What Does "Inductor Testing" Mean in a Laboratory?
An inductor is a passive component — a coil of wire (often on a magnetic core) that stores energy in a magnetic field and opposes changes in current — characterised by its inductance (L, in henry), quality factor (Q), DC resistance (R_DC), self-resonant frequency (SRF) and saturation current (I_sat). "Inductor testing" in a laboratory is the measurement of these electrical parameters under defined conditions, on the correct instrument, against the applicable product standard. It is governed internationally by IEC 62024 (High-frequency inductive components — electrical characteristics and measuring methods; Part 1 for nH chip inductors 1 MHz–3 GHz, Part 2 for DC-DC converter inductor rated current) and IEC 61007 (Test methods and procedures for transformers and inductors used in electronic and telecommunication equipment); in China by GB/T 40853 (≡ IEC 62024, with Part 2-2023 effective for DC-DC inductor rated-current measurement) and GB/T 8554 (≡ IEC 61007, under revision to IEC 61007:2020 by TC89), plus the fixed-inductor industry specification SJ/T 11287-2003《电子设备用固定电感器》. It is distinct from the multimeter "inductor test" that dominates the consumer SERP — that is resistance / continuity troubleshooting, which detects only open or shorted windings. A laboratory inductor test measures the inductance and its frequency / current dependence on an LCR meter or impedance analyser — the difference between "is the wire broken?" and "does the inductance meet spec at the operating frequency and current?".
Why a Laboratory Test Is Not a Multimeter Resistance Check
The competitor SERP is dominated by how to test an inductor with a multimeter in ohms mode — probe the two terminals, expect ~2–3 Ω (just the wire), declare "open" if infinite or "shorted turns" if lower than expected. That test answers only one question: is the winding broken or shorted? It does not measure:
- Inductance (L) — the actual value the component is specified by; a multimeter cannot measure it.
- Quality factor (Q) / dissipation (D) — the loss; a low-Q inductor with correct L still fails in a filter.
- DC resistance (R_DC) — measured, but only a high-end DMM with 4-wire Kelvin does it accurately on sub-Ω windings.
- Self-resonant frequency (SRF) — where the parasitic capacitance resonates with L and the inductor stops being an inductor; measured by impedance sweep.
- Saturation current (I_sat) / DC superposition — how L collapses as DC current pushes the core toward saturation; the single most important parameter for a power inductor, and one no multimeter can measure.
The practical point: a multimeter-resistance "good" inductor can be wildly out of spec on inductance, saturated at its operating current, or self-resonant below the application frequency — and the circuit will fail.
What Are the Core Electrical Parameters and How Are They Measured?
The full inductor characterisation per IEC 62024 / GB/T 40853 covers:
- Inductance (L) — measured on an LCR meter (Hioki IM3533/IM3536/IM3570, or equivalent) at a defined frequency and signal level, reported as Ls (series equivalent) or Lp (parallel equivalent). The frequency is set below the self-resonant frequency (where L flat-lines vs frequency); the level is set below the core's saturation threshold. For a coil with current dependence (magnetic core), a constant-current (CC) mode keeps the core out of saturation during the measurement.
- Quality factor (Q) — L·ω / R_ESR, the inductor's "efficiency"; measured simultaneously with L on the LCR meter.
- DC resistance (R_DC) — the winding resistance; measured with a DC signal (the LCR meter alternates AC for L/Q then DC for R_DC). R_DC varies with temperature (copper's temperature coefficient), so temperature correction (e.g. Hioki IM3533) may be needed. Note: the AC-measured R_s / R_p is not R_DC — it includes skin-effect and proximity-effect losses at the test frequency.
- Self-resonant frequency (SRF) — the frequency at which L and the parasitic capacitance resonate; above SRF the component is capacitive, not inductive. Found by an impedance sweep (impedance analyser mode).
- Saturation current (I_sat) and DC superposition — the DC current at which L falls to a declared fraction (commonly −10 % or −20 %) of its zero-DC value, as the magnetic core saturates. Measured by superimposing a DC bias current on the AC test signal — the DC bias / superposition characteristic.
