.
• •RF circuits — Tuning and impedance matching in radio transmitters and receivers.
• •Transformers — Two coupled inductors form a transformer (covered in Lesson 5).