Understanding Inductance Units: Henry, Millihenry, and Microhenry Conversions

Inductance units, measured primarily in henries (H), quantify a component's ability to oppose changes in electrical current by storing energy in a magnetic field. When you select an inductor for a power supply or an audio crossover, you are essentially choosing how much 'electrical inertia' the circuit will possess. Just as a heavy mechanical flywheel resists changes in rotational speed, an inductor resists changes in electron flow, absorbing energy when current rises and releasing it when current falls.

To design or troubleshoot effectively, you must fluently navigate the metric prefixes of inductance. A misunderstanding of these scales is the fastest way to fry a switching transistor or design a filter that completely misses its target cutoff frequency.

The Core Inductance Units and Conversion Table

The base unit is the henry (H), named after Joseph Henry. However, a 1-henry inductor is physically massive and rarely used in modern electronics outside of heavy industrial mains filtering or large audio crossovers. Most bench work involves sub-multiples. Below is the definitive reference for inductance units you will encounter on schematics and component datasheets.

What Inductance Actually Changes in a Real Circuit

Inductance dictates the rate at which current can change over time. The fundamental governing equation is V = L × (di/dt), where V is the induced voltage, L is the inductance in henries, and di/dt is the rate of current change in amperes per second. This relationship means that if you force current through an inductor to stop abruptly, the inductor will generate whatever voltage is necessary to keep that current flowing, often resulting in destructive voltage spikes.

Worked Numeric Example: The Flyback Spike

Let's look at a real-world relay driving circuit. You have a 10 mH (0.01 H) relay coil carrying 2 A of steady-state DC current. When your driving transistor switches off, the current attempts to drop from 2 A to 0 A in just 5 microseconds (0.000005 s).

• di (change in current) = 2 A
• dt (change in time) = 0.000005 s
• L = 0.01 H

Plugging these into the formula:

V = 0.01 × (2 / 0.000005) = 0.01 × 400,000 = 4,000 Volts

Furthermore, inductance determines energy storage. The energy stored in the magnetic field is calculated as E = 0.5 × L × I². In our relay example, E = 0.5 × 0.01 × (2²) = 0.02 Joules (20 mJ). While 20 mJ sounds small, it is more than enough to pit and degrade mechanical relay contacts over time, or punch through the thin silicon oxide layer of a MOSFET.

Where You Meet Inductance Units in Practice

Different domains of electronics operate in entirely different inductance scales, driven by the operating frequencies and power levels involved. According to standard design practices outlined by resources like Electronics Tutorials, matching the inductance unit to the application is critical for efficiency and stability.

Switch-Mode Power Supplies (SMPS) — The µH Domain

If you are designing or repairing a buck or boost converter operating between 100 kHz and 2 MHz, you will live in the microhenry (µH) range. A typical 12V-to-5V buck converter running at 500 kHz requires an inductor between 4.7 µH and 10 µH. At these frequencies, core material matters immensely. You will typically use powdered iron or specialized ferrite blends (like Coilcraft's MSS1210 series) that can handle high DC bias currents without saturating. If the core saturates, the inductance drops to near zero, effectively turning your inductor into a short circuit and destroying the switching IC.

Audio Crossovers and Mains Filtering — The mH Domain

Audio frequencies (20 Hz to 20 kHz) and AC mains frequencies (50/60 Hz) are relatively slow. To create meaningful reactance at these low frequencies, you need high inductance values in the millihenry (mH) range. A passive low-pass crossover for an 8-ohm woofer cutting off at 800 Hz might require a 1.5 mH to 2.5 mH inductor. Because audio signals carry high peak currents, these inductors are often wound with thick enameled copper wire on large ferrite bobbins or use air-cores to completely eliminate saturation and hysteresis distortion.

RF and High-Speed Digital — The nH Domain

In VHF/UHF radio design or high-speed digital routing (like DDR4 memory traces), you deal in nanohenries (nH). At 2.4 GHz (Wi-Fi), a mere 5 nH of inductance presents significant impedance. Here, discrete components are often replaced by the physical geometry of the PCB itself. A standard 1-inch (25.4 mm) trace of 50-ohm microstrip on FR4 fiberglass has a parasitic inductance of roughly 20 nH. RF engineers use precise trace lengths and vias to create tuned matching networks without ever soldering a physical coil.

Common Confusions and Troubleshooting Pitfalls

When diagnosing circuits or reading datasheets, hobbyists and junior technicians frequently conflate related but distinct concepts. Clarifying these is essential for accurate troubleshooting.

Inductance (L) vs. Inductive Reactance (X_L)

This is the most common confusion. Inductance (measured in Henries) is a fixed physical property of the component, determined by its number of wire turns, core material, and physical geometry. It does not change with frequency. Inductive Reactance (measured in Ohms) is the actual opposition to alternating current at a specific frequency, calculated as X_L = 2πfL. A 10 µH inductor has the same inductance at 1 kHz and 1 MHz, but its reactance at 1 MHz is 1,000 times higher than at 1 kHz. As detailed in All About Circuits, treating henries and ohms as interchangeable will lead to catastrophic filter miscalculations.

Inductance vs. Capacitance

While both store energy, they do so in opposite domains. Capacitors store energy in an electric field and oppose changes in voltage. Inductors store energy in a magnetic field and oppose changes in current. If you are trying to smooth out voltage ripple on a DC power rail, you use a capacitor. If you are trying to smooth out current ripple or limit inrush current, you use an inductor.

The Measurement Trap: Testing at the Wrong Frequency

A frequent bench mistake is measuring a switching regulator inductor with a cheap multimeter's built-in inductance range. Most budget DMMs test inductance at 1 kHz or 120 Hz. However, a powdered-iron core inductor designed for a 500 kHz buck converter will exhibit a vastly different permeability (and thus a different inductance value) at 1 kHz than it does at its operating frequency. Furthermore, cheap meters cannot apply a DC bias current during the test. An inductor that reads a perfect 10 µH on your meter might drop to 2 µH when 3 A of DC current is pushed through it in-circuit due to core saturation. For accurate power-stage troubleshooting, you must use a dedicated LCR meter (like the DER EE DE-5000 or Keysight U1733C) capable of testing at 100 kHz+ and verify the component's saturation curve on the manufacturer's datasheet.