Unit inductance, measured in Henrys (H), quantifies a component's ability to oppose changes in electrical current by storing energy in a magnetic field. When you introduce inductance into a circuit, it fundamentally changes the phase relationship between voltage and current—causing current to lag behind voltage in AC systems—and introduces frequency-dependent reactance that blocks high-frequency signals while passing DC. Beginners frequently confuse inductance with resistance; while resistance opposes steady current and permanently dissipates energy as heat, inductance only opposes changing current and temporarily stores energy. It is also commonly confused with capacitance, which stores energy in an electric field and opposes changes in voltage rather than current.
The Henry (H) and Its Sub-Units
One Henry is defined as the inductance that induces one volt of electromotive force (EMF) when the current changes at a rate of one ampere per second. Because a full Henry is massive for most modern electronics, you will almost exclusively work with fractional sub-units. The physical inductance value is dictated by the number of coil turns, the cross-sectional area, the coil length, and the permeability of the core material (air, ferrite, or powdered iron).
| Unit Prefix | Symbol | Decimal Value | Typical Application & Component |
|---|---|---|---|
| Millihenry | mH | 10-3 H | Audio crossover networks, mains EMI chokes, large relay coils |
| Microhenry | µH | 10-6 H | Switch-mode power supply (SMPS) buck/boost converters, RF filters |
| Nanohenry | nH | 10-9 H | High-frequency RF decoupling, PCB trace parasitics, VHF antennas |
To visualize this, think of the water hammer effect in plumbing: inductance is like the inertia of water flowing through a heavy pipe. It takes sustained pressure to get the water moving, but once it is flowing, shutting the valve instantly causes a massive, destructive pressure spike because the water's momentum must go somewhere.
Worked Numeric Example: Calculating a Flyback Voltage Spike
The most critical formula for DIYers and bench technicians dealing with unit inductance is the transient voltage equation: V = L × (di/dt). This dictates the voltage spike generated when a magnetic field collapses.
The Scenario: You are using an ESP32 to drive a 12V automotive fuel injector (a highly inductive solenoid) via a logic-level MOSFET.
- Inductance (L): 100 mH (0.1 H)
- Steady-state current (i): 1.0 A
- Switching time (dt): The MOSFET turns off in 10 µs (0.00001 s)
The Calculation:
- Calculate the rate of current change: di/dt = 1.0 A / 0.00001 s = 100,000 A/s
- Multiply by inductance: V = 0.1 H × 100,000 A/s
- Resulting spike: 10,000V
Without a flyback diode placed in reverse-bias across the injector coil to safely recirculate that collapsing magnetic energy, that 10,000V spike will punch straight through the MOSFET's drain-source breakdown voltage (typically 30V-60V) and arc back into your ESP32, permanently bricking the microcontroller. According to All About Circuits, this collapse of the magnetic field is the foundational principle behind ignition coils and boost converters, but a destructive hazard in un-snubbed switching circuits.
Where You Meet Unit Inductance in Practice
You will encounter inductance in nearly every power and signal subsystem on your workbench:
- Switch-Mode Power Supplies (SMPS): In a buck converter, the inductor (typically 10µH to 47µH) acts as an energy bucket. It stores energy when the high-side FET is on and releases it to the load when the FET is off, smoothing the chopped DC into a steady lower voltage.
- Motor Control and VFDs: The stator windings of AC and BLDC motors possess high unit inductance. Variable Frequency Drives (VFDs) must account for this inductance when calculating PWM switching frequencies to prevent excessive heat and insulation breakdown.
- Audio Crossovers: Passive speaker crossovers use large air-core or ferrite-core inductors (often 1mH to 10mH) in series with woofers. Because inductive reactance (XL = 2πfL) increases with frequency, the inductor blocks high-frequency treble from reaching the woofer while passing bass frequencies.
- EMI/RFI Filtering: Common-mode chokes utilize mutual inductance to block high-frequency noise from entering or leaving a device, keeping switching noise out of sensitive analog sensor lines.
Inductance vs. Resistance vs. Capacitance
Understanding how unit inductance differs from the other two passive properties is crucial for troubleshooting AC phase shifts and transient responses.
| Property | Unit of Measure | Opposes Changes In... | Energy Storage Medium | AC Phase Shift |
|---|---|---|---|---|
| Resistance | Ohms (Ω) | Current (Steady & Changing) | None (Dissipates as Heat) | None (In phase) |
| Inductance | Henrys (H) | Current | Magnetic Field | Current lags Voltage by 90° |
| Capacitance | Farads (F) | Voltage | Electric Field | Current leads Voltage by 90° |
Frequently Asked Questions About Unit Inductance
How do you measure unit inductance with a standard multimeter?
Most basic digital multimeters (DMMs) cannot measure inductance; they only measure DC resistance. To measure the actual unit inductance (in Henrys), you need an LCR meter. A benchtop or handheld LCR meter applies a specific AC test frequency (often 100 Hz, 1 kHz, or 100 kHz) to the component and measures the phase angle and impedance to calculate the inductance. Note that measuring a coil's DC resistance with a standard DMM only tells you the wire's parasitic resistance, not its magnetic inductance.
Why does unit inductance cause a voltage spike when a circuit opens?
This is governed by Faraday's Law of Induction. An inductor stores energy in a magnetic field proportional to the current flowing through it. When the circuit opens, the current attempts to drop to zero instantly. The inductor resists this change by collapsing its magnetic field, which induces a massive electromotive force (EMF) to keep the current flowing. If there is no path for the current (like a flyback diode), the voltage will rise until it arcs across the switch contacts or breaks down the semiconductor junction, dissipating the stored energy violently.
Does the physical core material change the unit inductance value?
Yes, drastically. The inductance of a coil is directly proportional to the magnetic permeability (µ) of its core. An air-core inductor has a permeability very close to 1. If you replace the air core with a ferrite or powdered iron core, the permeability can increase by hundreds or thousands of times, multiplying the unit inductance by that exact factor. However, as detailed in Electronics Tutorials, high-permeability cores are susceptible to magnetic saturation; if the current exceeds the core's limit, the permeability drops back toward that of air, causing the inductance to plummet and potentially leading to catastrophic component failure in power supplies.
What is the difference between self-inductance and mutual inductance?
Self-inductance is the property of a single coil opposing changes in its own current, which is the standard value (in Henrys) printed on an inductor's datasheet. Mutual inductance occurs when the changing magnetic field of one coil induces a voltage in a physically separate, nearby coil. This is the foundational principle behind transformers, where the ratio of turns between the primary and secondary coils dictates the voltage step-up or step-down.
