The Short Answer: What Unit Inductance Is Measured In
Inductance is measured in Henrys (H), named after American scientist Joseph Henry. Because one Henry is a relatively massive amount of inductance for modern electronics, you will almost exclusively work with fractional sub-units on the bench:
- Millihenry (mH): 10-3 Henrys. Common in audio crossovers, line-frequency filters, and large power supply chokes.
- Microhenry (µH): 10-6 Henrys. The standard unit for switched-mode power supply (SMPS) inductors, buck/boost converters, and general-purpose RF chokes.
- Nanohenry (nH): 10-9 Henrys. Used in high-frequency RF circuits, VHF/UHF matching networks, and PCB trace parasitics.
By definition, an inductor has an inductance of 1 Henry when a current change of 1 ampere per second induces an electromotive force (EMF) of 1 volt across its terminals. If you are transitioning from DC resistance (Ohms) to AC impedance, think of inductance as electrical inertia. Just as a heavy mechanical flywheel resists changes in rotational speed, an inductor resists changes in current flow. This fundamental property is why we use them to smooth out current ripple in power supplies and block high-frequency noise in EMI filters.
Bench Setup: Configuring Your LCR Meter for Inductance
Standard digital multimeters (DMMs) measure DC resistance, but they cannot measure inductance. To test coils, you need a dedicated LCR meter (like the popular DER EE DE-5000 or a benchtop Keysight U1733C). An LCR meter applies an alternating current (AC) test signal at a specific frequency and calculates the impedance vector to isolate the inductive reactance. For a deeper look into how these instruments calculate impedance vectors, refer to this guide on LCR meter fundamentals.
Meter Configuration Block
- Dial / Mode: Set to L (Inductance). Ensure the meter is not in C (Capacitance) or R (Resistance) mode.
- Test Frequency: Select 1 kHz for general power inductors and audio chokes. Select 100 kHz for high-frequency SMPS chokes, ferrite beads, and RF components. (Measuring a 100 kHz choke at 120 Hz will yield wildly inaccurate readings due to core permeability variations).
- Lead Jacks / Fixtures: Use the guarded LCR terminals. For axial/radial leaded parts, use Kelvin (4-wire) test clips or SMD tweezers to eliminate lead resistance from the measurement.
- Range: Set to Auto for initial troubleshooting. If the meter struggles to lock, manually step down to the 2 mH or 200 µH range based on the component's physical size.
- Equivalent Circuit Model: Select Series (Ls) for low-impedance power inductors (typically < 100Ω). Select Parallel (Lp) for high-impedance RF chokes and ferrite beads.
Never measure inductance on an energized circuit. While your multimeter's voltage function may be rated for CAT III or CAT IV to safely measure 120V/240V mains, the LCR/inductance function injects its own AC test signal and lacks high-voltage isolation. Probing a live switched-mode power supply (SMPS) or motor drive will instantly destroy the meter's input protection FETs and pose a severe shock hazard. Always de-energize, lock out/tag out, and verify dead with a CAT-rated voltage tester before connecting LCR leads. Furthermore, large filter capacitors must be safely discharged with a bleed resistor, as residual DC voltage will also damage the LCR meter's sensitive bridge circuitry.
Step-by-Step Probe Placement and Measurement Technique
Getting a stable reading requires more than just touching the leads. Inductors are highly susceptible to parasitic effects and environmental interference. Follow this sequence to ensure accuracy:
- Isolate the Component: In-circuit measurements are notoriously unreliable for inductors. Parallel capacitance from the PCB and surrounding traces will skew the reading, often making a good inductor read as a capacitor or throwing an error. Desolder at least one leg of the inductor to lift it off the PCB pad.
- Null the Test Leads: Short your Kelvin clips or SMD tweezers together. Press the REL or Zero button on your LCR meter. This subtracts the residual inductance and resistance of the test leads (which can be 0.1 µH to 0.5 µH—enough to ruin a nanohenry measurement).
- Probe Placement: Attach the Kelvin clips directly to the bare copper wire leads of the inductor, not the solder joints. If using tweezers on an SMD component, grip the metal endcaps firmly and ensure your fingers are not bridging the component, as the capacitance of your skin will alter high-frequency readings.
- Elevate the Component: Hold the inductor at least 2 inches above the bench mat. If your mat has a steel core, or if there are other transformers nearby, the magnetic field from the LCR meter's test signal will couple with the bench, artificially inflating the inductance reading.
- Read and Record: Wait for the primary inductance (L) and the Quality Factor (Q) or Dissipation Factor (D) to stabilize. Record both values.
