The henry (H) is the SI unit of inductance, defined as the property of a closed circuit in which an electromotive force of one volt is induced when the electric current varies uniformly at a rate of one ampere per second. When you place an inductor in a circuit, this physical property fundamentally changes how the circuit handles current transitions: it actively opposes any change in current flow (di/dt), stores energy in a magnetic field, and forces a phase shift between voltage and current in AC systems. While the National Institute of Standards and Technology (NIST) defines the henry as the base SI unit, working engineers and hobbyists almost exclusively deal with fractional sub-multiples because a single whole henry represents a massive amount of magnetic storage for standard electronics.
The Henry (H) Explained: Base Units and Sub-Multiples
In schematic design and bench work, you will rarely see a component labeled in whole henrys unless you are dealing with heavy industrial motor chokes or massive power factor correction banks. For everything from RF filters to switching power supplies, the SI unit is scaled down using standard metric prefixes. Understanding these scales is critical when substituting parts or reading manufacturer datasheets.
| Unit Name | Symbol | Multiplier | Typical Application Domain |
|---|---|---|---|
| Henry | H | 10^0 (Base) | Industrial line reactors, massive audio crossovers |
| Millihenry | mH | 10^-3 | Relay coils, solenoid valves, low-frequency EMI filters |
| Microhenry | µH | 10^-6 | DC-DC buck/boost converters, switching power supplies |
| Nanohenry | nH | 10^-9 | RF impedance matching, high-speed digital decoupling |
What Inductance Actually Changes in a Real Circuit
Inductance dictates how a circuit responds to changing current. The governing equation is V = L(di/dt), meaning the induced voltage (V) equals the inductance (L) multiplied by the rate of current change over time. This creates two major behavioral shifts in real installations: it smooths out current ripple in power supplies, and it generates destructive voltage spikes when current is abruptly interrupted.
Worked Numeric Example: The Flyback Voltage Spike
Let us calculate exactly what the inductance SI unit translates to when you switch off an inductive load. Imagine you are driving a 12V relay coil using a logic-level MOSFET.
- Inductance (L): 50 mH (0.05 H)
- Steady-state current (I): 100 mA (0.1 A)
- Switching time (dt): The MOSFET turns off in 10 µs (0.00001 s)
When the MOSFET opens, the current drops from 0.1 A to 0 A. The rate of change (di/dt) is 0.1 A / 0.00001 s = 10,000 A/s.
Applying the formula: V = 0.05 H × 10,000 A/s = 500 Volts.
Where You Meet the Henry in Practice
You will encounter specific ranges of the henry depending on the frequency and power domain of your project. Selecting the right physical inductor requires matching the inductance value to the operating frequency and ensuring the saturation current rating exceeds your peak load.
- Switch-Mode Power Supplies (µH): In a 3D printer mainboard or a bench power supply, you will find shielded ferrite drum inductors (like the Wurth Elektronik WE-PD series) in the 4.7 µH to 100 µH range. These store energy during the MOSFET's 'on' time and release it to the load during the 'off' time.
- Audio Crossovers and Motor Drives (mH): Passive speaker crossovers use air-core or ferrite-core inductors ranging from 0.5 mH to 5 mH to block high frequencies from reaching a woofer. Similarly, VFDs (Variable Frequency Drives) use line reactors measured in millihenrys to smooth the PWM waveform sent to AC induction motors.
- RF and High-Speed Digital (nH): On a 2.4 GHz PCB trace (like an ESP32 antenna match), you deal in nanohenrys. A simple 10 mm length of straight PCB trace possesses a parasitic inductance of roughly 10 nH. At microwave frequencies, this tiny fraction of the SI unit creates enough inductive reactance to detune an antenna if not properly modeled.
Common Confusions: Inductance vs. Reactance vs. Capacitance
When discussing the fundamentals of inductors, beginners frequently conflate the physical property with its AC opposition, or mix up magnetic storage with electric storage. Here is the exact breakdown of what people commonly confuse the inductance SI unit with.
| Property | Symbol | SI Unit | What It Opposes | Energy Storage Medium |
|---|---|---|---|---|
| Inductance | L | Henry (H) | Changes in current (di/dt) | Magnetic Field |
| Inductive Reactance | X_L | Ohm (Ω) | Alternating Current (AC) flow | N/A (It is a resistance equivalent) |
| Capacitance | C | Farad (F) | Changes in voltage (dv/dt) | Electric Field |
Inductance (Henry) is a fixed physical trait determined by the coil's turns, core material, and geometry. Inductive Reactance (Ohms) is a dynamic behavior calculated as X_L = 2πfL; it changes depending on the AC frequency applied to that fixed inductance.
Frequently Asked Questions About the Inductance SI Unit
Is the henry the only SI unit for inductance?
Yes, the henry (H) is the sole base SI unit for inductance recognized by the International System of Units. However, because one henry is impractically large for most modern electronics, the millihenry (mH), microhenry (µH), and nanohenry (nH) are universally used as standard decimal sub-multiples in engineering and schematic design.
How do I measure the inductance SI unit with a standard multimeter?
You cannot measure inductance with a standard digital multimeter (DMM). Standard DMMs only measure DC resistance (Ohms), which will only tell you the parasitic wire resistance of the coil, not its magnetic inductance. To measure henrys, you must use a dedicated LCR meter (like the Uni-T UT612 or a benchtop Keysight U1733C), which applies an AC test signal at a specific frequency (usually 1 kHz or 100 kHz) to calculate the reactive component.
Why do schematics use 'L' for the component but 'H' for the unit?
This follows standard scientific notation conventions. 'L' is the algebraic symbol used to represent the physical property of inductance in equations (like V = L(di/dt)) and is used as the reference designator on PCBs (e.g., L1, L2). 'H' is the symbol for the unit of measurement (the henry), just as 'R' represents resistance on a board while 'Ω' (ohms) is the unit.
Does the physical inductance value in henrys change with AC frequency?
Ideally, no; the physical inductance is determined by the number of wire turns and the core geometry. However, in real-world components, the measured inductance can appear to drop at high frequencies due to core material limitations (ferrite roll-off) and parasitic inter-winding capacitance. Furthermore, if the core material saturates due to excessive DC bias current, the effective inductance in henrys will plummet drastically, often leading to catastrophic failure in switching regulators.
