Electronics

Inductor Calculator

Calculate inductance in series and parallel, inductive impedance at frequency, RL time constant, and energy stored in an inductor.

Quick Answer

Series: L_total = L1 + L2 + ... (values add). Parallel: 1/L_total = 1/L1 + 1/L2 + ... (opposite of capacitors). Impedance: X_L= 2πfL. Time constant: τ = L/R.

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Inductor Values (mH, comma-separated)

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79.00 mH

Total (Series)

Inductors

About This Tool

The Inductor Calculator covers the essential inductance calculations needed for circuit design: combining inductors in series and parallel, computing inductive impedance at a given frequency, finding the RL time constant, and calculating stored energy. Inductors behave as the dual of capacitors in many ways, and understanding both is fundamental to electronics.

Inductors in Series

Series inductors add directly: L_total = L1 + L2 + L3. This assumes no mutual coupling between the inductors. If inductors are physically close and share magnetic flux, mutual inductance must be accounted for, which can either increase or decrease the total depending on the relative winding orientation.

Inductors in Parallel

Parallel inductors combine like parallel resistors: 1/L_total = 1/L1 + 1/L2 + .... The total is always less than the smallest individual inductor. Parallel inductors are less common in practice than parallel capacitors, but they appear in some power supply and filter designs.

Inductive Impedance

An inductor's impedance increases with frequency: X_L = 2πfL. At DC (0 Hz), an ideal inductor is a short circuit. At high frequencies, it looks like an open circuit. Combined with the DC resistance of the wire, the total impedance is Z = √(R² + X_L²). This frequency-dependent behavior is what makes inductors useful in filters, where they block high-frequency signals while passing low-frequency ones.

RL Time Constant

The RL time constant τ = L/R describes how quickly current builds up in an inductor through a resistor. After one time constant, the current reaches 63.2% of its final value. After five time constants, it is effectively at steady state. This is analogous to the RC time constant but with current instead of voltage.

Frequently Asked Questions

Why do inductors in series add directly but parallel ones don't?

In series, the same current flows through all inductors and the voltages add. In parallel, the voltage is the same across all inductors and the currents split. The reciprocal formula for parallel inductors is the mathematical result of summing currents at the same voltage. It mirrors how resistors combine, but is the opposite of capacitors.

What is inductive reactance?

Inductive reactance (XL) is the opposition an inductor presents to alternating current. It equals 2*pi*f*L and increases linearly with frequency. Unlike resistance, reactance doesn't dissipate energy as heat. Instead, it stores and returns energy to the circuit each AC cycle.

How does an inductor store energy?

An inductor stores energy in its magnetic field. The energy E = 0.5*L*I^2 depends on both the inductance and the square of the current. When current is interrupted, this stored energy must go somewhere, which is why inductors can produce large voltage spikes (back-EMF) when switched off suddenly.

What is the RL time constant used for?

The RL time constant determines how quickly current builds up or decays in an inductive circuit. It is used in relay driver circuits to calculate snubber component values, in switching power supply design, and whenever you need to predict how fast an inductive load responds to a voltage change.

What is mutual inductance?

When two inductors are placed near each other, their magnetic fields interact. This coupling is called mutual inductance. It can increase the effective inductance when the fields aid each other, or decrease it when they oppose. Transformers deliberately maximize mutual inductance between their windings.

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