The Inductance Unit of Measure: Henrys in the Real World

The base inductance unit of measure is the Henry (H), named after Joseph Henry. By definition, one Henry is 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 the rate of one ampere per second. However, on the electronics workbench, a 1 Henry inductor would be the size of a microwave oven. In practical circuit design and repair, you will almost exclusively deal in fractional units: millihenrys (mH), microhenrys (µH), and nanohenrys (nH).

The conversion ladder is strictly base-1000:

  • 1 Henry (H) = 1,000 millihenrys (mH)
  • 1 millihenry (mH) = 1,000 microhenrys (µH)
  • 1 microhenry (µH) = 1,000 nanohenrys (nH)

Understanding these prefixes is critical because a misread decimal on your meter can lead you to replace a 100µH switching power supply (SMPS) choke with a 100mH line filter, which will immediately saturate and fail under load. While a standard digital multimeter (DMM) can measure the DC resistance (DCR) of an inductor's wire, it cannot measure inductance. For that, you need a dedicated LCR meter or a DMM with a specific inductance testing function, which applies an AC test signal to calculate the reactance.

Meter Setup and Probe Placement for Inductance Testing

Measuring inductance accurately requires isolating the component from the rest of the circuit. Parallel traces, capacitors, and semiconductor junctions on a PCB will completely skew the AC test signal, giving you a meaningless reading.

Safety Category (CAT) Warning: Always measure inductance out-of-circuit. However, before you desolder the inductor, you must verify the board is de-energized and bulk capacitors are discharged. If you are probing a mains-derived SMPS board to verify it is dead, your multimeter must be rated CAT III 1000V or CAT IV 600V to survive accidental contact with live transients. Never use a cheap, unrated meter for mains-adjacent voltage verification.

Meter Setup Block

Configure your LCR meter (e.g., DER EE DE-5000 or Keysight U1733C) using the following baseline settings for general power inductors:

  • Dial / Mode: Select 'L' (Inductance). Ensure the meter is not in 'C' (Capacitance) or 'R' (Resistance).
  • Test Frequency: Set to 1 kHz for general audio and line-frequency chokes. Set to 10 kHz or 100 kHz if you are testing ferrite-core SMPS inductors, as their permeability characteristics change at higher frequencies.
  • Test Signal Level: 1 Vrms is standard. For high-current power inductors, some advanced meters allow a DC bias current; without it, the core may be unsaturated during testing but saturate in real operation.
  • Range: Use Auto-Range for initial sweeps. If the reading is fluctuating, lock the manual range to the expected decade (e.g., the 200µH range for a 47µH part).
  • Lead Jacks: Use the dedicated 4-wire Kelvin terminals if your meter supports them. This eliminates the resistance and inductance of the test leads from the measurement.

Probe Placement

For 2-wire testing, clip the probes directly to the inductor's bare metal terminals. If the inductor has thick leads, scrape away any enamel or oxidation to ensure a solid metal-to-metal connection. For surface-mount device (SMD) inductors, use fine-point tweezers probes and press firmly on the end caps. Before measuring, short the probes together and use the meter's 'Relative' (REL) or 'Zero' function to null out the parasitic inductance of the leads (typically 30nH to 80nH).

Expected Readings: Good vs. Bad Inductor Values

An inductor can fail in three primary ways: an open winding (broken wire), a shorted turn (insulation breakdown between wire wraps), or terminal corrosion. A good inductor will read very close to its nominal stamped value (usually within a ±10% to ±20% tolerance) and exhibit a low DC Resistance (DCR).

Expected Inductance and DCR Readings by Application
Inductor Type Nominal Inductance Expected Good DCR Bad Reading (Fault Indicated)
SMPS Power Choke 10µH – 100µH < 0.05Ω (very low) OL (Open) or Inductance reads 50% of nominal (Shorted turns)
AC Line Filter 1mH – 10mH 1.0Ω – 8.0Ω < 0.5mH (Massive inter-turn short) or OL (Blown thermal fuse inside)
RF Choke / Tuned 10nH – 500nH < 1.0Ω Wildly fluctuating L value or OL (Hairline fracture in fine wire)
Motor Stator Winding 5mH – 50mH 0.5Ω – 5.0Ω Phase-to-phase imbalance > 5% (Shorted turns in one coil)

The most insidious failure is the inter-turn short. If the thin enamel insulation between two adjacent wraps of wire melts, the current bypasses the rest of the coil. The DC resistance might only drop by a fraction of an ohm—often too small for a standard DMM to reliably detect—but the inductance will plummet dramatically because the magnetic field generation is proportional to the square of the number of turns ($L \propto N^2$). A 100µH inductor with a single shorted turn might drop to 60µH, which is an immediate fail for a switching regulator.

