Measuring an inductor’s value requires an AC signal, which is why a standard multimeter can only check an inductor’s DC resistance, not its actual inductance. To get a true inductance reading, you need either a dedicated LCR meter, an oscilloscope paired with a function generator, or a resonant circuit with a known capacitor. Each method suits different budgets and accuracy needs.
Why a Standard Multimeter Falls Short
Inductance is a property that only reveals itself when current is changing. A standard digital multimeter measures DC resistance, so connecting it across an inductor will tell you the resistance of the wire inside the coil (typically fractions of an ohm to a few ohms) but nothing about its inductance. That DC resistance reading is still useful as a quick continuity check to confirm the inductor isn’t open, but it won’t help you identify an unknown value or verify that a component meets its spec.
Some higher-end handheld multimeters do include an inductance function. These work by applying a small AC test signal internally, similar to a basic LCR meter. If your multimeter has an “L” or “mH” position on the dial, you can use it for rough measurements of low-frequency inductors. For anything requiring precision or high-frequency accuracy, you’ll want one of the methods below.
Using an LCR Meter
An LCR meter is the simplest and most common tool for measuring inductors. It applies a small AC signal at a controlled frequency and directly reads back the inductance, resistance, and quality factor of the component. Benchtop models offer the highest accuracy, while handheld versions are affordable and practical for most hobbyist and repair work.
To take a measurement, connect the inductor to the meter’s test leads, select inductance mode, and choose your test frequency. That last step matters more than people realize: an inductor’s measured value shifts with frequency because of parasitic effects inside the component. For the most meaningful reading, set the test frequency to match the frequency the inductor will actually operate at in your circuit. A 100 µH power inductor meant for a 100 kHz switching converter should ideally be tested near 100 kHz, not at 1 kHz.
The meter displays inductance in henries (H), millihenries (mH), or microhenries (µH). Most meters also show the quality factor (Q) and equivalent series resistance (ESR), which tell you how much energy the inductor wastes as heat. A higher Q means less loss. These parasitic values change with frequency too, so again, testing at your operating frequency gives the most realistic picture of how the inductor will perform in your design.
Understanding Q Factor and ESR
Every real inductor has some internal resistance from its wire and core losses. This is called the equivalent series resistance, or ESR. A perfect inductor would have zero ESR, but in practice every component has a measurable amount. The quality factor, Q, is simply the ratio of the inductor’s reactive impedance to its ESR. A Q of 50 means the inductor stores 50 times more energy per cycle than it dissipates.
Why does this matter? In filter circuits, oscillators, and RF applications, a low-Q inductor will broaden filter responses and reduce circuit performance. In power supplies, ESR translates directly to heat. An LCR meter at the right test frequency gives you both numbers. For high-frequency RF inductors with very high Q values, even LCR meters can lack the resolution to measure ESR accurately. In those cases, specialized resonant-line test equipment is the industry standard, though that’s mainly a concern for component manufacturers rather than bench-level testing.
The Oscilloscope and Function Generator Method
If you don’t have an LCR meter but do have an oscilloscope and a function generator, you can measure inductance using the voltage-current (I-V) method. This approach is more involved but gives you flexibility, especially for measuring inductors at specific frequencies or under specific drive conditions.
The setup works like this: connect a precision resistor in series with the inductor under test, then drive the series combination with a sine wave from the function generator. Use two oscilloscope channels to probe the voltage across the resistor and the voltage across the inductor. The voltage across the known resistor tells you the current flowing through the circuit (since V = IR). With both voltage and current waveforms on screen, you can calculate the inductor’s impedance.
At a single frequency, the inductor’s impedance has two components: the resistive part (ESR) and the reactive part (which depends on inductance). By measuring the amplitude ratio and phase difference between the voltage and current waveforms, you can separate these two components. The inductance equals the reactive portion of the impedance divided by 2π times the test frequency. Many modern oscilloscopes have built-in math functions or power analysis software that can automate these calculations, but even with a basic scope you can read the amplitude and time shift from the screen and work through the math by hand.
