An inductance that grows with the test frequency
Symptom
Section titled “Symptom”An LCR meter reads a winding with many turns, a flyback’s primary for example, well above the inductance the design expects, and it reads a different value at each test frequency: the higher the frequency, the higher the reading.
The meter reports the part’s effective value at its test frequency, parasitics included [Keysight Technologies, Impedance Measurement Handbook]. The capacitance between the turns sits in parallel with the inductance and resonates with it (self-resonant frequency). Below that resonance the pair behaves as a larger inductance, and the reading rises steeply as the test frequency approaches it (apparent inductance). Many turns bring a large inductance, which grows with the square of the turns (inductance from the inductance factor), together with the capacitance between them, so their resonance sits low.
For example, a winding of 1 mH with 20 pF across it resonates at 1.13 MHz. A meter reads it as 1.008 mH at 100 kHz, and as 4.75 mH at 1 MHz.
How to confirm
Section titled “How to confirm”Read the winding at two or more of the meter’s test frequencies. A reading that climbs steeply as the frequency rises comes from the resonance: near it the apparent inductance grows without bound (apparent inductance), far more than a core’s own change with frequency. To find the resonance, sweep the winding’s impedance with an impedance analyzer: above the resonance the part is capacitive (self-resonant frequency).
Measure at a test frequency well below the self-resonance: the reading is then high by about the square of the ratio of the two frequencies, a hundredth at a tenth of the resonance (apparent inductance). If the meter’s test frequencies all sit too close, find the self-resonance with a sweep and correct the reading with the same equation.
References
Section titled “References”- [Keysight Technologies, Impedance Measurement Handbook]: an impedance meter reads the effective value at its test frequency, parasitics included.
- Measuring magnetics: the apparent inductance, worked through, and a bench exercise.
The numbers in the examples, presets and exercises on this site are synthetic: round teaching values, not data from any real design.