Ls vs Lp, Frequency and Signal Level — the Setting Decisions
Three settings determine whether an L measurement is meaningful:
- Series (Ls) vs parallel (Lp) equivalent circuit — use Ls for low-impedance inductors (≈ ≤ 100 Ω, where R_s cannot be ignored) and Lp for high-impedance inductors (≈ ≥ 10 kΩ, where R_p cannot be ignored). The choice is dictated by which loss resistance dominates; in the uncertain 100 Ω–10 kΩ band, check with the component manufacturer. Wrong choice gives a small but systematic L error.
- Measurement frequency — must be well below the SRF; the rule is to find the frequency range where L flat-lines (Fig. 1 of the Hioki note) and measure there. Measuring at an arbitrary standard frequency can give an L value that differs materially from the value at the circuit's operating frequency — a recurring theme in application notes.
- Signal level / current — must be below the core's saturation threshold for a magnetic-core inductor; for an air-core inductor, set the level for best instrument accuracy (e.g. 1 V on the Hioki IM3500 series, +1 dBm on the IM7580 series). CC mode keeps the current constant as the DUT impedance varies.
Why DC Bias / Superposition Is the Critical Power-Inductor Test
For a power inductor or choke, the inductance under DC load current is the parameter that matters, and it is not what a small-signal LCR meter measures. The Voltech DC1000A analysis is explicit: as DC current flows, the core is pushed toward saturation, effective permeability falls, and inductance can collapse — an inductor that measures 100 µH small-signal may drop 20 % at rated current and to 20 µH at twice rated. Conventional LCR meters test at ≤ 100 mA; a general-purpose bench supply makes the measurement worse, because its large output capacitor shunts the AC test signal away from the inductor.
The correct method is a dedicated DC bias current source (e.g. Voltech DC1000A, 0–25 A per unit, parallelable to 200 A) that presents a high AC impedance to the LCR meter's test signal while supplying the DC — with active AC-correction circuitry that drives the AC voltage across the inductor toward zero, reducing AC error current by up to 100×. With the DC bias source + any LCR meter, the inductance-vs-current curve is measured across the operating range, giving the true I_sat and the DC-superposition characteristic.
What Are the Reliability and Physical Tests?
Beyond the electrical characterisation, inductors carry physical and reliability requirements:
- Saturation / temperature derating — L vs temperature and L vs current, to confirm the part holds spec across the operating envelope.
- Hi-pot / dielectric withstand — winding-to-core and winding-to-winding insulation, the safety barrier (especially for mains-frequency chokes and transformers).
- Temperature rise / thermal — the winding and core temperature at rated current, the long-term-reliability driver.
- Solderability / resistance to soldering heat — per the IEC 60068 / GB/T 2423 environmental methods for surface-mount parts.
- Vibration / mechanical shock — for harsh-environment and automotive inductors.
How Do Chip, Power and RF Inductor Tests Differ?
| Type | Dominant parameters | Distinctive test |
|---|---|---|
| Chip / SMD (nH–µH) | L, Q, SRF, R_DC | GB/T 40853.1 (IEC 62024-1), 1 MHz–3 GHz; fixture open/short calibration for nH-level L |
| Power inductor / choke | L, I_sat, R_DC, temperature rise | DC bias / superposition (DC1000A + LCR); GB/T 40853.2 (IEC 62024-2) rated-current |
| RF inductor | L, SRF, Q at frequency | Impedance analyser sweep; SRF is the upper-frequency limit |
| Common-mode / signal choke | CM impedance, DM impedance | Impedance analyser, common-mode rejection |
What Belongs on the Report?
A compliant inductor test report states the inductor type (chip / power / RF / choke), the standard (GB/T 40853.1/.2 / IEC 62024, GB/T 8554 / IEC 61007, SJ/T 11287), the parameters measured (L, Q, R_DC, SRF, I_sat), the test conditions (frequency, signal level, Ls/Lp, DC bias current if applied), and the result against the datasheet limit. For power inductors the DC-superposition curve (L vs DC current) and the I_sat at the declared −10/−20 % point are the headline; for chip inductors the L/Q/SRF at the operating frequency are the headline. Conflating a multimeter resistance reading with an LCR inductance measurement is the cardinal error.
