Measuring Inductance: LCR Meter Setup, Expected Values, and Fault Diagnosis

To accurately measure inductance, you must use a dedicated LCR meter (such as the DER EE DE-5000 or Keysight U1733C) set to the correct test frequency, with the component completely isolated from the circuit. Standard digital multimeters (DMMs) cannot measure inductance unless they feature a specific, often inaccurate, Lx setting. A good reading for a standard 47µH power choke is between 42.3µH and 51.7µH (±10% tolerance) with a low DC resistance (DCR) under 0.5Ω. This guide details the exact meter setup, probe placement, and decision paths required to diagnose failing inductors in switch-mode power supplies (SMPS) and RF circuits.

Meter Setup and Probe Placement Protocol

Inductance is highly dependent on test frequency and signal level. A haphazard setup will yield phantom readings. Follow this exact configuration for bench-grade LCR meters like the widely used DER EE DE-5000.

Probe Placement Per Test Point

1. Isolate the Component: Desolder at least one leg of the inductor from the PCB. Measuring in-circuit will parallel the inductor with bypass capacitors and semiconductor junctions, completely invalidating the reading.
2. Zero the Leads: Short the Kelvin clips or test probes together. Press the REL or NULL button to subtract lead resistance and parasitic inductance from the baseline.
3. Connect at the Body: Attach the probes directly to the component leads, as close to the inductor body as possible. Do not probe the PCB pads, as trace inductance will skew high-frequency readings.

Safety Categories and Pre-Test Discharge

Inductors themselves do not store lethal charge in the same way capacitors do, but they are frequently located on the primary side of switch-mode power supplies (SMPS) alongside high-voltage bulk capacitors. Furthermore, an inductor can generate a massive voltage spike (flyback) if current is interrupted while testing.

Expected Readings: Good vs. Failing Inductors

An inductor fails in three primary ways: open winding, shorted turns, or core degradation (cracking/saturation). The table below maps expected numerical values for a typical 47µH SMPS power choke against known failure modes.

Parameter	Good Reading (47µH Choke)	Failing / Bad Reading	Physical Cause of Fault
Inductance (L)	42.3µH to 51.7µH (±10%)	< 35µH or wildly fluctuating	Cracked ferrite core, air gap shift, or partial shorted turns reducing effective permeability.
DC Resistance (DCR)	0.05Ω to 0.45Ω	OL (Infinite) or < 0.01Ω	OL indicates a broken wire (open winding). Near-zero indicates severe turn-to-turn shorting.
Quality Factor (Q)	> 20 (at 1 kHz)	< 5	High core losses, degraded potting compound, or moisture ingress in the winding.

As noted in Electronics Tutorials inductor theory, the Q factor is the ratio of inductive reactance to resistance. A sudden drop in Q while inductance remains nominally stable is a classic early indicator of core degradation before the component fails catastrophically.

Five Mistakes That Give Misleading Inductance Readings

If your meter displays a value that defies physics (e.g., a negative inductance or a 10µH coil reading as 500nH), you have fallen victim to one of these common bench errors:

1. Measuring In-Circuit: PCB traces and parallel ceramic decoupling capacitors create an LC tank. The meter's test signal will resonate with the parasitic capacitance, yielding wildly inaccurate inductance values. Always lift one leg.
2. Wrong Test Frequency: Inductance is not a static number; it changes with frequency due to core material properties. Measuring a 100mH iron-core choke at 10 kHz will show a fraction of its true value due to eddy current losses and core roll-off. Match the test frequency to the component's operating frequency.
3. Skipping the Null/Zero Step: Test leads have their own inductance (typically 50nH to 200nH) and resistance. If you are measuring RF coils in the nanohenry (nH) range, failing to short the probes and press REL will result in a 20%+ measurement error.
4. Magnetic Coupling: Holding the inductor in your hand while wearing a steel watch, or resting it on a steel workbench, alters the magnetic flux path. Always test inductors on a non-magnetic surface (wood, plastic, or ESD mat).
5. Core Saturation from Test Signal: Some older or high-end LCR meters output a 1V or 2V RMS test signal. On very small ferrite beads or low-turn RF coils, this voltage can drive enough current to partially saturate the core, artificially lowering the measured inductance. Drop the test signal level to 0.5V or 0.1V if your meter supports it.

Decision Tree: Which Tool and Method to Use

Selecting the right tool and test parameters depends entirely on the physical size and application of the inductor. Use this decision path to lock in your setup.

Component Type & Value	Required Tool Feature	Test Frequency	Concrete Pick / Default Action
RF / IF Coils (< 10µH, air/ferrite core)	High-frequency LCR meter, 4-terminal SMD tweezers or Kelvin clips.	10 kHz to 1 MHz	Use Keysight U1733C (or similar 100kHz capable meter). Null leads at 100kHz.
SMPS Power Chokes (10µH to 10mH, powdered iron/ferrite)	Standard bench LCR, DCR measurement capability.	1 kHz	Use DER EE DE-5000. Set to Auto L/DCR. Isolate from PCB.
Line Filters / Transformers (> 10mH, laminated iron)	Low-frequency LCR, high voltage tolerance.	50 Hz / 60 Hz / 100 Hz	Use DER EE DE-5000 at 100Hz. If meter lacks 100Hz, use 50Hz/60Hz setting.
Quick Field Checks (Verifying open/short only)	Standard DMM with resistance (Ω) mode.	N/A (DC only)	Use Fluke 117. Measure DCR. If OL, it's dead. If near 0Ω on a large coil, it's shorted.