Inductance is measured by applying an alternating current (AC) test signal to the component and analyzing the voltage-current phase shift using an LCR (Inductance, Capacitance, Resistance) meter. The meter calculates the reactive impedance ($X_L = 2\pi fL$) to derive the inductance value in Henries (H). For accurate bench work, you must match the meter's test frequency to the inductor's intended operating frequency—typically 1 kHz for power inductors (>100 µH) and 100 kHz for RF or high-speed switching inductors (<10 µH).
Reference Data: Expected Inductance Values by Application
Before probing, you need to know what the meter should display. An inductor's value shifts based on test frequency, core material, and DC bias. Powdered iron cores (often identified by Micrometals color codes like yellow/white -26) exhibit a rolling drop in inductance under DC bias, whereas ferrite cores maintain flat inductance until a sharp saturation cliff. Below is a data-dense reference for common inductor types you will encounter on the bench.
| Application / Example Part | Nominal Value | Test Frequency | Typical DCR | Expected Q Factor |
|---|---|---|---|---|
| 5V Buck Converter (e.g., Coilcraft MSS1260) | 4.7 µH | 100 kHz | 12.5 mΩ | > 40 |
| PFC Choke (e.g., Würth Elektronik 744774) | 330 µH | 1 kHz | 0.45 Ω | > 50 |
| Audio Crossover (e.g., Jantzen Air Core) | 1.5 mH | 1 kHz | 0.28 Ω | > 80 |
| RF Matching Network (e.g., Murata LQG15) | 10 nH | 100 MHz | 0.15 Ω | > 60 |
| Common Mode EMI Choke (e.g., TDK ACM2012) | 90 µH | 1 kHz | 1.2 Ω | N/A (measured as Z) |
LCR Meter Setup and Probe Placement Protocol
Getting a stable, repeatable reading requires precise meter configuration and proper fixture compensation. A standard multimeter cannot measure inductance; you need a dedicated LCR meter like the Keysight U1733C or BK Precision 879B.
Meter Setup Block
- Dial Position / Mode: Set to 'L' (Inductance). Toggle to 'Ls' (Series equivalent) for low-impedance inductors (<100 Ω, like power chokes) where winding resistance dominates. Use 'Lp' (Parallel equivalent) for high-impedance RF chokes (>100 Ω) where core losses and parallel capacitance dictate behavior.
- Lead Jacks: Use the guarded 4-terminal Kelvin jacks (Hc, Hp, Lc, Lp) to eliminate lead resistance from the measurement path. This is mandatory for measuring the DCR of sub-milliohm power inductors.
- Range: Auto-range is acceptable for initial scouting, but lock to a manual range (e.g., 200 µH) for stable, noise-free readings on small SMD parts.
- Test Frequency: 1 kHz for >100 µH; 100 kHz for 1 µH to 100 µH; 1 MHz+ for <1 µH.
Probe Placement per Test Point
- Leaded (THT) Inductors: Use Kelvin clips. Clip directly to the bare wire leads, not the solder joints, to avoid measuring parallel PCB trace capacitance or joint resistance.
- SMD Power Inductors: Use SMD tweezers. Press the ceramic tips firmly against the metal end-caps. Apply consistent pressure; variable pressure changes the contact resistance and ruins the Q-factor reading.
- In-Circuit vs. Out-of-Circuit: Always measure out-of-circuit (desoldered or with one leg lifted) for absolute accuracy. In-circuit measurements will read artificially high due to parallel PCB capacitance and parallel power traces skewing the phase angle.
Diagnosing Faults: Good vs. Bad Inductor Readings
Inductors rarely fail open unless subjected to massive current spikes that melt the internal enamel wire. More commonly, they degrade due to core saturation, thermal breakdown, or shorted turns. A good reading numerically means the inductance falls within the manufacturer's tolerance (typically ±10% to ±20% for power inductors, ±2% for RF) and the DCR matches the datasheet specification.
| Condition | Inductance (L) Reading | DCR Reading | Q Factor / ESR | Physical Symptom |
|---|---|---|---|---|
| Good / Nominal | Within ±10% to ±20% of spec | Matches datasheet DCR | High Q, low ESR | None |
| Shorted Turns (Core/Winding fault) | 30% to 60% LOWER than spec | Noticeably LOWER than spec DCR | Q drops near zero | Inductor runs hot, converter switches erratically |
| Open Winding (Blown) | OL (Overload) or Error | Infinite (OL) | N/A | No output voltage, dead PCB |
| Core Saturation / Cracking | Nominal at low signal, drops under bias | Nominal | Normal at 1kHz, poor at load | Audible whining, thermal shutdown |
Mistakes That Give Misleading Readings
- Ignoring Parallel Capacitance: Measuring a 10mH choke in-circuit with a 100nF bypass cap across it will cause the LCR meter to read a resonant peak or display a negative inductance/capacitance error. The meter gets confused by the phase angle shift caused by the parallel LC tank.
- Magnetic Coupling: Measuring two inductors placed physically close together (like in an interleaved PFC circuit or a coupled-inductor SEPIC converter) without isolating them. The mutual inductance ($M$) will skew the reading higher or lower depending on the winding polarity.
- Skipping Open/Short Compensation: Failing to zero the meter with the probes touching (short) and apart (open) injects 5-10% error into sub-microhenry RF measurements by failing to subtract the fixture's parasitic capacitance and lead inductance.
Safety Categories (CAT Ratings) and Mains Measurement Rules
While inductors are passive components, the circuits they live in are not. If you are probing a Power Factor Correction (PFC) choke in a switched-mode power supply (SMPS) or a line-filter choke in an AC-DC inverter, you are working in high-energy environments. According to Fluke's measurement category guidelines, your test equipment must be rated for the environment's transient potential.
CAT Rating Requirement: Your multimeter or LCR meter must be rated CAT II (minimum) for appliance-level SMPS repairs, and CAT III if you are measuring line-side filter inductors connected directly to branch circuit wiring. A CAT III 600V rating ensures the meter's internal arc-gaps can survive a 4000V transient spike if you accidentally probe a live node while the meter is in LCR mode.
Discharging Filter Caps: SMPS inductors are often paired with large electrolytic filter capacitors (e.g., 400V DC bus). An inductor will not give a valid reading if a parallel capacitor is holding a 300V charge, and the residual voltage will destroy your meter. Bleed all bus capacitors with a high-wattage resistor (e.g., 10kΩ 5W) before clipping on the Kelvin leads. For deeper magnetics design theory and core loss calculations, refer to the electronics tutorials on inductors and manufacturer tools like the Coilcraft Inductor Finder to verify expected DCR and saturation currents before testing.
