Inductance Converter

Convert between inductance units including henry, microhenry, millihenry, nanohenry, and weber per ampere.

Understanding Inductance

What is Inductance?

Inductance is the property of an electrical conductor that opposes changes in current. When current through an inductor changes, it creates a magnetic field that induces a voltage opposing that change (Lenz's Law). The SI unit is the henry (H).

Named after Joseph Henry, who discovered self-inductance independently of Michael Faraday.

Faraday's Law of Induction

The voltage induced across an inductor is proportional to the rate of change of current.

Where V is voltage (volts), L is inductance (henrys), and dI/dt is the rate of current change (amperes per second).

Energy Stored in an Inductor

An inductor stores energy in its magnetic field. The energy depends on inductance and current.

Where E is energy (joules), L is inductance (henrys), and I is current (amperes).

Inductive Reactance

In AC circuits, inductors have impedance that depends on frequency.

Where X_L is inductive reactance (ohms), f is frequency (hertz), and ω is angular frequency (radians/second). Higher frequency → higher impedance.

Self-Inductance

Self-inductance is the inductance of a single coil due to its own magnetic field. For a solenoid (long coil):

Where μ₀ is permeability of free space, μ_r is relative permeability of core material, N is number of turns, A is cross-sectional area, and l is length.

Mutual Inductance

When two coils are near each other, current change in one induces voltage in the other. This mutual inductance (M) is the basis for transformers.

The coupling coefficient k relates mutual inductance to self-inductances: M = k√(L₁L₂), where 0 ≤ k ≤ 1.

Inductors in Series and Parallel

Abhenry (abH)

The abhenry is the CGS electromagnetic unit of inductance.

One abhenry equals one nanohenry. The abhenry is rarely used in modern practice.

Stathenry (statH)

The stathenry is the CGS electrostatic unit of inductance.

The stathenry is extremely large and practically never used.

Types of Inductors

Air Core Inductors

• Construction: Coil wound without magnetic core
• Advantages: No core losses, linear behavior, high Q-factor
• Disadvantages: Lower inductance per size
• Applications: RF circuits, high-frequency oscillators

Ferrite Core Inductors

• Construction: Coil wound on ferrite (ceramic magnetic material)
• Advantages: High inductance, compact size
• Disadvantages: Non-linear at high currents, temperature sensitive
• Applications: Power supplies, EMI filters, transformers

Iron Core Inductors

• Construction: Coil wound on laminated iron core
• Advantages: Very high inductance, handles high power
• Disadvantages: Heavy, core losses at high frequency
• Applications: Power transformers, low-frequency chokes

Toroidal Inductors

• Construction: Coil wound on donut-shaped core
• Advantages: Self-shielding, high efficiency, compact
• Disadvantages: Harder to wind, more expensive
• Applications: Power supplies, audio equipment

SMD/Chip Inductors

• Construction: Ceramic or ferrite with thin-film or multilayer winding
• Advantages: Tiny size, suitable for automated assembly
• Disadvantages: Limited current capacity, lower Q
• Applications: Smartphones, portable electronics, RF circuits

Quality Factor (Q)

The quality factor measures inductor efficiency—the ratio of energy stored to energy dissipated.

Where R is the series resistance (ESR - equivalent series resistance). Higher Q means lower losses. Typical values:

• Low Q (<10): Iron core power inductors
• Medium Q (10-100): Ferrite core inductors, typical chip inductors
• High Q (>100): Air core RF inductors, silver wire coils

Parasitic Capacitance

Real inductors have parasitic capacitance between windings, creating a self-resonant frequency (SRF).

Above the SRF, the inductor behaves like a capacitor! Always check that your operating frequency is well below the SRF.

Saturation Current

Inductors with magnetic cores can saturate when the magnetic field becomes too strong. At saturation:

• Inductance drops dramatically
• Core may overheat
• Circuit performance degrades

Always check the inductor's saturation current (I_sat) rating—the current at which inductance drops by 10-30%. Air core inductors don't saturate.

DC Resistance (DCR)

The DC resistance of the wire winding causes power loss.

Higher inductance coils typically have more turns → more resistance → more loss. Choose an inductor with DCR appropriate for your current level.

Common Applications

Lenz's Law

Lenz's Law states that the induced voltage always opposes the change in current. This is why:

• Current through an inductor cannot change instantaneously
• Sudden disconnection creates a voltage spike (back-EMF)
• Inductors "smooth out" current changes
• Flyback diodes are needed when switching inductive loads

Time Constant (τ)

In an RL circuit (resistor + inductor), the time constant describes how quickly current changes.

After time τ, current reaches 63% of its final value. After 5τ, it's essentially at steady state (99%).

Most Common Conversions

Quick Reference Cards

Typical Inductance Values