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Power Inductor Checker

February 26, 2013


There are three kind of passive electronic component, Inductor, Capacitor and Resistor. The inductor is an only component that can be built easy and many type of core and bobbin kits are available. When wind it yourself, an inductance meter is needed to measure if it is in desired specs or not. However measurement of inductance is relatively difficult because the inductance varies depends on measurement frequency and DC bias current. For tuning inductors in radio circuit, it should be measured at working frequency. And for choke inductors used in SMPS, variation of inductance by load current, especially point of core-saturation, must be known.

This project build an inductor measurement adapter specified for power inductors. Also an oscilloscple, 20MHz or higher, is needed for measurement.

Theory

When a constant DC voltage E is applied to an inductor L like shown in right image, the current I increases proportional to the time past, I = E * t / L. If the inductance is constant independet of DC current, ploted line on the graph will be straight. However it is affected by DC current, it will plot a curved line.

Thus the inductance and its current-dependency can be easily measured by applying a voltage pulse to the inductor and displaying the current on the oscilloscope.

Hardware

PCB
Circuit Diagram

Right photo shows the build inductor checker. It applies voltage pulses to the inductor to be checked 50 times per second. The inductor current is sensed by series resistor Rs and the waveform is displayed on the scope. The inductance can be read by: L = Vs * Δt / ΔI

The pulse width is adjusted by a pot and a jumper switch for wide inductance range of inductors, and the test voltage is supplyed by an external voltage source. The test voltage is decoupled by very low ESR capacitors C1 and C2 to retain a constant voltage against high current pulses.

Examples

TDK TSL0808-220K1R7
A: 21.5μH
B: Rated current = 1.7A
C: Saturated at 3A (ferrite core saturats sharply)
D: 3.2μH (after core-saturation, it works as air-core inductor, so the peak current must be kept lower than core-saturation point)
Yuden LHLZ06NB221K
A: 230μH
B: Rated current = 360mA
C: Saturated at 650mA
D(E): 43μH (inductor current is limited at Vs / DCR)
TOKIN HP053Z
A: 85μH at 0A
B: 50μH at 5A
C: 27μH at 10A
D: Rated current = 5A (dust core has high tolerance for peak current by its gradual saturation characteristics)
Junk 100μH
A: 108μH
B: Saturated at 2.5A
C: 16μH
Removed from a junk M/B
A: 2.3μH at 10A.
B: 1μH at 40A.
C: Current limitter triggered at 60A.
D: Vs dips approx. 0.5V at peak current. Also drop out voltage at Rs should be considered.
Rewinded the inductor in above (6T to 18T)
A: 20μH at 3A (3 times turns makes 9 times of inductance. It is well corresponding to the theory due to very low leakage flux of troid-core.)
B: 12μH at 8A.
Small signal core material (FT-50-#77)
A: 90uH
B: High-μ gap-less core saturates easy, so that it cannot handle large power.

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