The Short Answer: What Inductance Is Measured In
Inductance is measured in Henrys (H), named after Joseph Henry, who discovered electromagnetic induction independently of Michael Faraday. One 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 the rate of one ampere per second.
Because one full Henry is a massive amount of inductance for modern electronics, you will almost never see a bare 'H' on a schematic or a meter display. Instead, standard SI metric prefixes are used to break the values down into manageable numbers:
- Millihenry (mH): 10^-3 Henrys. Common in power supply chokes, audio crossovers, and mains EMI filters.
- Microhenry (µH): 10^-6 Henrys. The standard range for switch-mode power supply (SMPS) buck/boost inductors and RF chokes.
- Nanohenry (nH): 10^-9 Henrys. Used in high-frequency RF matching networks, VHF/UHF antennas, and parasitic trace inductance calculations.
If you are looking at a standard digital multimeter (DMM) like a Fluke 87V, you will not find an inductance setting. Standard DMMs measure voltage, current, resistance, and sometimes capacitance. To measure inductance in Henrys, you need a dedicated LCR meter (measuring Inductance, Capacitance, and Resistance).
Meter Setup and Probe Placement for Inductance Testing
Getting an accurate inductance reading requires more than just touching probes to a component. Inductance is frequency-dependent, meaning the value you read changes based on the test signal frequency your meter injects into the part.
Meter Setup Block
- Dial / Mode: Select 'L' (Inductance). Ensure the secondary display is set to show 'Q' (Quality Factor) or 'D' (Dissipation Factor) to assess core health.
- Test Frequency: Set to 1 kHz for general-purpose mH/µH inductors. Drop to 100 Hz / 120 Hz for large iron-core transformers and mains chokes. Bump to 100 kHz for nH RF chokes and ceramic SMD inductors.
- Range: Use Auto-range for initial sweeps, but lock to a manual decade range (e.g., 200µH) for stable, flicker-free readings on low-value SMD parts.
- Lead Jacks: For 4-terminal Kelvin measurements, use Hcur (High Current), Hpot (High Potential), Lcur (Low Current), and Lpot (Low Potential). For standard 2-terminal, use the basic L and H jacks.
Probe Placement
Never measure inductance in-circuit. Parallel PCB traces, bypass capacitors, and semiconductor junctions will create parallel impedance paths that completely invalidate the reading, or worse, feed residual voltage back into your LCR meter's sensitive front-end. Desolder at least one leg of the inductor, or remove it entirely.
For through-hole toroids and axial chokes, use standard alligator clips or Kelvin clips. For SMD components, use fine-point tweezers probes, ensuring you squeeze the tips firmly to break through any oxidation on the solder pads.
Expected Readings: Good vs. Bad Inductance Values
When troubleshooting, you need to know what a 'good' reading looks like numerically versus a 'bad' one. Inductors typically fail in two ways: open circuit (the wire snaps internally, reading 'OL' or Overlimit) or shorted turns (the enamel insulation melts, causing adjacent windings to short, which drastically drops the inductance value).
| Component Type | Expected Nominal | Acceptable Tolerance | 'Bad' Reading (Fault Indication) |
|---|---|---|---|
| SMPS Buck Power Inductor | 4.7 µH | ±20% (3.76 - 5.64 µH) | < 2.0 µH (Shorted turns) or OL (Open) |
| EMI Toroidal Line Choke | 2.5 mH | ±30% (1.75 - 3.25 mH) | < 1.0 mH (Insulation breakdown) or OL |
| 3-Phase AC Motor Winding | 45.0 mH | ±5% Phase-to-Phase Balance | > 10% imbalance between Phase A, B, and C |
| RF Ceramic SMD Chip | 10.0 nH | ±5% (9.5 - 10.5 nH) | < 5 nH or reading fluctuates wildly (Cracked core) |
A 'good' reading is one that falls within the manufacturer's stated tolerance. However, for motor windings and multi-phase transformers, the absolute number matters less than the balance. If Phase A reads 45.2 mH, Phase B reads 44.9 mH, and Phase C reads 38.1 mH, Phase C has shorted turns, even if 38 mH looks like a 'normal' number in isolation.
