Inductance is the property of an electrical conductor that opposes any change in the current flowing through it by generating a self-induced electromotive force (back-EMF). In a real circuit or installation, inductance doesn't block steady DC current like a resistor does; instead, it delays current rise times, stores energy in a magnetic field, and violently fights sudden current interruptions, which fundamentally changes how you must snub, filter, or switch AC and DC loads.
The Core Inductance Def and the Henry (H)
If you need a working inductance def for the bench, think of it as electrical inertia. The standard unit of inductance is the Henry (H), named after Joseph Henry. By definition, a circuit has an inductance of 1 Henry when a current change of 1 Ampere per second induces an electromotive force of 1 Volt across the component. The governing equation is:
V = L × (di/dt)
Where V is the induced voltage, L is the inductance in Henrys, and di/dt is the rate of current change over time. Because physical components span massive orders of magnitude, you will rarely work with whole Henrys outside of heavy power factor correction banks. Instead, bench work relies on millihenrys (mH), microhenrys (µH), and nanohenrys (nH).
Below is a reference table mapping common inductance ranges to their physical forms and typical applications, which is critical when selecting parts for a build.
| Inductance Range | Typical Physical Form | Core Material | Primary Application |
|---|---|---|---|
| 1 nH – 100 nH | Air-core coil / PCB trace | Air / FR4 Fiberglass | RF tuning, VHF/UHF filters, impedance matching |
| 1 µH – 100 µH | Molded axial / SMD chip | Ferrite / Iron powder | Switch-mode power supply (SMPS) buck/boost converters |
| 1 mH – 10 mH | Toroidal / Bobbin wound | Silicon steel / N87 Ferrite | Audio crossovers, AC line chokes, EMI filtering |
| 100 mH – 10 H | Large laminated E-I core | Laminated electrical steel | Power factor correction, heavy motor starting, ballasts |
When sourcing components, always check the core material's saturation current (I_sat). A 10 µH inductor might drop to 5 µH if you push 2A through a core rated for only 1.5A, causing your SMPS controller to fault or overheat. For high-frequency switching (500 kHz+), look for low-loss ferrite mixes like TDK's N87 or powdered iron.
Worked Numeric Example: The Relay Coil Flyback
To understand what inductance changes in a real circuit, let's look at what happens when you interrupt an inductive DC load. This is the most common way hobbyists and junior engineers destroy microcontrollers and MOSFETs.
Imagine you are driving a standard 12V DC automotive relay using an N-channel MOSFET. The relay coil has a measured DC resistance of 80 Ω, drawing a steady-state current (I) of 150 mA (0.15 A). The coil's inductance (L) is measured at 50 mH (0.05 H).
When your microcontroller turns off the MOSFET, the current drops from 150 mA to 0 A. Modern MOSFETs switch incredibly fast. Let's assume the current falls to zero in just 1 microsecond (dt = 0.000001 s).
Using our formula V = L × (di/dt):
V = 0.05 H × (0.15 A / 0.000001 s)V = 0.05 × 150,000V = 7,500 Volts
The Fix: This is why we use flyback diodes. Placing a standard 1N4007 rectifier diode (or a faster 1N4148 / SS14 Schottky for high-speed PWM) in reverse-parallel across the relay coil clamps this spike to roughly 0.7V (or 0.3V for Schottky), safely recirculating the stored magnetic energy back into the coil until it dissipates as heat.
In a fluid system, inductance acts exactly like the mass of water flowing through a long pipe. If you slam a valve shut, the water's momentum creates a massive pressure spike known as "water hammer." In a circuit, slamming a switch open against an inductor's magnetic momentum creates a voltage spike (back-EMF). The diode acts as a pressure-relief bypass valve.
Where You Meet This in Practice (and Common Confusions)
You will encounter inductance in nearly every power and signal application, whether you intentionally design for it or fight it as a parasitic element.
Practical Applications
- Switch-Mode Power Supplies (SMPS): Inductors are the primary energy storage elements in buck, boost, and buck-boost converters. They charge up when the switch is closed and dump energy into the output capacitor when the switch opens.
- Motor Drives and VFDs: AC motors are highly inductive. Variable Frequency Drives (VFDs) must manage the inductive reactance of both the motor windings and the long cable runs between the drive and the motor, often requiring output
dV/dtfilters to prevent insulation breakdown. - Parasitic Trace Inductance: A straight piece of copper wire or PCB trace has roughly 1 nH of inductance per millimeter of length. At 1 MHz, this is negligible. At 100 MHz (like an ESP32's RF antenna trace or a fast DDR memory bus), a 10mm trace introduces 10 nH, which can cause severe signal ringing and ground bounce if not properly terminated.
What People Commonly Confuse It With
When studying the inductance def, beginners frequently mix up three related concepts:
- Inductance vs. Resistance: Resistance dissipates energy as heat and opposes all current flow equally, regardless of whether the current is steady or changing. Inductance stores energy in a magnetic field and only opposes changes in current. A perfect inductor has 0 Ω DC resistance.
- Inductance vs. Capacitance: These are electrical duals. Capacitance opposes changes in voltage by storing energy in an electric field. Inductance opposes changes in current by storing energy in a magnetic field.
- Inductance (L) vs. Inductive Reactance (X_L): Inductance is a fixed physical property of the component (measured in Henrys). Inductive reactance is the actual AC resistance the component presents at a specific frequency, calculated as
X_L = 2πfL(measured in Ohms). An inductor's reactance increases as AC frequency increases, which is why they are used as high-frequency chokes.
Frequently Asked Questions
Does inductance change with frequency or current?
The physical geometry defines the baseline inductance, but the effective inductance changes in practice. If you push too much DC current through a ferrite or iron-core inductor, the core reaches magnetic saturation. Once saturated, the permeability drops to near that of air, and the inductance plummets. Similarly, at very high frequencies, parasitic winding capacitance can cause the inductor to reach self-resonance, turning it into a capacitor above the Self-Resonant Frequency (SRF).
How do I measure inductance without an LCR meter?
While a dedicated LCR meter (like a Keysight U1733C or a budget DER EE DE-5000) is best, you can estimate it using a function generator, an oscilloscope, and a known resistor. Build an RL voltage divider, apply a known AC sine wave, and measure the voltage across the resistor and the inductor. Using the voltage divider ratio and the known frequency, you can back-calculate X_L and solve for L. For a quick DC bench test, apply a step voltage through a known resistor and measure the time constant (τ = L/R) on a scope to find L.
Why do solid-state relays (SSRs) specify minimum load currents?
Many AC SSRs use TRIACs or SCRs internally, which require a minimum "holding current" to stay latched during the AC cycle. If you switch a highly inductive load with a very low steady-state current, the phase shift between voltage and current caused by the inductance can drop the current below the holding threshold before the half-cycle ends, causing the SSR to misfire or drop out unexpectedly.
For deeper reading on magnetic component design and core saturation curves, consult the Electronics Tutorials inductor guide or the All About Circuits DC textbook chapter on inductors. Always verify your specific core material's B-H curve in the manufacturer's datasheet before finalizing a power supply design.
