Inductance is measured in Henrys (H), named after American scientist Joseph Henry. Because one full Henry represents a massive amount of inductance rarely seen outside of heavy industrial machinery, practical electronics almost exclusively use fractional sub-units: millihenrys (mH, 10⁻³), microhenrys (µH, 10⁻⁶), and nanohenrys (nH, 10⁻⁹). To measure these values accurately, you cannot use a standard multimeter; you must use a dedicated LCR meter capable of applying an AC test signal to calculate the component's reactance.
The Henry: Understanding Inductance Units and Scale
By definition, an inductor has an inductance of 1 Henry when a current changing at a rate of 1 ampere per second induces a voltage of 1 volt across it ($V = L \times di/dt$).
Think of inductance as the electrical equivalent of mechanical inertia. Just as a heavy flywheel resists changes in rotational speed, an inductor resists changes in current flow. If you try to stop the current instantly, the magnetic field collapses and generates a massive voltage spike to keep the current moving. For a concrete bench example: if you switch off a 10mH relay coil carrying 100mA in just 1 microsecond, the induced voltage spike is $V = 0.01 \times (0.1 / 0.000001) = 1,000V$. This is why flyback diodes are mandatory across relay coils.
The Inductance Scale in Practice
- Nanohenrys (nH): Used in RF circuits, VHF/UHF filters, and high-speed switching power supplies (e.g., 10nH to 500nH).
- Microhenrys (µH): The standard for DC-DC buck/boost converters, EMI chokes, and audio crossovers (e.g., 1µH to 500µH).
- Millihenrys (mH): Found in mains-frequency ballasts, large solenoid valves, and low-frequency filtering (e.g., 1mH to 100mH).
- Henrys (H): Rare in discrete components, but common when describing the magnetizing inductance of large power transformers.
Meter Setup and Probe Placement for Inductance Testing
Meter Setup Block
- Dial / Function: Set the meter to L (Inductance). Do not leave it on Auto if you are troubleshooting, as the meter might accidentally lock onto the DC resistance (DCR) instead.
- Test Frequency: Set to 1 kHz for general-purpose inductors (µH range). If measuring large iron-core chokes (>10mH), drop the frequency to 100 Hz or 120 Hz to mimic their actual operating conditions and avoid core saturation skewing the reading.
- Equivalent Circuit Mode: Select Series (Ls) for low-impedance inductors (most power and RF chokes). Select Parallel (Lp) only for high-impedance, low-value RF inductors where parasitic capacitance dominates.
- Lead Jacks & Range: Plug leads into the dedicated LCR terminals. Use Auto-ranging for initial sweeps, but switch to Manual range (e.g., the 20µH range) to stabilize the display and reduce noise when logging values.
Probe Placement and Technique
For through-hole inductors, use Kelvin (4-terminal) test clips if your meter supports them. Standard alligator clips introduce lead resistance that can skew the Quality Factor (Q) reading, though the primary inductance (L) reading will remain mostly accurate. Place the probes directly on the component leads, not on the PCB pads.
Never measure inductance in-circuit. Parallel PCB traces, bypass capacitors, and semiconductor junctions will create alternative current paths, completely invalidating the LCR meter's AC test signal and yielding wildly inaccurate (usually much lower) readings.
Expected Readings: Good vs. Bad Inductor Values
Inductors rarely fail 'open' unless subjected to extreme overcurrent that melts the internal wire. The most common failure mode is shorted turns, where the thin enamel insulation between wire windings breaks down due to heat or voltage spikes. This reduces the total number of active turns, drastically dropping the inductance value.
| Component Type | Nominal Value | Typical Tolerance | Good Reading (Pass) | Bad Reading (Fail / Shorted Turns) |
|---|---|---|---|---|
| SMD Buck Inductor (e.g., 4.7µH) | 4.7 µH | ±20% | 3.8 µH to 5.6 µH | < 2.0 µH or OL (Open) |
| EMI Common Mode Choke | 10 mH | ±30% | 7.5 mH to 13.0 mH | < 5.0 mH (inter-winding short) |
| RF Air-Core Coil | 150 nH | ±5% | 142 nH to 158 nH | < 120 nH (deformed coil) |
| Iron-Core Mains Ballast | 2.5 H | ±15% | 2.1 H to 2.8 H | < 1.5 H (lamination/insulation failure) |
Mistakes That Give Misleading Readings
- Ignoring DCR (DC Resistance): A reading of '0.5 Ω' on your LCR meter isn't inductance; it's the wire's DC resistance. Always verify you are reading 'L', not 'R'.
- Test Frequency Mismatch: Measuring a 50Hz iron-core transformer primary at 100kHz will yield a falsely low inductance reading due to high-frequency core losses and eddy currents.
- Magnetic Interference: Holding the inductor too close to a steel bench vise, a soldering station transformer, or another powered inductor will couple external magnetic fields into your test, causing the reading to fluctuate wildly.
Safety Categories (CAT Ratings) and Mains Verification
To verify the absence of voltage on mains-connected inductive components, your verification multimeter (e.g., a Fluke 117 or 87V) and its test leads must carry a minimum CAT III 600V or CAT IV 600V safety rating. This ensures the meter can safely absorb transient voltage spikes (like inductive kickback from neighboring equipment on the same bus) without arcing across the internal PCB. Once verified dead and locked out, you may safely use your LCR meter (which is typically unrated for CAT environments, as it is a low-voltage bench instrument) to measure the inductance.
Frequently Asked Questions
What is inductance measured in on a standard multimeter?
It isn't. A standard digital multimeter lacks the internal AC oscillator and phase-detection circuitry required to measure inductance. If you place an inductor across a standard DMM's ohms (Ω) setting, you are only measuring the DC Resistance (DCR) of the copper wire. While a DCR reading of '0.0 Ω' indicates a dead short, and 'OL' indicates a broken wire, a normal DCR reading (e.g., 1.2 Ω) tells you absolutely nothing about whether the inductor's actual Henry value is intact or if it suffers from shorted turns.
What is the unit of inductance in the SI system?
The base SI (International System of Units) derived unit for inductance is the Henry (H). In SI base units, it can be expressed as $kg \cdot m^2 \cdot s^{-2} \cdot A^{-2}$. For practical engineering and circuit analysis, we use the sub-multiples (mH, µH, nH) to avoid writing excessive strings of leading zeros.
Why does my inductor read a different value at 100 Hz versus 1 kHz?
Inductance is not a perfectly static number; it varies with frequency due to the magnetic core material's permeability curve and parasitic winding capacitance. Ferrite cores designed for switching power supplies (operating at 100kHz+) will often read artificially high at 100Hz because the core is fully saturated by the low-frequency test signal. Always match your LCR meter's test frequency to the component's intended operating frequency for the most accurate troubleshooting data.
What is a good Q factor reading for an RF inductor?
The Quality Factor (Q) is the ratio of inductive reactance to resistive loss ($X_L / R$). For standard power inductors, Q is largely ignored. However, for RF tank circuits and antenna matching networks, you want a high Q to minimize signal loss. A good Q factor for an RF air-core or ceramic inductor at VHF frequencies is typically between 50 and 150. If your LCR meter shows a Q below 20 at the target RF frequency, the component will introduce unacceptable insertion loss into your filter or oscillator circuit.