Common Mistakes That Give Misleading Inductance Readings
If your LCR meter is giving you numbers that don't match the schematic, you are likely falling victim to one of these bench-level mistakes. According to LCR measurement best practices, environmental and setup variables heavily skew inductance.
- Ignoring Lead Inductance: Test leads have their own inductance. Two inches of standard banana-to-alligator wire adds roughly 50 nH to your reading. If you are measuring a 22 nH RF choke, your leads are doubling the value. Fix: Use the meter's 'Relative' or 'Zero' function with the probes shorted together before measuring low-value parts.
- Core Saturation (Lack of DC Bias): Ferromagnetic cores (iron, ferrite) lose permeability as current increases. A bench LCR meter injects a tiny AC signal (usually < 1V at 1mA). The inductor might read 100 µH on the bench, but drop to 20 µH when 5A of DC current flows through it in-circuit. Fix: If your application involves high DC current, you must use an LCR meter with a DC Bias capability, or calculate the derating curve from the datasheet.
- Parasitic Capacitance at High Frequencies: Every inductor has parasitic parallel capacitance between its windings. If you test a large mH choke at 100 kHz, you might be approaching its Self-Resonant Frequency (SRF). Near the SRF, the capacitive reactance cancels the inductive reactance, and the meter will display garbage data or an error. Fix: Always test large inductors at low frequencies (100 Hz - 1 kHz).
Decision Tree: Which LCR Meter Should You Buy?
Choosing the right tool depends entirely on the decade range of the components you test most often. Use this decision path to select your bench equipment:
| If Your Primary Work Is... | Then You Need... | Recommended Tool Category |
|---|---|---|
| Audio crossovers, power transformers, HVAC contactors (> 1 mH) | High accuracy at 120 Hz, basic 2-terminal testing | Entry-level handheld LCR |
| SMPS repair, buck/boost converters, general bench work (1 µH to 10 mH) | Auto-ranging, 1 kHz / 100 kHz selectable, good Q-factor readout | Mid-range handheld LCR with tweezers |
| RF design, VHF/UHF filters, SMD matching networks (< 100 nH) | 4-terminal Kelvin, 1 MHz+ test frequency, DC bias, SMD fixture | Benchtop LCR Meter |
Safety Categories and Mains-Adjacent Inductors
When measuring inductors that live in mains-voltage environments—such as EMI line filters in computer power supplies, HVAC contactor coils, or variable frequency drive (VFD) line reactors—safety protocols are non-negotiable.
Your LCR meter is a low-voltage diagnostic tool. It is not designed to be connected to live circuits, and most LCR meters do not carry a CAT safety rating for live mains troubleshooting. Therefore, the safety category (CAT rating) applies to your verification process, not the inductance measurement itself.
- De-energize and Lockout: Turn off the main breaker and apply a lockout/tagout (LOTO) device.
- Verify Dead with a CAT-Rated DMM: Use a CAT III 600V (or CAT IV 600V for service entrance) multimeter, like a Fluke 117 or 87V, to verify that the circuit is completely dead. Test line-to-line and line-to-ground.
- Discharge Capacitors: Mains line filters contain large X and Y capacitors wired in parallel with the inductors. These can hold lethal charges. Use a high-wattage discharge resistor (e.g., 10kΩ 5W) to bleed the caps before connecting your LCR meter.
- Isolate and Measure: Once verified dead and discharged, disconnect the inductor from the circuit and proceed with your LCR meter measurement.
By understanding what inductance is measured in, configuring your LCR meter for the correct test frequency, and respecting the physical limitations of magnetic cores, you can accurately diagnose failing magnetics and validate your custom-wound transformers on the first try.
