What Is the Unit of Measurement for Inductance? Testing Guide

The standard unit of measurement for inductance is the Henry (H), named after American scientist Joseph Henry. One Henry is defined as the inductance of a closed circuit that produces an electromotive force of one volt when the electric current in the circuit varies uniformly at a rate of one ampere per second. In practical bench and jobsite electronics, a full Henry is a massive amount of inductance. You will almost exclusively work with sub-multiples: millihenries (mH, 10^-3 H), microhenries (µH, 10^-6 H), and nanohenries (nH, 10^-9 H).

While a standard digital multimeter (DMM) can measure the DC resistance (DCR) of an inductor's wire to check for shorts or opens, it cannot measure the actual inductance. To measure Henrys, you must use a dedicated LCR meter, which injects an AC test signal at a specific frequency to calculate the component's reactive impedance. Below is your complete guide to the units, meter setups, and expected readings for testing inductors in the real world.

The Henry: Understanding Inductance Units in Practice

Inductance is the property of a conductor that opposes a change in current flow, storing energy in a magnetic field. The physical size and core material of an inductor dictate its value in Henrys. When reading schematics or datasheets, you will see the unit expressed with standard SI prefixes. Confusing a microhenry (µH) with a millihenry (mH) is a common mistake that will result in a circuit failing to oscillate or a power supply collapsing under load.

To anchor these units in reality, here is a data-dense reference table mapping common inductor applications to their typical unit ranges, physical forms, and the required LCR test frequencies. Matching your meter's test frequency to the inductor's intended operating frequency is critical for an accurate reading.

Table 1: Inductance Units by Application and Test Parameters

Application	Typical Range	Primary Unit	Physical Form Factor	LCR Test Frequency
Switch-Mode Power Supply (SMPS) Output Filter	1.0 to 100	µH (microhenries)	Ferrite drum core, shielded SMD	1 kHz or 100 kHz
Mains EMI Common Mode Choke	1.0 to 50	mH (millihenries)	Toroidal high-permeability core	100 Hz or 120 Hz
RF Choke (VHF/UHF Transmitters)	10 to 500	nH (nanohenries)	Air core or ceramic multilayer	1 MHz or higher
Audio Crossover Network	0.5 to 5.0	mH (millihenries)	Iron powder or laminated steel core	1 kHz
Precision Timing / Resonant Tank	100 to 470	µH (microhenries)	Shielded bobbin, tight tolerance	10 kHz to 100 kHz

As noted in the All About Circuits inductor guide, the core material heavily influences how the inductor behaves at different frequencies. An iron-powder core choke rated for 10 mH at 120 Hz might read drastically lower if your LCR meter tests it at 100 kHz due to core losses and parasitic capacitance.

Meter Setup and Probe Placement for Inductance Testing

Standard multimeters like the Fluke 87V lack the internal AC bridge circuitry required to measure inductance. You need an LCR meter, such as the budget-friendly Der EE DE-5000 or the professional-grade Keysight U1733C. Setting up the meter incorrectly is the number one cause of misleading readings on the bench.

Probe Placement and Execution Steps

1. Isolate the Component: Never measure an inductor in-circuit. Parallel PCB traces, bypass capacitors, and semiconductor junctions will skew the phase angle, giving you a wildly inaccurate inductance value. Desolder and lift at least one leg of the component.
2. Zero the Meter: Short your test probes together and press the 'Zero' or 'Null' button on the LCR meter to subtract the lead inductance (which can be 0.5 µH to 2 µH on its own—massive when measuring nanohenry RF chokes).
3. Connect Probes: For through-hole inductors with thick wire, standard alligator clips are fine. For SMD inductors or low-DCR power chokes, use Kelvin clips (4-wire measurement) to separate the current-carrying and voltage-sensing paths, eliminating lead resistance errors.
4. Read and Record: Wait for the reading to stabilize (usually 2-3 seconds). Note both the primary inductance value (L) and the Quality Factor (Q) or Dissipation Factor (D) if your meter displays it.

