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Inductor Energy Calculator

Calculate the energy stored in an inductor's magnetic field in joules and millijoules from its inductance and current

Frequently Asked Questions

How is energy stored in an inductor?

An inductor stores energy in the magnetic field created by current through its coil. The stored energy equals one half of the inductance times the square of the current.

Why does interrupting inductor current cause a voltage spike?

The stored magnetic energy must go somewhere when current stops suddenly, producing a large voltage. This is why flyback diodes protect circuits that drive relays, motors, and other inductive loads.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

An inductor stores energy in the magnetic field created by current flowing through its coil. The stored energy equals one half of the inductance times the square of the current. Because the energy depends on the square of the current, even modest currents in a large inductor can store significant energy.

Where It Matters

Stored magnetic energy is central to switching power supplies, where an inductor charges and discharges many thousands of times per second to convert voltages efficiently. It also explains the voltage spike that appears when current through an inductor is interrupted suddenly, which is why flyback diodes protect circuits driving relays and motors.

How To Use It

Enter the inductance in millihenries and the current in amperes. The calculator returns the stored energy in joules and millijoules. Try changing the current to see how the squared relationship rapidly increases the stored energy.

Tips

Doubling the current quadruples the stored energy.
Inductance in millihenries is divided by 1000 to reach henries.
Never open a current-carrying inductor without a discharge path.

Inductors in Power Conversion

Switching power supplies depend on an inductor as the core energy-transfer element. In a boost converter, the switch closes and current ramps up in the inductor, storing energy in the magnetic field. When the switch opens, the inductor forces that current to continue flowing through the output diode, delivering energy to the load at a higher voltage than the input. In a buck converter, the inductor is placed in series with the output and smooths the pulsed current from the switching node into a steady DC current drawn by the load. The inductance value and switching frequency together set the current ripple: larger inductance or higher frequency produces less ripple, which means less output voltage variation and lower RMS current through capacitors.

Core Saturation

Every magnetic core has a saturation flux density B_sat. Ferrite cores used in switch-mode supplies typically saturate between 0.3 T and 0.5 T. When the core saturates, the incremental permeability drops to near that of air and the inductance collapses - current then rises almost without limit, which can destroy the switching transistor. Powdered iron cores have a distributed air gap baked into the material, which causes them to saturate more softly and gradually rather than abruptly. This makes powdered iron popular for EMI filters and differential-mode inductors where a gradual saturation characteristic is acceptable. Air-core coils never saturate because there is no magnetic material to become saturated, but they offer much lower inductance for a given size and produce significant external magnetic field.

DCR and Copper Loss

Real inductors have a winding resistance called DC resistance (DCR). Power dissipated in DCR equals I² × DCR, exactly like a resistor. For a high-current power rail inductor even a few milliohms of DCR can mean meaningful power loss at tens of amperes. Always check the DCR specification in the datasheet when selecting inductors for high-current rails, and compare efficiency at the intended operating current.

Energy Storage vs. Impedance Applications

Not all inductors are intended to store energy. Flyback transformers store energy in an intentional air gap during the switch-on phase and release it to the secondary during switch-off; the gap is essential to the design. Differential inductors placed in series with motor phase leads limit the rate of current change (di/dt), reducing motor winding stress in variable-frequency drives. Line chokes on power supply inputs filter differential-mode noise from the supply current. In resonant tank circuits, an inductor and capacitor exchange energy back and forth at the resonant frequency f = 1 / (2π√(LC)); the circuit naturally selects that frequency with a bandwidth determined by the losses in the circuit.

Inductor Q Factor

The quality factor Q = ωL / R measures how efficiently an inductor stores energy relative to the power it dissipates. High-Q coils built with air cores or litz wire (stranded wire that reduces skin effect losses) are used in RF oscillators and filters where narrow bandwidth and low insertion loss matter. Low-Q inductors are perfectly adequate for power supply filtering, where the goal is energy transfer rather than frequency selectivity. Audio crossover inductors typically target a moderate Q that shapes the filter response predictably.

Practical Inductor Specifications

When selecting an inductor from a catalog, the key specifications are: inductance value (at a defined test frequency and DC bias), rated current (both the saturation current where inductance drops by a defined amount, and the RMS current where temperature rise reaches a limit), DCR, self-resonant frequency (SRF), and Q factor. The SRF is particularly important: above the SRF, the distributed winding capacitance dominates and the component behaves as a capacitor rather than an inductor. For high-frequency filter applications, the SRF must be well above the operating frequency. Operating temperature range and maximum ambient temperature also matter for reliability.

Magnetically shielded inductors enclose the core in a conductive or high-permeability shell, reducing the external magnetic field that can couple into neighboring components or PCB traces. Unshielded inductors are smaller and cheaper for the same inductance but require careful placement to avoid interference. Shielding typically reduces the achievable inductance slightly for a given package size.

Winding and Core Types

Toroidal inductors wind the coil around a donut-shaped core; because the flux is largely contained within the toroid, they produce very little external field and are excellent for sensitive circuits. E-core inductors are easy to wind on a bobbin and are the dominant form for power transformers and larger filter inductors. Rod-core inductors with a straight ferrite rod are used in AM radio antennas where the rod concentrates the external magnetic field for reception. Ferrite beads are a special case: rather than acting as true inductors with a well-defined inductance, they present a resistive impedance at high frequencies that absorbs and dissipates EMI energy rather than reflecting it. Ferrite beads are used on power supply rails and signal lines specifically to suppress broadband high-frequency noise.

Frequently Asked Questions

What happens if current exceeds the inductor's rated value? The core saturates and inductance collapses toward the air-core value, which may be 10 to 100 times lower. The reduced impedance allows current to spike far beyond the design level, potentially destroying the switching transistor or triggering overcurrent protection. Always choose an inductor whose saturation current is comfortably above the peak inductor current in the circuit, including startup and fault conditions.

Why do switching supplies run at high frequency? The energy stored per cycle is E = ½LI², and the inductor must store and release that energy each cycle. At higher switching frequencies, the required inductance to achieve a given current ripple decreases in proportion to frequency, and the required output capacitance also decreases. Smaller passives mean smaller and lighter designs. The trade-off is that switching losses in the transistors increase with frequency, so the designer optimizes for minimum total loss at the target operating point.