If you need to verify an inductor, the direct answer depends on what you are actually trying to measure. To check for basic physical faults (broken wire or dead shorts), use a Digital Multimeter (DMM) to measure DC Resistance (DCR). To measure the actual inductance value (Henries) and verify the component is functioning within its AC parameters, you must use an LCR meter. A standard DMM cannot measure inductance.
Inductors fail in specific, predictable ways: the core saturates, the winding insulation melts causing shorted turns, or the wire snaps from thermal cycling. This guide provides the exact bench procedures, expected numeric values, and decision paths to diagnose inductor faults confidently.
The Direct Answer: DMM vs. LCR Meter for Inductor Measurement
A common bench mistake is attempting to measure an inductor's value with a standard multimeter. A DMM applies a DC voltage and measures current to calculate resistance (Ohms). Inductance, however, is a property that opposes changes in AC current. Therefore, measuring inductance requires an alternating test signal.
- Use a DMM (e.g., Fluke 87V) when you are in the field and need to confirm a coil isn't open-circuit (infinite resistance) or dead-shorted (near 0.0Ω). You are measuring DC Resistance (DCR), not inductance.
- Use an LCR Meter (e.g., DER EE DE-5000 or Keysight U1733C) when you are at the bench and need to verify the actual inductance (e.g., confirming a 47µH power inductor hasn't dropped to 12µH due to core cracking or saturation).
Meter Setup and Probe Placement Protocol
DMM Setup (For DCR / Continuity Checks)
- Dial Position: Set to Ohms (Ω) or the lowest resistance range (e.g., 200Ω or 600Ω auto-range). Do not use the continuity beeper; it only tells you a path exists, not the exact resistance.
- Lead Jacks: Black lead in COM, Red lead in V/Ω.
- Zeroing: Touch the probe tips together. Note the lead resistance (usually 0.1Ω to 0.3Ω). You must subtract this from your final reading.
LCR Meter Setup (For Actual Inductance)
- Dial/Menu: Set to 'L' (Inductance).
- Test Frequency: Set to 100Hz or 120Hz for power/mains inductors and chokes. Set to 1kHz or higher for RF and high-frequency signal inductors.
- Circuit Mode: Select Series (SER) mode for low-impedance power inductors (DCR < 100Ω). Select Parallel (PAR) mode for high-impedance RF chokes (> 10kΩ).
- Lead Jacks: Use the dedicated Kelvin clips or standard LCR test leads.
Probe Placement
Place one probe on each of the inductor's two termination pads or leads. Polarity does not matter for standard, non-tapped inductors. Ensure your fingers are not touching both metal probe tips simultaneously, as your body's resistance and capacitance will introduce parallel errors, especially on high-frequency LCR measurements.
Expected Readings: Good vs. Bad Inductor Values
What constitutes a "good" reading depends entirely on the inductor's physical size and application. Below is a reference table for common inductor types. A good inductor will read within ±10% to ±20% of its rated inductance and match its specified DCR.
| Inductor Type | Typical Rated Value | Expected DCR (Good) | Expected LCR (Good) | Bad Reading (Fault) |
|---|---|---|---|---|
| Molded Power (e.g., Coilcraft MSS1210) | 100µH | 0.043Ω | 95µH - 105µH (at 100Hz) | DCR = 0.01Ω (Shorted turns) |
| Toroidal Choke (Mains Filter) | 10mH | 1.5Ω - 3.0Ω | 9mH - 11mH (at 120Hz) | DCR = OL (Open winding) |
| RF Signal Inductor (0805 SMD) | 10nH | 0.15Ω - 0.50Ω | 9nH - 11nH (at 100MHz) | LCR reads 2nH (Core cracked) |
| Buck Converter Choke | 4.7µH | 0.015Ω - 0.030Ω | 4.2µH - 5.2µH (at 100kHz) | LCR drops to 1µH (Saturated) |
Troubleshooting Decision Path
Use this decision tree to determine your next step when an inductor is suspected of causing a circuit failure. This path terminates in a concrete action or replacement part.
