The Henry: Understanding the Unit of Measurement for Inductance
The fundamental unit of measurement for inductance is the Henry (H), named after the American scientist Joseph Henry who discovered electromagnetic induction independently around the same time as Michael Faraday. In practical circuit design and bench testing, a single Henry represents a massive amount of inductance. Because of this, you will almost exclusively work with sub-multiples: the millihenry (mH, 10-3 H), the microhenry (µH, 10-6 H), and the nanohenry (nH, 10-9 H).
Inductance is a component's inherent opposition to a change in current. If resistance is a narrow pipe restricting water flow, inductance is a heavy, iron water wheel placed inside that pipe. Once the water (current) gets the wheel spinning, it flows easily. But if you try to suddenly stop or reverse the water flow, the heavy wheel's momentum will fight you, creating a massive pressure spike (voltage flyback). This principle is why we use inductors in buck converters to smooth current, and why they generate high-voltage spikes in ignition coils.
According to the NIST guide on SI Units, 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 a rate of one ampere per second. On the bench, however, we don't measure the voltage spike; we use an LCR meter to inject an AC test signal and measure the resulting impedance phase shift to calculate the Henry value.
Expected Readings: Good vs. Bad Inductor Values
When you put probes to a component, you need to know what the meter should say before you can trust the result. A 'good' reading numerically means two things: first, the measured inductance falls within the component's specified tolerance (typically ±10% to ±20% for power inductors, and ±1% to ±5% for precision RF coils); second, the DC resistance (DCR) is appropriately low for the wire gauge used.
Below is a data-dense reference table detailing what you should expect when testing common inductor types found in modern electronics. Keep this handy when troubleshooting switch-mode power supplies (SMPS) or audio crossovers.
| Component Application | Nominal Value (Unit) | Expected Tolerance Range | Typical DCR (Good) | Failure Mode Reading |
|---|---|---|---|---|
| SMPS Buck Converter Inductor | 4.7 µH | 4.22 µH – 5.17 µH (±10%) | < 0.05 Ω | OL (Open wire) or 0.00 µH / near-zero DCR (Shorted turns) |
| Audio Crossover Air-Core Coil | 1.5 mH | 1.42 mH – 1.57 mH (±5%) | 0.3 Ω – 0.8 Ω | OL (Wire break) or > 2.0 mH (Core shift/debris in air gap) |
| EMI Common Mode Choke | 10 mH | 8.0 mH – 12.0 mH (±20%) | < 1.2 Ω per winding | Asymmetric L between windings (>10% mismatch indicates partial short) |
| RF Impedance Matching Coil | 47 nH | 45.5 nH – 48.4 nH (±2%) | < 0.1 Ω | Reads as capacitance (parasitic C dominates past self-resonant frequency) |
What a bad reading looks like: If your LCR meter reads 'OL' (Over Limit) on the inductance scale, the internal wire is broken. If the inductance reads perfectly nominal but the DCR is unusually high, the coil has likely suffered thermal damage, corroding the internal wire or compromising the solder joints. If the inductance reads significantly lower than nominal while DCR drops to near zero, you have shorted turns—insulation between the windings has melted, effectively bypassing a chunk of the coil.
Meter Setup and Probe Placement for Inductance Testing
Standard multimeters (like the Fluke 117 or Brymen BM869s) cannot measure inductance; they only measure the DCR. To measure the actual unit of measurement for inductance, you need a dedicated LCR meter, such as the DER EE DE-5000 or the Keysight U1733C. These devices apply an alternating test voltage and measure the phase angle between voltage and current to isolate the inductive reactance.
Dial/Mode: Set to 'L' (Inductance). Disable 'Auto' mode if you need to force a specific test frequency to match the manufacturer's datasheet.
Lead Jacks: For high-precision 4-terminal Kelvin measurements (essential for sub-microhenry RF coils), use the Hcur/Hpot and Lcur/Lpot terminals with specialized Kelvin clips. For standard 2-terminal handheld testing, insert leads into COM and V/Ω/L.
