The SI unit of measure for inductance is the Henry (H), named after American scientist Joseph Henry. One Henry is defined as the inductance of a closed circuit in which an electromotive force of one volt is produced when the electric current in the circuit varies uniformly at a rate of one ampere per second. Because one full Henry represents a massive amount of magnetic energy storage, practical electronics almost exclusively rely on fractional sub-units: millihenries (mH), microhenries (µH), and nanohenries (nH).
Understanding the unit is only the first step. On the bench, knowing what the unit of measure for inductance is doesn't help you if you cannot accurately verify a component's value. Standard digital multimeters (DMMs) cannot measure inductance; they only measure DC resistance and basic AC/DC voltage. To measure Henrys, you need a dedicated LCR (Inductance, Capacitance, Resistance) meter. Below is the exact reference data and bench procedure for measuring inductance, diagnosing faults, and avoiding the parasitic traps that give misleading readings.
The Henry and Its Practical Sub-Units
When reading a schematic or a component datasheet, you will rarely see a whole number followed by 'H'. The sub-units dictate the physical size of the component and its intended application. According to the NIST Guide to the SI, metric prefixes scale the base unit of the Henry linearly. Here is how those units translate to real-world bench components.
| Unit Name | Symbol | Multiplier (Henry) | Typical Component Form Factor | Common Application |
|---|---|---|---|---|
| Henry | H | 1 | Massive iron-core line reactors | 480V 3-phase motor drives, harmonic filtering |
| Millihenry | mH | 10-3 (0.001) | Toroidal or E-core chokes | Audio crossover networks, mains EMI filters |
| Microhenry | µH | 10-6 (0.000001) | Ferrite drum cores, SMD shielded coils | Switch-mode power supplies (SMPS), buck converters |
| Nanohenry | nH | 10-9 (0.000000001) | Air-core coils, ceramic multilayer chips | RF matching networks, VHF/UHF antennas |
LCR Meter Setup and Probe Placement
Measuring inductance requires applying an AC test signal to the component and calculating the phase shift between voltage and current. If you use the wrong test frequency or the wrong equivalent circuit model, your Henry reading will be useless. For general bench work, an instrument like the DER EE DE-5000 or the Keysight U1733C is standard.
Meter Setup Block
- Function/Dial Position: Set to L (Inductance). Do not use C or R.
- Equivalent Model: Select Ls (Series Inductance) for low-impedance components (typical power inductors < 100mH). Select Lp (Parallel Inductance) for high-impedance components (RF chokes, high-value filter inductors).
- Test Frequency: Set to 1 kHz for general purpose mH/µH inductors. Use 100 Hz for large iron-core line reactors. Use 100 kHz for SMPS ferrite power inductors to simulate actual switching conditions.
- Range: Set to Auto initially. If the reading jumps erratically, lock it to the expected manual range.
- Lead Jacks: Use the dedicated LCR terminal block, not the standard banana jacks used for voltage.
Probe Placement and Fixture Selection
For through-hole inductors with wire leads, standard alligator clips introduce variable contact resistance that ruins the DCR (DC Resistance) and Q-factor readings. You must use Kelvin (4-wire) test clips. The Kelvin clips separate the current-forcing path from the voltage-sensing path at the very tip of the jaw, eliminating lead resistance from the measurement.
Numbered Test Sequence:
- Power off the LCR meter and connect the Kelvin clip fixture.
- Short the tips of the Kelvin clips together and run the meter's Short Zero calibration to null out the fixture's residual inductance (usually around 0.1 µH to 0.5 µH).
- Isolate the inductor from the circuit. Never measure inductance in-circuit (see mistakes section below).
- Clamp the jaws directly onto the component leads, as close to the component body as physically possible.
- Read the primary display for Inductance (L) and the secondary display for DCR or Q-factor.
Expected Readings: Good vs. Bad Inductors
What does a good reading look like numerically? A healthy inductor will display an inductance value within its stated tolerance (typically ±10% to ±20% for ferrite power inductors, and ±1% to ±5% for precision RF coils). Furthermore, the DCR should be low, and the Q-factor (Quality factor) should be high. If an inductor has suffered thermal damage or core saturation cracking, the inductance might read nominally correct, but the DCR and Q-factor will expose the fault.
According to Fluke's guidelines on LCR meter basics, evaluating the secondary parameters is just as critical as the primary Henry reading.
| Parameter | Good Reading (Pass) | Bad Reading (Fail/Replace) | Physical Failure Mode |
|---|---|---|---|
| Inductance (Ls) | 42.3 µH to 51.7 µH (within ±10%) | < 35 µH or > 60 µH | Core air-gap shift, cracked ferrite, or shorted internal turns. |
| DCR (DC Resistance) | 0.05 Ω to 0.15 Ω | > 0.50 Ω or Open (OL) | Corroded lead joint, thermal stress fracture, or burnt wire. |
| Q-Factor | > 40 (at 100 kHz) | < 15 | High core losses, moisture ingress, or degraded winding insulation. |
| ESR (Eq. Series Res.) | < 0.20 Ω | > 1.0 Ω | Skin effect anomalies, proximity effect losses, or core degradation. |
Measurement Mistakes and Safety Categories
Inductance is highly sensitive to its environment. A reading that looks 'bad' might actually be a perfectly good inductor being measured incorrectly. Here are the most common mistakes that yield misleading Henry values:
- Measuring In-Circuit: This is the most common bench error. If the inductor is soldered into a PCB, parallel PCB traces, bypass capacitors, and semiconductor junctions will create alternative AC current paths. This will drastically lower the measured inductance and skew the DCR. Always desolder at least one leg of the inductor before testing.
- Ignoring Test Frequency: Inductance is not a static number; it changes with frequency due to the magnetic permeability of the core material. A ferrite core inductor might measure 100 µH at 120 Hz, but only 45 µH at 100 kHz. Always match your LCR meter's test frequency to the operating frequency of the circuit.
- Magnetic Coupling: If you are testing an inductor while it is sitting next to another magnetized component, a transformer, or even a steel bench mat, the external magnetic field will alter the permeability of the core, shifting your reading. Keep the test area clear of ferrous metals.
- Using Lp instead of Ls: If you measure a low-impedance power inductor using the Parallel (Lp) model, the meter's math will factor in the parallel parasitic capacitance of the windings, giving you a falsely inflated inductance reading. Stick to Ls for power components.
Safety Category (CAT) and Mains Warning
When measuring small signal inductors on a PCB, your environment is CAT I. However, if you are testing line reactors, mains EMI chokes, or ballast inductors connected to 120V/240V/480V systems, you are operating in a CAT III or CAT IV environment.
Mandatory Procedure: Never connect an LCR meter to an energized circuit. LCR meters inject their own sensitive AC test signals and will be instantly destroyed (and potentially explode) if subjected to mains voltage. You must de-energize the panel, apply Lockout/Tagout (LOTO), and verify the circuit is dead using a properly rated CAT III/IV voltage tester before disconnecting the reactor for LCR testing. Local electrical codes (NEC-style guidance) dictate that live panel work requires a licensed electrician; your local AHJ has final authority.
By understanding that the Henry is a dynamic, frequency-dependent unit rather than a static resistance value, you can move beyond simply checking for continuity. Setting up your LCR meter with the correct frequency, utilizing Kelvin connections, and evaluating the DCR alongside the primary inductance reading will allow you to accurately diagnose power supply failures, filter degradation, and RF mismatches on the bench.
