The self inductance unit, the Henry (H), measures a coil's ability to oppose changes in electrical current by generating a back-electromotive force (EMF) proportional to the rate of current change. When you place an inductor in a circuit, this property fundamentally changes the time constant of RL networks, dictates the energy storage capacity in switch-mode power supplies (SMPS), and sets the cutoff frequencies in analog filters. Think of inductance like a massive, heavy water wheel in a pipe: it takes significant effort to get the water flowing (current rising), but once spinning, the wheel violently resists any attempt to suddenly stop the flow.
The Henry (H) and Real-World Sub-Units
One Henry is defined as the inductance that induces one volt of electromotive force when the current changes at a rate of one ampere per second. In practical electronics, a full Henry is an enormous amount of inductance, typically only seen in massive utility-scale smoothing chokes or heavy industrial motor starters. For benchtop and PCB-level work, you will almost exclusively use fractional sub-units.
| Unit | Symbol | Scientific Notation | Typical Application Domain |
|---|---|---|---|
| Millihenry | mH | 10⁻³ H | Audio crossovers, 50/60Hz line filters, relay coils, solenoid drivers |
| Microhenry | µH | 10⁻⁶ H | Switch-mode power supplies (Buck/Boost), DC-DC converters, EMI chokes |
| Nanohenry | nH | 10⁻⁹ H | RF impedance matching, VHF/UHF tuning, high-speed digital signal integrity |
According to standard component datasheets from manufacturers like Coilcraft, selecting the correct sub-unit range is your first critical filter. Using a 10mH inductor in a 1MHz buck converter will result in catastrophic core saturation and switching losses, while a 10nH inductor in an audio crossover will act as a simple piece of wire.
Worked Numeric Example: Calculating Inductive Kickback
The most visceral way to understand the self inductance unit is to calculate what happens when you try to force the current to change instantly. The governing equation for the voltage generated across an inductor is:
V = L × (di / dt)
Where:
V = Induced voltage (Volts)
L = Self inductance (Henrys)
di = Change in current (Amperes)
dt = Time taken for the change (Seconds)
The Math:
- di = 0.1 A (current goes from 0.1A to 0A)
- dt = 0.000001 s
- di/dt = 100,000 A/s
- V = 0.050 H × 100,000 A/s = 5,000 Volts
The inductor generates a 5,000V spike to keep the current flowing. This is why flyback diodes are non-negotiable across relay coils. Without a diode to provide a safe recirculation path, this 5kV spike will instantly avalanche and destroy your switching transistor, and in high-power contactors, it can sustain dangerous arc flashes across mechanical switch contacts.
Where You Meet the Self Inductance Unit in Practice
You will encounter specific inductance ranges repeatedly depending on your project domain. Understanding these contexts prevents costly over-engineering or catastrophic under-specification.
Switch-Mode Power Supplies (SMPS)
In a buck or boost converter, the inductor is the primary energy storage element. The value (typically 1 µH to 47 µH) is calculated based on the switching frequency, input/output voltages, and desired ripple current. If the inductance is too low, ripple current spikes, causing excessive heat in the output capacitors. If too high, the converter's transient response becomes sluggish, and the physical size of the component becomes impractical.
EMI and RFI Filtering
Common-mode chokes and differential-mode inductors rely on high self inductance (often 1 mH to 10 mH) to present a high impedance to high-frequency noise while passing DC or 50/60Hz AC power. Here, the exact inductance value matters less than the core material's behavior at the target noise frequency.
RF and High-Speed Digital
At VHF/UHF frequencies or on PCIe/USB3.0 data lines, parasitic trace inductance (measured in nH) becomes the dominant factor. A mere 5mm trace of FR4 PCB can exhibit 3 nH to 5 nH of self inductance, which can cause signal ringing and impedance mismatches if not properly terminated.
Common Confusions: Reactance, Mutual Inductance, and Impedance
Even experienced makers frequently mix up related magnetic terms. Clearing up these confusions is vital for accurate circuit analysis, as detailed in foundational texts like All About Circuits.
- Inductance (H) vs. Inductive Reactance (Ω): Inductance is the physical property of the coil (measured in Henrys). Inductive reactance ($X_L$) is the opposition to AC current that results from that property, measured in Ohms. Reactance changes with frequency ($X_L = 2\pi fL$); inductance does not.
- Self vs. Mutual Inductance: Self inductance is a single coil interacting with its own magnetic field. Mutual inductance (also measured in Henrys) describes how the magnetic field of one coil induces a voltage in a second, adjacent coil. Transformers rely on mutual inductance; chokes rely on self inductance.
- Ideal Inductance vs. Real Impedance: A real inductor is not just an 'L'. It is an inductor in series with a resistor (the copper wire's DCR) and in parallel with a capacitor (parasitic winding capacitance). At the Self-Resonant Frequency (SRF), the inductor stops acting like an inductor and becomes a capacitor.
Decision Path: Selecting the Right Inductor Value and Package
Do not just pick an inductor based on the Henry value. You must evaluate Saturation Current ($I_{SAT}$), RMS Current ($I_{RMS}$), and Self-Resonant Frequency (SRF). Use the decision tree below to terminate your search with a concrete part family.
| If your application is... | And your priority is... | Then select this topology/package | Concrete Part Series Recommendation |
|---|---|---|---|
| High-Frequency SMPS (1MHz - 3MHz) | Low profile, minimal AC core losses, high SRF | Molded Ferrite / Metal Alloy SMD | Coilcraft XEL Series (e.g., XEL3530-471ME) |
| General Purpose Buck Converter (100kHz - 500kHz) | High energy storage, cost-efficiency, high $I_{SAT}$ | Shielded Drum Core SMD | Würth Elektronik WE-PD (e.g., 74477410) |
| RF Impedance Matching / VHF Filtering | Tight tolerance (±2%), high Q-factor, nH range | Multilayer Ceramic or Wirewound 0402/0603 | Murata LQH Series (e.g., LQH32CN1R0K53L) |
| Audio Crossover / 50Hz Line Choke | High inductance (mH), high current, no saturation | Air-core or Gapped Toroidal Through-Hole | Jantzen Audio Air Core (e.g., 000-1212 1.5mH) |
Frequently Asked Questions
Can I measure the self inductance unit with a standard multimeter?
No. Standard digital multimeters (DMMs) measure DC resistance, voltage, and current. To measure inductance in Henrys, you need an LCR meter (like the Keysight U1733C or a benchtop DE-5000) which applies an AC test signal at a specific frequency (usually 1kHz or 100kHz) and calculates the phase shift between voltage and current.
Does the self inductance of a coil change if I increase the current?
For air-core inductors, no; the inductance remains constant regardless of current. However, for inductors with magnetic cores (ferrite, iron powder), pushing high current drives the core toward magnetic saturation. As saturation approaches, the effective permeability drops, and the measured self inductance decreases drastically. This is why datasheets specify $I_{SAT}$ at the point where inductance drops by 20% or 30%.
Why do high-speed digital traces act like inductors?
Every conductive path possesses inherent self inductance, dictated by its geometry and the magnetic field it generates. A straight PCB trace generates roughly 1 nH per millimeter of length. At low frequencies (DC or 60Hz), this is negligible. But at high-speed digital edge rates (where $dt$ is in picoseconds), that 1 nH of trace inductance generates massive $V = L(di/dt)$ voltage spikes, causing ground bounce and signal ringing.