Common Mistakes That Give Misleading Inductance Readings

Even with a high-end bench meter, operator error can make a perfectly good inductor look defective, or mask a fatal flaw. Watch out for these bench traps:

Mistake 1: Measuring In-Circuit. If you measure an inductor while it is still soldered to the PCB, the meter's AC test signal will travel through parallel bypass capacitors and transformer windings. You will likely read a massive, incorrect inductance value, or the meter will simply display an error. Always desolder at least one leg of the component.

Mistake 2: Ignoring Test Frequency. Inductance is not a static number; it changes with frequency due to core material properties and parasitic self-capacitance. A ferrite bead that reads 600Ω of impedance at 100 MHz might only exhibit a few nanohenrys of actual inductance at 1 kHz. Always match your meter's test frequency to the inductor's intended operating frequency band. Refer to the Keysight LCR Measurement Basics guide for deep-dive frequency dependencies.

Mistake 3: Forgetting to Zero the Leads. At the nanohenry scale, the test leads themselves act as an inductor. If you are measuring a 15nH RF choke and you forget to short the probes and hit the 'REL' button, the meter will add the 40nH of the leads to your reading, telling you the part is 55nH. You will throw away a good part based on bad math.

Mistake 4: Core Saturation from Test Voltage. Some high-permeability toroidal cores will saturate if the LCR meter applies too high of an open-circuit test voltage. When the core saturates, its permeability drops, and the measured inductance artificially collapses. If a part reads low, check the datasheet to see if it requires a specific low-voltage test signal.

Decision Path: Diagnosing and Replacing Faulty Inductors

When you are troubleshooting a dead power supply or a failing motor drive, use this decision tree to determine your next move. Do not guess; follow the meter's data to a concrete conclusion.

Symptom / Meter Reading Diagnosis Action & Concrete Replacement Pick
L reads 'OL' (Overlimit)
DCR reads 'OL'		 Open winding. The internal wire has snapped, or the thermal fuse embedded in the winding has blown due to overcurrent. Replace. The part is dead. For a standard 100µH SMPS shielded power inductor replacement, default to the Bourns SRP1265A-100M (100µH, 14A saturation current, shielded to prevent EMI).
L reads 40% to 80% of nominal
DCR reads slightly lower than spec		 Inter-turn short. The enamel insulation has broken down, bypassing a section of the coil and destroying the magnetic field geometry. Replace immediately. Do not reuse. If the exact OEM part is unavailable, match the inductance (µH) and ensure the new part's Isat (saturation current) exceeds the original. The Wurth Elektronik 7443538100 series is an excellent high-current flat-wire alternative.
L reads nominal (e.g., 47µH)
DCR reads 5x higher than spec		 Terminal corrosion or a micro-fracture at the solder joint. The copper wire is intact, but the connection to the end-cap is degrading, causing massive $I^2R$ heating. Resolder or Replace. If it's a through-hole part, desolder, clean the leads with flux, and reflow. If it's an SMD part with corroded end-caps, replace with a fresh TDK SPM6530T-470M.
L reads nominal
DCR reads nominal (e.g., 0.02Ω)		 The inductor is electrically healthy. The fault lies elsewhere in the circuit (likely a shorted switching MOSFET or a failed output capacitor). Stop. Put the inductor back in the circuit. Move your oscilloscope probes to the switching node to check for gate drive signals.

By strictly adhering to the correct inductance unit of measure conversions, zeroing your test leads, and measuring out-of-circuit at the appropriate frequency, you eliminate the guesswork from magnetics troubleshooting. When the meter indicates an open or a shorted turn, trust the data, pull the part, and drop in a verified, high-current shielded replacement like the Bourns SRP series to get the board back online.