If you are asking what are inductors measured in, the base SI unit is the Henry (H). However, because one Henry represents a massive amount of inductance, practical electronics rely on sub-multiples: millihenries (mH), microhenries (µH), and nanohenries (nH). An inductor opposes changes in current, and its measurement tells you how much voltage it will induce per rate of current change ($V = L \cdot di/dt$). While a basic digital multimeter (DMM) can measure an inductor's DC resistance, verifying its actual inductance requires an LCR meter and an understanding of how test frequencies interact with magnetic cores.
The Units of Inductance: Henries and Sub-Multiples
Named after American physicist Joseph Henry, the 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. In practical bench work, you will almost never encounter a 1 Henry inductor outside of massive industrial power factor correction banks or specialized audio crossovers. According to Electronics Tutorials, the physical size of an inductor scales with its inductance and current handling, meaning smaller units dominate modern PCB design.
| Unit Name | Symbol | Scientific Notation | Typical Bench Value | Common Application & Form Factor |
|---|---|---|---|---|
| Henry | H | $10^0$ H | 1H to 10H | Mains filter chokes, tube amplifier power supplies (massive iron laminations) |
| Millihenry | mH | $10^{-3}$ H | 1mH to 500mH | Buck/boost converter power stages, audio crossover networks (toroidal/bobbin cores) |
| Microhenry | µH | $10^{-6}$ H | 1µH to 900µH | Switching regulator SMDs, RF chokes, EMI suppression beads (ferrite drum/shielded) |
| Nanohenry | nH | $10^{-9}$ H | 1nH to 900nH | VHF/UHF impedance matching, RF filters (air-core coils, ceramic multilayer SMDs) |
Meter Setup and Probe Placement for LCR Testing
Standard multimeters cannot measure inductance. You need a dedicated LCR meter, such as a budget-friendly DER EE DE-5000 or a professional Keysight E4980A. Inductance is not a static number; it changes based on the test frequency due to core material permeability roll-off and parasitic winding capacitance. Therefore, configuring your meter correctly is just as critical as the physical connection.
LCR Meter Configuration Block
Dial / Mode: Set to L (Inductance). Ensure the meter is set to measure the series equivalent circuit (Ls) for low-impedance power inductors, or parallel (Lp) for high-impedance RF chokes.
Test Frequency: Set to 100 Hz or 120 Hz for iron/ferrite power inductors and mains chokes. Set to 1 kHz or 10 kHz for signal-path and RF inductors. Testing a 10µH RF choke at 100 Hz will yield highly inaccurate results due to core physics.
Range: Start on Auto-range to find the magnitude, then switch to Manual range (e.g., the 2 mH range) to maximize the meter's ADC resolution and stabilize the reading.
Lead Jacks: Use the dedicated LCR 4-terminal Kelvin jacks if available. If using a 2-wire setup, plug into the standard COM and V/Ω jacks using the shortest possible test leads.
Probe Placement Procedure
- De-energize and Discharge: Turn off the circuit. Inductors in power supplies are often parallel to large filter capacitors. Discharge all capacitors with a high-wattage bleeder resistor before touching the inductor.
- Isolate the Component: Desolder at least one leg of the inductor from the PCB. Measuring in-circuit will parallel the inductor with trace capacitance and IC protection diodes, ruining the reading.
- Zero the Leads: Short the test clips together and press the "Zero" or "REL" button on your LCR meter to subtract the inherent inductance of the copper test leads (which can add 0.5µH to 1.5µH of error).
- Connect Probes: For axial/radial leaded inductors, use Kelvin clips. Clip the current-drive jaws on the outside of the lead, and the voltage-sense jaws on the inside, closer to the component body. This eliminates the resistance of the test leads from the Q-factor calculation.
Expected Readings: Good vs. Bad Inductor Values
What does a good reading look like numerically? A healthy inductor will read within ±10% to ±20% of its stamped or datasheet value, depending on its tolerance band (e.g., J = ±5%, K = ±10%, M = ±20%). However, inductance alone does not tell the whole story. A component can have the correct inductance but still fail under load due to high DC resistance or a degraded core. As noted in the All About Circuits AC textbook, the Quality factor (Q) and DC Resistance (DCR) are vital secondary measurements.
| Parameter | Good Reading (Numeric Expectation) | Bad Reading | Probable Failure Mode |
|---|---|---|---|
| Inductance (L) | Within ±10% of spec (e.g., 95µH for a 100µH part) | Reads 0.00, OL (Infinite), or wildly out of spec (e.g., 2µH) | Internal winding short (drops L), broken wire (OL), or cracked ferrite core. |
| DC Resistance (DCR) | Low, matching wire gauge (e.g., 0.05Ω to 2.5Ω) | Infinite (OL) or significantly higher than datasheet (e.g., 50Ω) | Broken winding, corroded terminal cap, or poor internal solder joint. |
| Quality Factor (Q) | High (e.g., >40 for RF coils, >15 for power chokes at test freq) | Drops below 5 or reads negative | Inter-winding short, core saturation, or severe parasitic capacitance dominating. |
When testing common-mode chokes on AC input stages, you must measure the two windings independently. A good common-mode choke will show identical inductance on both windings (e.g., 10mH ± 5%). If one winding reads 10mH and the other reads 2mH, the core is cracked or one winding is partially shorted, which will destroy the EMI filtering and potentially cause asymmetric heating.
Safety Categories and Mistakes That Skew Readings
When working with inductors pulled from mains-powered equipment, safety and measurement integrity must be managed simultaneously. If you are probing an inductor in a live circuit (which is not recommended for LCR testing, but sometimes done with specialized oscilloscopes and current probes), your meter and test leads must carry the appropriate Safety Category rating. According to Fluke's safety guidelines, measuring at the power supply input requires CAT II rated equipment, while measuring at the building distribution panel level requires CAT III. However, for accurate LCR bench measurements, the circuit must be dead, and the component should be isolated.
Common Mistakes That Give Misleading Readings
- Measuring In-Circuit: PCB traces, bypass capacitors, and semiconductor junctions create parallel impedance paths. An inductor that reads as a 0.5Ω short in-circuit might be perfectly healthy once desoldered. Always lift one leg.
- Using the Wrong Test Frequency: Ferrite materials are highly frequency-dependent. A 10mH EMI choke designed to suppress 100 kHz noise might only read 4mH if your LCR meter tests it at 1 MHz because the core's permeability has rolled off. Always match the test frequency to the component's intended operating band.
- Magnetic Bench Interference: Inductors are magnetic antennas. If you hold a microhenry RF coil directly over a steel workbench, near a soldering iron transformer, or next to a speaker magnet, the external magnetic field will alter the core's permeability, skewing the reading by 5% to 15%. Test in free air.
- Ignoring Lead Inductance: At the nanohenry scale, the test leads themselves act as inductors. A standard pair of 12-inch multimeter leads possesses roughly 150nH to 200nH of self-inductance. If you are measuring a 10nH VHF matching coil without using 4-terminal Kelvin fixtures and performing a short-circuit zero calibration, your reading will be 2000% off.
Understanding what inductors are measured in is only the first step. Mastering the physical setup of your LCR meter, respecting the magnetic environment of your workbench, and interpreting the DCR and Q-factor alongside the Henry value will ensure you accurately diagnose power supply failures, RF mismatches, and audio crossover faults.