For the related passive-component framework, see Electronic Component Testing; for transformer/magnetic-component testing, the same GB/T 8554 framework applies.
FAQ
What is the difference between testing an inductor with a multimeter and with an LCR meter?
A multimeter in ohms mode measures only the winding's DC resistance (≈ 2–3 Ω) — it detects an open (infinite) or a turn-to-turn short (lower than expected), but it cannot measure inductance, Q, SRF or saturation current. An LCR meter measures L, Q and R_DC at a defined frequency and level — the actual parameters the inductor is specified by. A multimeter-resistance "good" inductor can be far out of spec on inductance.
Why must the LCR test frequency be below the self-resonant frequency?
Because above the SRF, the inductor's parasitic capacitance resonates with L and the component behaves as a capacitor, not an inductor — the measured "L" is meaningless. The correct frequency is found by sweeping and identifying the band where L is flat vs frequency; measuring at an arbitrary standard frequency can give an L value that differs from the operating-frequency value.
What is saturation current (I_sat), and why is a DC bias source needed to measure it?
I_sat is the DC current at which L falls to a declared fraction (commonly −10 % or −20 %) of its zero-DC value, as the magnetic core saturates. Small-signal LCR meters test at ≤ 100 mA and cannot reach saturation; a general-purpose bench supply makes the measurement worse (its output capacitor shunts the AC test signal). A dedicated DC bias current source (e.g. Voltech DC1000A) supplies the DC while presenting high AC impedance, giving the true L-vs-current curve.
When should Ls (series) vs Lp (parallel) equivalent circuit be used?
Use Ls for low-impedance inductors (≈ ≤ 100 Ω, where R_s cannot be ignored) and Lp for high-impedance inductors (≈ ≥ 10 kΩ, where R_p cannot be ignored). The choice depends on which loss resistance dominates; in the uncertain 100 Ω–10 kΩ band, check with the component manufacturer. Wrong choice gives a systematic L error.
Which standards govern inductor testing?
Internationally, IEC 62024 (Part 1 nH chip inductors 1 MHz–3 GHz; Part 2 DC-DC converter inductor rated current) and IEC 61007 (transformers and inductors test methods). In China, GB/T 40853 (≡ IEC 62024, Part 2-2023 in force) and GB/T 8554 (≡ IEC 61007, under revision to IEC 61007:2020 by TC89), plus the fixed-inductor industry spec SJ/T 11287-2003. TC89 (全国磁性元件与铁氧体材料标准化技术委员会) is the responsible body.
Our Testing Capabilities
As an ISO/IEC 17025-accredited third-party laboratory, Beijing ZKGX Research provides inductor testing across chip, power, RF and choke types and both commercial and high-reliability frameworks:
- L / Q / R_DC measurement — on LCR meters (Hioki IM3533/IM3536/IM3570 class) at defined frequency and signal level, with Ls/Lp selection, below-SRF frequency, and temperature-corrected R_DC, to GB/T 40853.1 / IEC 62024-1.
- Self-resonant frequency (SRF) — by impedance-analyser sweep, for chip and RF inductors.
- Saturation current (I_sat) / DC superposition — on a DC bias current source (Voltech DC1000A class, 0–25 A, parallelable) + LCR meter, producing the L-vs-DC-current curve and the I_sat at declared −10/−20 %, per GB/T 40853.2-2023 / IEC 62024-2.
- Hi-pot / dielectric, temperature rise, solderability, vibration — per IEC 60068 / GB/T 2423 and the SJ/T 11287 fixed-inductor specification.
- Power-inductor / SMPS magnetics — load-condition validation, L under rated current, temperature rise; GB/T 8554 / IEC 61007 transformer-and-inductor test methods.
Sample types include chip / SMD inductors (nH–µH), power inductors and chokes, RF inductors, common-mode chokes, and transformer/magnetic-component assemblies. If you have a specific inductor type, standard (GB/T 40853 / IEC 62024 / GB/T 8554 / IEC 61007 / SJ/T 11287), parameter requirement (L / Q / R_DC / SRF / I_sat), or application (DC-DC / SMPS / RF / automotive / signal), contact the laboratory to confirm the exact test set and reporting format before testing.