Range: Start on Auto-Range. If the display fluctuates wildly or fails to lock, manually step down to the 2mH or 200µH range.
Test Frequency: Set to 1 kHz for mH/µH power and audio inductors. Set to 100 kHz for nH RF inductors. Testing an RF coil at 120 Hz will yield completely useless data.
Inductors in offline switch-mode power supplies (SMPS), motor drives, or induction heaters are connected to high-voltage DC buses or mains AC. Never test in-circuit on a live board. If probing an unpowered board that connects to mains, your meter and test leads MUST be rated CAT II (minimum 600V) or CAT III to withstand transient let-through current and voltage spikes. De-energize, lock out, and verify dead with a CAT-rated voltage tester before attaching LCR probes. Large inductors can also store residual magnetic energy; short the terminals briefly with an insulated resistor before handling.
Numbered Steps for Probe Placement
- Isolate the Component: Inductors are highly susceptible to parallel circuit paths. If the inductor is in-circuit, parallel capacitance and low-resistance traces will skew the reading. Desolder and lift at least one leg of the inductor off the PCB pad.
- Discharge and Demagnetize: Ensure the component is fully discharged. If testing a large iron-core choke, ensure it hasn't retained a massive magnetic flux field from a recent DC fault, which can saturate the core and temporarily lower the measured inductance.
- Probe Placement: Place one probe on each of the two axial or radial leads. Polarity does not matter for basic inductance (L) measurements. Ensure your fingers are not touching the bare metal of the probes or the component leads; your body's capacitance and resistance will introduce parallel errors, especially on high-impedance RF coils.
- Zero the Leads: If measuring very low values (under 1 µH), short the probe tips together and use the meter's 'Relative' or 'Zero' function to subtract the test lead inductance (which is typically around 0.05 µH to 0.1 µH for standard banana-to-alligator leads).
Common Mistakes That Skew Inductance Measurements
When a reading doesn't match the expected values in the table above, the component isn't always the culprit. Misleading readings usually stem from environmental or setup errors. According to the Keysight LCR Meter Measurement Handbook, understanding the test conditions is just as critical as the reading itself.
1. Test Frequency Mismatch
The permeability of an inductor's core material (ferrite, powdered iron, or laminated steel) changes with frequency. A 10 µH inductor measured at 120 Hz might read 11.5 µH, but when measured at 100 kHz, it might drop to 8.2 µH due to core losses and the onset of self-resonance. Always check the manufacturer's datasheet to see what test frequency (e.g., 1 kHz vs 100 kHz) was used to establish the nominal value, and match it on your meter.
2. Core Saturation from Test Leads
Some cheap LCR meters apply a surprisingly high open-circuit test voltage. If you are measuring a tiny, un-gapped ferrite bead or a high-permeability toroidal core, the test signal itself might drive the core into magnetic saturation. When a core saturates, its permeability drops toward that of air, and your meter will display an inductance value far lower than the actual small-signal operating value. If you suspect this, look for a meter with a selectable test voltage (e.g., 0.5Vrms vs 1.0Vrms) and drop it to the lowest setting.
3. Magnetic Coupling to the Environment
Inductors work by projecting a magnetic field. If you hold a radial inductor an inch away from your steel workbench, a metal project box, or another transformer while taking a measurement, the external metal will couple with the magnetic field, altering the inductance and introducing eddy current losses. Always test inductors suspended in free air, at least a few inches away from any ferromagnetic materials or other active magnetic components.
4. Ignoring the Self-Resonant Frequency (SRF)
Every physical inductor has parasitic capacitance between its windings. At a specific frequency (the SRF), the inductive reactance and capacitive reactance cancel out, and the component acts like a pure resistor. Above the SRF, the component actually behaves as a capacitor. If your LCR meter is set to 1 MHz and you test a large 10 mH choke (which likely has an SRF around 500 kHz), the meter will either throw an error, display a negative inductance value, or incorrectly read it as a capacitor. Always ensure your test frequency is well below the component's SRF.