| Symptom / Initial Reading | Secondary Test | Result | Concrete Action / Part Pick |
|---|---|---|---|
| DMM reads "OL" (Open) | Visually inspect leads under magnification. | Pad cracked or wire snapped. | Replace. Match exact footprint and Isat (e.g., Wurth Elektronik 744774 series). |
| DMM reads 0.0Ω (Dead Short) | Measure DCR with LCR meter (4-wire Kelvin). | DCR is significantly below datasheet spec. | Replace. Internal winding insulation has melted. Buy a higher temperature rated part (e.g., 155°C Class F). |
| DCR is normal, but power supply outputs low voltage / high ripple. | Measure Inductance (L) with LCR meter. | Inductance is >20% below rated value. | Replace. Core is cracked or permanently saturated. Upgrade to a part with a higher Isat rating (e.g., Coilcraft XEL series). |
| DCR and L are normal, but circuit still fails. | Check for AC losses / ESR at operating frequency. | High AC resistance / overheating in operation. | Replace. Switch from ferrite to powdered iron core, or use Litz wire wound inductor to reduce skin effect losses. |
Default Bench Recommendation: If your DMM shows normal DCR but the circuit misbehaves, stop guessing. Purchase a DER EE DE-5000 (approx. $130 USD). It is the undisputed king of hobbyist and mid-tier bench LCR meters, offering 100Hz to 100kHz test frequencies and auto-ranging Series/Parallel modes that will instantly reveal hidden core saturation or shorted turn faults.
Five Mistakes That Give Misleading Inductor Readings
- Measuring In-Circuit: The surrounding PCB traces, capacitors, and MOSFETs create parallel resistance and capacitance networks. An in-circuit DCR reading of 0.5Ω might just be a parallel voltage divider, not the inductor. Always desolder at least one leg.
- Ignoring Lead Resistance on Low-DCR Parts: A modern 4.7µH buck converter inductor might have a DCR of 0.015Ω. If your DMM leads have 0.2Ω of resistance, your meter will read 0.215Ω, leading you to falsely condemn a perfectly good part. Use an LCR meter with Kelvin (4-wire) clips to eliminate lead resistance.
- Using the Wrong LCR Test Frequency: Inductance is not a static number; it varies with frequency due to core material properties. Measuring a 100kHz switching regulator choke at 1kHz might yield a falsely high inductance reading. Always match the LCR test frequency to the component's operating frequency where possible.
- Mismatching Series/Parallel Modes: LCR meters calculate values based on equivalent circuit models. If you measure a low-impedance power inductor in 'Parallel' mode, the meter's math will yield wildly inaccurate, often negative or infinite, inductance values. Rule of thumb: < 100Ω impedance = Series mode; > 10kΩ impedance = Parallel mode.
- Magnetic Saturation from Handling: If you are testing high-permeability toroidal cores, simply holding them tightly against a steel workbench or near a strong neodymium magnet can partially saturate the core, temporarily lowering the measured inductance. Test them on a non-magnetic wooden or plastic surface.
Safety Protocols: CAT Ratings and Flyback Hazards
Inductors store energy in their magnetic fields ($E = \frac{1}{2}LI^2$). If you disconnect a probe or break a circuit while current is flowing through an inductor, the rapid collapse of the magnetic field ($V = L \frac{di}{dt}$) generates a massive voltage spike. This "flyback" voltage can easily exceed 1,000V, destroying your meter's input protection, arcing across your probes, and delivering a lethal shock.
Always de-energize the circuit and safely discharge filter capacitors before probing inductors.
When measuring inductors located on the mains-voltage side of a power supply (such as EMI filter chokes or Power Factor Correction (PFC) inductors), your test equipment must be rated for the environment. According to Fluke's safety guidelines on measurement categories, working on mains-connected equipment requires a minimum of a CAT III 1000V or CAT IV 600V rated multimeter and test leads.
Standard CAT II meters are not rated for the transient overvoltages present on the primary side of switch-mode power supplies. Furthermore, never attempt to measure the DCR of a mains choke while the equipment is plugged in, even if the device is "switched off." The EMI choke is directly across the AC line, and a slip of the probe can result in a catastrophic short circuit and arc flash. Always pull the plug, verify dead with a proven voltage tester, and short the inductor leads with an insulated discharge tool before attaching your meter leads.
For deeper theory on how inductors store energy and behave in AC/DC circuits, refer to the foundational texts at All About Circuits. Understanding the physics of core saturation and magnetic hysteresis will make your bench measurements significantly more accurate and your troubleshooting much faster.