Mistakes That Give Misleading Readings
- Testing at the Wrong Frequency: An iron-powder core inductor designed for a 500 kHz buck converter will read significantly higher than its rated value if tested at 120 Hz because the core's permeability peaks at lower frequencies before rolling off.
- Ignoring the Q Factor: An inductor might read the correct nominal inductance (e.g., 10 µH) but have a Q factor near zero. This indicates the magnetic core is lossy or the wire enamel has degraded, rendering the part useless for filtering despite the 'correct' L value.
- Magnetic Coupling: Leaving the inductor resting next to a relay or another transformer on the workbench will induce mutual inductance, causing the meter to display erratic, fluctuating values.
Expected Readings: Good vs. Bad Inductor Values
Inductors rarely fail open unless subjected to massive current spikes that melt the internal wire. The most common failure mode is an inter-turn short, where the thin enamel insulation between wire windings breaks down due to heat or voltage transients. This effectively reduces the number of active turns, causing the inductance to drop. For more on how magnetic core materials handle these stresses, review this overview of inductor characteristics.
| Component Type | Nominal Value | Acceptable Range (Good) | Failed Reading (Bad) | Typical Failure Mode |
|---|---|---|---|---|
| SMPS Power Choke (Buck) | 4.7 µH | 4.4 µH – 5.0 µH (±10%) | < 2.5 µH or OL | Inter-winding short drops L; open solder joint yields OL. |
| EMI Line Filter Toroid | 2.5 mH | 2.2 mH – 2.8 mH | OL or < 1.0 mH | Wire fracture from mechanical vibration (OL) or severe thermal degradation. |
| RF Ferrite Bead | 600 Ω @ 100MHz | Z within datasheet curve | 0 Ω (Short) or OL | Cracked ceramic body from board flex (Open) or solder bridge (Short). |
| Contactor / Solenoid Coil | 450 mH | 400 mH – 500 mH | < 200 mH or OL | Moisture ingress shorts turns (drops L); coil burnout from stuck armature (OL). |
Pro-Tip: Always Measure DCR. Inductance is only half the story. After measuring L, switch your standard DMM to the Ohms (Ω) range and measure the DC Resistance (DCR) of the coil. A 4.7 µH power choke should have a DCR between 10 mΩ and 50 mΩ. If your LCR meter reads 4.7 µH but your DMM reads 0.2 Ω, the inductor has shorted turns and will overheat and fail under load.
Frequently Asked Questions About Inductance Measurement
Can I measure inductance with a standard digital multimeter?
Generally, no. Standard DMMs only output a DC test voltage, which allows them to measure resistance but cannot calculate inductive reactance. To measure inductance, you need an LCR meter, a dedicated component tester (like the TC1 or Mega328-based testers), or a high-end bench DMM (like certain Keysight 34465A configurations with the LCR module installed). Some hobbyists use the 'RC/L time constant' method with an oscilloscope and a function generator, but an LCR meter is the only practical tool for quick bench verification.
Why does my inductor read a different value at 120 Hz versus 1 kHz?
This is normal and expected, especially for inductors with magnetic cores (iron powder, ferrite, or laminated steel). The effective permeability of the core material changes with frequency. Additionally, at higher frequencies, the 'skin effect' forces current to the outer edge of the wire, increasing AC resistance, and parasitic capacitance between the windings begins to resonate with the inductance. Always test the component at the frequency closest to its actual operating frequency in the circuit.
What does it mean if inductance is measured in millihenrys but the meter reads 'OL'?
'OL' (Over Limit) on an LCR meter means the impedance is too high for the selected range, or the circuit is open. If you are testing a small signal choke and see OL, the internal wire has likely snapped due to mechanical stress or a manufacturing defect. If you are testing a massive iron-core transformer primary, the inductance might simply exceed the meter's maximum range (often capped at 100H or 1000H on handheld units). Try switching to a lower test frequency (like 120 Hz) to see if the meter can resolve the value.
How do I know if an inductor is shorted if the inductance reading looks normal?
A single shorted turn in a 50-turn inductor might only drop the total inductance by 4%, which falls within the manufacturer's ±10% tolerance. The LCR meter will display a 'good' inductance value. However, that single shorted turn acts as a transformer secondary with a heavy load, creating massive eddy currents that will cause the inductor to overheat and melt in-circuit. To catch this, you must check the Q factor (Quality Factor) on your LCR meter. A shorted turn will cause the Q factor to plummet. If the datasheet specifies a Q of 40 and your meter reads 5, the inductor is internally compromised and must be replaced.
