To perform an accurate inductor measure, you need two tools: an LCR meter to read inductance (L) and equivalent series resistance (ESR), and a digital multimeter (DMM) to measure DC resistance (DCR). A good inductor reads within ±10% of its nominal L value, with near-zero ESR and a low DCR that matches its wire gauge. If your inductance is significantly low, the core is likely shorted or saturated; if DCR is infinite, the winding is open.
Meter Setup and Probe Placement for Inductor Testing
Measuring inductors is not as simple as poking two probes onto a resistor. The test frequency and equivalent circuit model you select on your LCR meter will drastically change the reading. Handheld DMMs with an 'L' setting are generally useless for power electronics because they test at a fixed, often undocumented frequency and cannot measure ESR.
LCR Meter Setup Block
- Dial/Function: Set to L (Inductance) or Z (Impedance). Most bench meters allow you to display L and ESR (or Q factor) simultaneously.
- Test Frequency: Use 1 kHz for general-purpose and RF chokes. Use 100 Hz or 120 Hz for large power inductors (like PFC chokes or audio crossovers) to avoid core eddy-current skewing the reading.
- Equivalent Circuit Mode: Select Series (SER) mode for low-impedance components (power inductors, DCR < 100Ω). Select Parallel (PAR) mode for high-impedance components (small RF chokes, > 1 kΩ).
- Test Level (Voltage): Set to 1 Vrms. Going higher risks saturating high-permeability ferrite cores during the test, which artificially lowers the inductance reading.
DMM Setup for DCR
Set your DMM dial to Ohms (Ω) and select the lowest manual range or auto-range. Critical step: Short your test leads together and use the 'REL' (Relative) or 'Zero' button to null out the lead resistance. Standard test leads add 0.2Ω to 0.5Ω of resistance, which will completely mask the true DCR of a heavy-gauge power inductor.
Probe Placement
For RF chokes and high-DCR inductors, standard sharp-tipped probes are fine. For power inductors where DCR is under 0.1Ω, you must use Kelvin (4-wire) test clips. Kelvin clips separate the current-carrying path from the voltage-sensing path at the exact point of contact, eliminating lead resistance from the measurement. Place the jaws directly on the inductor's terminal pads, avoiding any oxidized solder if possible.
Expected Readings: Good vs. Bad Inductor Values
The table below provides baseline expectations for common inductor types. A 'good' reading is not just about hitting the exact microhenry target; it is about the relationship between L, ESR, and DCR. According to fundamental magnetics theory outlined by All About Circuits, an inductor's efficiency is heavily dependent on minimizing parasitic resistances.
| Component Type | Nominal L | Expected DCR | Expected ESR (at test freq) | Bad Reading (Failure Mode) |
|---|---|---|---|---|
| RF Choke (Ferrite) | 10 µH | 0.4Ω - 0.8Ω | ~1.2Ω @ 1 MHz | L reads < 5 µH (shorted turns) or infinite DCR (broken wire). |
| Buck Converter Choke (Iron Powder) | 4.7 µH | 0.010Ω - 0.025Ω | ~0.04Ω @ 100 kHz | DCR > 0.1Ω (internal joint failure) or ESR > 0.5Ω (core degradation). |
| PFC Choke (Sendust/Ferrite) | 300 µH | 0.15Ω - 0.30Ω | ~0.8Ω @ 100 Hz | L drops to 50 µH (core cracked/saturated) or DCR is open. |
| Common Mode Choke (Nanocrystalline) | 10 mH | 1.2Ω - 1.8Ω | Varies widely | Asymmetrical DCR between windings (one winding burned open). |
Interpreting the Numbers
If your Buck Converter Choke reads 4.6 µH (well within the typical ±20% manufacturing tolerance) but the ESR is reading 0.8Ω instead of the expected 0.04Ω, the inductor is failing. High ESR in power inductors usually indicates that the internal copper windings have suffered thermal stress, or the core material has micro-fractured, leading to massive localized eddy current losses. It will overheat rapidly under load.
Safety Categories and In-Circuit Measurement Traps
Measuring inductors while they are still soldered to a PCB is the fastest way to get a misleading reading and potentially destroy your meter. However, if you must probe in-circuit, you need to understand the safety environment.
If you are measuring a PFC choke or common mode choke on the primary side of an offline switch-mode power supply (SMPS), you are working in a CAT III or CAT IV environment. Your LCR meter and DMM must be rated for this category. Never use a cheap, unrated handheld LCR meter on mains-connected magnetics. Always de-energize the circuit, lock out the breaker, and manually discharge bulk capacitors with a high-wattage bleeder resistor before connecting test leads. NEC-style guidance dictates that if you are unsure of the isolation, defer to a licensed professional.
Mistakes That Give Misleading Readings
- Parallel PCB Traces: If you measure an inductor in-circuit, the surrounding copper traces, capacitors, and semiconductor junctions create parallel impedance paths. A 10µH inductor might read as 2µH simply because a parallel bypass capacitor is altering the phase angle the LCR meter detects. Fix: Always desolder at least one leg of the inductor to lift it from the circuit.
- Residual Core Magnetism: If an inductor was subjected to a massive DC fault current, the core might be permanently magnetized (saturated). An LCR meter's tiny 1Vrms test signal won't be enough to push the core out of saturation, resulting in an artificially low inductance reading. Fix: Demagnetize the core using a degaussing tool or a slowly decaying AC current source before re-testing.
- Wrong Test Frequency: Measuring a 300µH iron-powder PFC choke at 1 kHz will yield heavy core losses, making the inductance appear lower and the ESR appear much higher than it operates in reality. Always match the LCR test frequency to the inductor's intended operating frequency band.
Step-by-Step Inductor Verification Workflow
Follow this exact sequence on the bench to confidently diagnose any inductor, from tiny 0805 SMD chokes to massive toroidal transformers.
- Isolate the Component: Desolder one or both pads to remove the inductor from the PCB. This eliminates parallel trace interference.
- Visual Inspection: Check for melted enamel, cracked ferrite cores, or lifted pads. A cracked ferrite core alters the air gap and ruins the inductance value.
- Null the DMM: Short your DMM leads, press 'REL', and verify the display reads 0.00Ω.
- Measure DCR: Apply Kelvin clips or sharp probes directly to the inductor terminals. Compare the reading to the manufacturer datasheet or the baseline table above. If it reads 'OL' (Open Loop), the internal wire has snapped; the part is trash.
- Configure LCR Meter: Set the meter to Series mode, 1 Vrms, and select the appropriate frequency (1 kHz for signal/RF, 100 Hz for power/audio).
- Measure L and ESR: Attach the LCR leads. Allow the reading to stabilize for 2-3 seconds. Note the primary inductance (L) and the secondary parameter (ESR or D/Q factor).
- Compare and Conclude: Verify L is within the manufacturer's tolerance (usually ±10% to ±20%). Verify ESR is low. If L is correct but ESR is 10x higher than expected, reject the part—it will cause thermal runaway under load.
For deeper analysis of core losses and high-frequency behavior, referencing application notes like the Fluke Electrical Safety Categories guide ensures your bench practices remain safe when scaling up to higher voltage magnetics. By combining a rigorous DCR check with frequency-appropriate LCR testing, you eliminate the guesswork and stop replacing inductors that are actually perfectly healthy.
