The Direct Answer: Inductance Is Measured In What Units?
Inductance is measured in Henrys (H), named after American scientist Joseph Henry. Because one full Henry represents a massive amount of magnetic energy storage, practical electronics almost exclusively rely on fractional sub-units. When you are testing components on a bench, you will encounter:
- Millihenrys (mH): $10^{-3}$ Henrys. Common in audio crossovers, line-frequency filters, and low-frequency power supplies.
- Microhenrys (µH): $10^{-6}$ Henrys. The standard for switch-mode power supply (SMPS) buck/boost chokes and general RF filtering.
- Nanohenrys (nH): $10^{-9}$ Henrys. Used in high-frequency RF matching networks, antenna tuning, and GHz-range signal integrity applications.
Understanding the unit is only the first step. Unlike resistance, which is a static DC property, inductance is highly dependent on the test frequency, the AC test voltage, and the magnetic core material. To get a reliable measurement, you need the right meter setup and a strict testing protocol.
Meter Setup and Safety Categories for Inductance Testing
You cannot accurately measure inductance with a standard multimeter unless it has a dedicated 'L' function (which is rare and usually limited in resolution). For reliable bench work, you need a dedicated LCR meter, such as the DER EE DE-5000 or the Keysight U1733C.
Meter Configuration Block
| Parameter | Setting / Position | Reasoning |
|---|---|---|
| Dial / Function | L (Inductance) or Z (Impedance) | Isolates the inductive reactance ($X_L$) from capacitive and resistive vectors. |
| Lead Jacks | COM and V/Ω/L (or 4-terminal Kelvin) | Standard 2-wire for mH; 4-wire Kelvin clips mandatory for µH and nH to eliminate lead resistance/inductance. |
| Test Frequency | 100 Hz / 120 Hz for mH 1 kHz for general µH 10 kHz - 100 kHz for RF nH | Core permeability drops at higher frequencies. Testing a 10mH iron-core choke at 100 kHz will yield a falsely low reading. |
| Test Level (Voltage) | 0.5V to 1.0V RMS | Standard signal level. Higher voltages can saturate small ferrite cores, skewing the reading. |
| Range | Auto (or manual starting at highest) | Prevents meter overload if the inductor is shorted or massively out of spec. |
Step-by-Step Probe Placement and Measurement
Follow this exact sequence to eliminate parasitic errors from your test leads and environment.
- Isolate and Discharge: Remove power. Short the inductor leads briefly with an insulated resistor to ensure no trapped magnetic field or parallel capacitor charge remains.
- Short the Probes (Zeroing): Touch your test probes or Kelvin clips directly together. Press the REL or Zero button on your LCR meter. This subtracts the inherent inductance of your test leads (which is typically 0.5µH to 1.5µH for standard alligator clips).
- Connect to the Test Points:
- Leaded/Radial Inductors: Grip the wire leads as close to the component body as possible. Do not touch the bare metal of the probes with your fingers; your body's capacitance will detune high-frequency (nH) readings.
- SMD Inductors: Use fine-point SMD tweezers or a dedicated SMD Kelvin test fixture. Standard alligator clips will introduce massive parasitic errors on 0805 or 0603 packages.
- Read and Record: Wait for the primary display (Inductance, L) and secondary display (DC Resistance, DCR, or Quality Factor, Q) to stabilize. Record both values.
Expected Readings: Good vs. Bad Inductor Values
An inductor can fail in three ways: an open winding (OL), an inter-turn short (lower inductance), or core degradation (inductance drops under load but looks fine on a bench meter). The table below defines what a good reading looks like numerically for common applications.
| Component Type | Nominal Value | Good Reading (±20% Tol) | Bad Reading / Failure Mode | Expected DCR |
|---|---|---|---|---|
| SMPS Buck Choke (e.g., 4.7µH) | 4.7 µH | 3.76 µH to 5.64 µH | < 2.0 µH (inter-turn short) or OL (broken wire) | 5 mΩ to 25 mΩ |
| Audio Crossover Coil | 1.5 mH | 1.2 mH to 1.8 mH | OL (thermal fuse blown inside winding) | 0.2 Ω to 0.8 Ω |
| EMI Common Mode Choke | 10 mH (per winding) | 8.0 mH to 12.0 mH | Imbalance > 10% between Winding A and Winding B | 0.5 Ω to 2.0 Ω |
| RF Matching Choke (0805) | 100 nH | 95 nH to 105 nH (±5%) | < 50 nH (cracked ceramic core) or OL | < 0.5 Ω |
Common Mistakes That Give Misleading Readings
If your readings are bouncing around or seem physically impossible, you have likely fallen into one of these measurement traps:
- Measuring In-Circuit: This is the most common error. Inductors on a PCB are surrounded by decoupling capacitors and semiconductor junctions. Parallel capacitance creates an LC tank circuit, causing the LCR meter to read a wildly inflated inductance value or throw a phase error. Rule: Always lift at least one leg of the inductor off the pad before measuring.
- Ignoring Test Frequency: Ferrite materials are highly frequency-dependent. If you measure a 100µH RF choke at 120 Hz, the meter might read 150µH due to low-frequency core permeability spikes. Always match your test frequency to the component's operating frequency (check the manufacturer datasheet).
- Forgetting to Zero the Leads: Standard banana-to-alligator test leads have about 1.2µH of self-inductance. If you are measuring a 2.2µH switching regulator choke and forget to hit the REL/Zero button, your meter will read 3.4µH, leading you to falsely reject a good part.
- Magnetic Coupling on Common Mode Chokes: A common mode choke has two windings on the same core. If you measure Winding A while Winding B is left floating, parasitic coupling can skew the result. Short the leads of the unmeasured winding together to stabilize the magnetic field.
Decision Tree: Troubleshooting and Concrete Replacement Picks
When an inductor fails, you cannot simply swap it for any part with the same Henry value. Core material, saturation current ($I_{sat}$), and physical footprint dictate the replacement. Use this decision path to select the exact part you need.
| Diagnostic Symptom | Root Cause | Action / Concrete Replacement Pick |
|---|---|---|
| Reading is OL (Open Loop) | Wire snapped internally or thermal fuse tripped due to overcurrent. | Replace with exact DCR match. For power rails, use the Bourns SRP1265A series (shielded, high $I_{sat}$). For audio, rewind or source a Jantzen Air Core coil. |
| Reading is 50% to 80% of nominal | Inter-turn short. Insulation enamel melted, causing adjacent windings to bypass. | Replace immediately. Do not reuse. For SMPS buck converters, drop in a Wurth Elektronik WE-PD series part with an $I_{sat}$ rating 20% higher than your peak switch current. |
| Reading is nominal, but circuit overheats | Core saturation. The inductor is too small for the DC bias current, causing inductance to collapse to near-zero under load. | Upgrade core material. Switch from standard ferrite to a powdered iron core like the Coilcraft XEL series, which offers a much softer saturation curve and handles high DC bias without collapsing. |
| Reading is nominal, but RF circuit is noisy | Low Quality Factor (Q). The meter reads the right µH/nH, but the DCR is too high for the RF frequency. | Swap to high-Q RF part. Use the Coilcraft 0805CS series (ceramic core, extremely low DCR) instead of standard multilayer ferrite chip inductors. |
By understanding that inductance is measured in Henrys—and more importantly, how those Henrys behave under different test frequencies and bias currents—you can move beyond simple continuity checks and accurately diagnose complex magnetic failures on the bench.
References:
1. Fluke Corporation. "What is Inductance?" fluke.com.
2. All About Circuits. "Inductance and Inductors." allaboutcircuits.com.