Expected Readings: Good vs. Bad Inductor Values

Knowing what a good reading looks like numerically requires understanding the manufacturer's tolerance. Unlike 1% precision resistors, inductors are inherently sloppy components. A standard ferrite drum core inductor might have a ±20% tolerance, while a precision molded choke might be ±5%. Furthermore, a 'good' inductance reading doesn't guarantee the part is healthy; you must also check the DC Resistance (DCR) to ensure the internal wire hasn't partially shorted.

Table 2: Expected Readings and Fault Diagnostics

Component Type	Nominal Value	Good Reading (L)	Bad Reading (Fault Mode)	DCR Check (DMM)
SMPS Toroidal Choke	47 µH (±15%)	40.0 µH to 54.0 µH	< 35 µH (Core cracked / saturation damage)	0.05Ω to 0.2Ω (Open = Bad)
Mains EMI Filter Choke	10 mH (±20%)	8.0 mH to 12.0 mH	> 15 mH (Moisture ingress altering core µ)	2.0Ω to 15.0Ω (Shorted turns = < 1Ω)
RF Ceramic SMD Choke	100 nH (±5%)	95 nH to 105 nH	150+ nH (Parasitic shift from pad damage)	< 0.5Ω (Open = OL on DMM)
Audio Crossover Coil	1.5 mH (±10%)	1.35 mH to 1.65 mH	0.8 mH (Shorted winding layers)	0.2Ω to 0.8Ω (High DCR = corroded wire)

Mistakes That Give Misleading Readings

If your LCR meter is spitting out numbers that don't match the schematic, check these common traps:

• Testing at the Wrong Frequency: Measuring a 10 µH RF choke at 120 Hz will often yield an error or a massively inflated number because the meter's test signal is too far below the component's self-resonant frequency (SRF).
• Ignoring Lead Inductance: If you are measuring a 5 nH inductor using 6-inch test leads without zeroing the meter, the leads themselves will add 15 nH of inductance, making your reading 300% too high.
• Magnetic Interference: Holding the inductor too close to a steel bench vise, a soldering station transformer, or another active inductor will couple external magnetic fields into the component, shifting the reading erratically.
• Core Saturation Bias: Standard LCR meters use a very low AC test voltage (usually 0.5V to 1V). They cannot apply the DC bias current that an inductor sees in a real SMPS circuit. An inductor might read 47 µH on the bench, but drop to 10 µH under a 5A load in-circuit due to core saturation. (Advanced bench LCR meters have a DC bias port to simulate this, but handhelds do not).

Safety Categories and Live Circuit Protocols

Inductors are frequently found in high-voltage environments, such as the input EMI filters of AC-DC power supplies, motor drive inverters, and fluorescent lamp ballasts. This introduces a critical safety boundary regarding CAT ratings.

When working with inductors in mains-connected equipment (like a 120V/240V AC line filter choke), you are operating in a CAT II or CAT III environment. According to Fluke's safety measurement guidelines, your workflow must strictly follow this sequence:

1. De-energize and Lock Out: Turn off the mains power and unplug the equipment. If hardwired, lock out the breaker.
2. Verify Dead with a CAT-Rated DMM: Use a CAT III or CAT IV rated multimeter (like a Fluke 117 or 87V) to verify that zero voltage exists across the inductor terminals and the surrounding bus.
3. Discharge Stored Energy: Inductors can ring and generate high-voltage spikes when current is interrupted. More importantly, the capacitors in the surrounding filter network will hold a lethal charge. Use a properly rated discharge resistor tool to bleed the bus capacitors before touching the board.
4. Isolate and Test: Once the board is verified dead and discharged, desolder the inductor and use your low-voltage LCR meter to take your measurements.

By understanding that the Henry is a reactive unit requiring AC excitation to measure, and by respecting the boundary between low-voltage test gear and high-voltage environments, you can accurately diagnose inductor faults without damaging your equipment or yourself.