Choose your precision resistor value carefully. It should be close to the expected impedance of the inductor at your test frequency so that the voltage divides roughly equally between the two components, giving you good measurement resolution on both channels.
The Resonant Frequency Method
This is a clever technique that requires only a known capacitor, a signal source, and a way to detect when the circuit hits resonance. It works because an inductor and capacitor in parallel (or series) resonate at a specific frequency determined by both values. If you know the capacitance and can find the resonant frequency, you can solve for the inductance.
The formula linking the three values is:
f = 1 / (2π√(LC))
Rearranged to solve for inductance:
L = 1 / (4π²f²C)
Here, f is the resonant frequency in hertz, C is the capacitance in farads, and L is the inductance in henries. For example, if you pair an unknown inductor with a 100 µF capacitor and find the circuit resonates at 200 Hz, the inductance works out to approximately 6.37 mH.
To find the resonant frequency, sweep a sine wave from a function generator through the circuit while monitoring the output amplitude. At resonance, a parallel LC circuit shows peak impedance (maximum voltage), while a series LC circuit shows minimum impedance (maximum current). The frequency where you see that peak or dip is your resonant frequency. Use a capacitor with a tight tolerance (1% or better) since any error in the capacitance value feeds directly into your inductance calculation.
This method is particularly useful for RF inductors where parasitic capacitance and other effects make low-frequency LCR readings misleading. By measuring at or near the intended operating frequency, you get a more realistic inductance value.
Bridge Circuits for Precision Work
For high-accuracy lab measurements, bridge circuits like the Maxwell bridge have been used for decades. A Maxwell bridge balances an unknown inductor against a known variable inductor (or capacitor, depending on the variant) and a set of precision resistors. You adjust the variable components until a detector in the bridge reads zero, indicating balance. At that point, the unknown inductance can be calculated from the known component values.
The balance equations are straightforward. In a Maxwell inductance bridge, the unknown inductance L₁ equals the ratio of two known resistors (R₃/R₄) multiplied by a known reference inductance L₂. The resistors are typically selected from standard decade values: 10, 100, 1,000, or 10,000 ohms. This method is largely academic today since modern LCR meters achieve comparable accuracy with far less effort, but understanding the principle helps if you encounter bridge-based instruments in calibration labs or older test setups.
Checking Saturation Current
For power inductors used in switching regulators and converters, the inductance value at low current is only part of the story. Every inductor has a saturation current: the point where the magnetic core can’t hold any more flux and the inductance drops sharply. Exceeding this limit causes current to spike, which can damage other components in the circuit.
Measuring saturation requires driving increasing DC current through the inductor while monitoring how its inductance changes. On a bench setup, you can use an oscilloscope with a differential voltage probe across the inductor and a current probe on the lead. By plotting the relationship between magnetic flux and current (called a B-H curve), you can see exactly where the core begins to saturate. The curve starts as a steep, roughly linear slope, then flattens as the core approaches its limit.
Some oscilloscopes with power analysis software can generate B-H plots automatically. You’ll need to enter the inductor’s physical parameters: number of turns, magnetic path length, and core cross-sectional area (all found in the component datasheet). The software then calculates the magnetic flux density from the measured voltage and overlays it against current. If the peak flux density in your circuit is approaching the saturation flux density listed in the core’s datasheet, the inductor is undersized for your application.
Choosing the Right Method
- Quick checks and general use: A handheld LCR meter is the fastest path to a reliable inductance reading. Test at or near your operating frequency for best accuracy.
- No LCR meter available: The oscilloscope I-V method works well with equipment most electronics labs already have. It takes more setup time but gives you impedance, ESR, and inductance in one measurement.
- Unknown inductor with minimal equipment: The resonant frequency method requires only a known capacitor and a signal source. Accuracy depends heavily on capacitor tolerance and your ability to pinpoint the resonant peak.
- Power inductor validation: Use an oscilloscope with current and voltage probes to check saturation behavior under realistic operating conditions. The inductance value on the label means nothing if the core saturates at your working current.