The basic principle of inductance meter measurement is the constant current source method. Because the op amp and peripheral components form a constant frequency AC constant current source, and then measure the voltage across the inductor in series in this constant current source circuit, the inductance of the inductor is obtained, that is, the inductance of the inductor is measured indirectly.
Calibration of inductance meter
After zeroing the inductance meter, insert a standard inductance in the inductance socket, for example, insert an 18μH inductance in the 20pμH block, adjust the resistance R5 to display 18μH, and then calibrate the other blocks in the same way. Because the 20μH block current is relatively large, about 4mA, the output limit of the op amp has been reached, and this resistor is smaller than other block resistors. The remaining 4 block resistors are basically in a relationship of 10 times. The other blocks are deduced by analogy.
Notes on inductance meter measurement
Precautions for measuring inductance with DC resistance and parasitic capacitance:
Because the inductor has DC resistance and parasitic capacitance, the inductance meter measures the total impedance, so when the parasitic capacitance and DC resistance of the inductor are too large. The measurement error will also be larger. In order to reduce the influence of parasitic capacitance, when the inductance is large, the parasitic capacitance is also often large, which can only be measured with low frequency. To reduce the error caused by the capacitor. For DC resistance, if the resistance is less than 10% of the inductive reactance at the measurement frequency, the measurement error
Precautions when using an inductance meter to measure the inductance with a magnetic core:
When using an inductance meter to measure the inductance with the magnetic core, the influence of the magnetic permeability of the magnetic core must also be considered. The magnetic permeability of the magnetic core is not a constant, but a function of the magnetic flux density. When the magnetic flux density is zero, the magnetic core has an initial permeability. When the magnetic flux density increases, the magnetic permeability rises and has a maximum value, and then decreases as the magnetic flux density increases. When the magnetic core reaches magnetic saturation, the magnetic permeability tends to l. When measuring the same inductance with a magnetic core, the readings of each block are often different, and some errors are up to tens or even several times. This is not an error of the inductance meter. But the working current of each block is different, so the magnetic flux density of the magnetic core is different, resulting in different magnetic permeability. The inductance is different.