Measurement of small inductors

How do we measure small inductors?

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In the practice of radio amateurs, very often there is the problem of making and measuring some inductors with a value between 1.5 - 16 uH. A simple device, easy to make, designed to measure the inductance of the coils made is shown in the figure.

How does the assembly work?

As can be seen, the diagram comprises an oscillator piloted with a quartz resonator, which will discharge a maximum voltage at the output when the coil with unknown inductance, in series with the variable capacitor Cv, will resonate on the crystal frequency Q1.

This maximum voltage is rectified by D1 and filtered by C5; C6; R7 and indicated by the analog instrument with maximum 100 uA the head of the ladder.

The value of Q1 is not critical. It is recommended to use a quartz with a frequency between 6.5 and 8 MHz. Some attention will be paid to the variable capacitor Cv. It will be ensured that the used specimen has the residual capacity (value at "fully open") as small as possible.

The maximum value ("completely closed") will be 180-220 pF. Basically these minimum Cv and maximum Cv values ​​will determine the range of inductance value that can be measured with the device.

How do we calibrate the assembly for a correct measurement of small inductors?

The calibration of the device will be done by inserting between the terminals Lx coils with known inductance (standard) of 2,5; 5; 7,5; 10; 12,5 and 15 μH; the Cv button is operated until the maximum indication of the uA-meter, marking around the axis of the variable capacitor the positions corresponding to each standard.

The device made with the values ​​of the components from the scheme covers the field of interest of radio amateurs.

How do we measure larger inductors using the same assembly?

For those who want to measure inductors greater than 15 uH Q1 will change to a 1-3 MHz crystal, and Cv will be 500 - 1000 pF and vice versa, for coils with inductance less than 2 uH, Q1 will be 10-13 MHz, and Cv of maximum 80 pF.

Of course, the interested amateur can mount Q1 in the socket, and with a Cv of 100 pF and a set of fixed capacitors of 100 pF in parallel through a switch will cover a much wider range of measurable inductors.

In this case, for each Q1 and position of the switch (capacitor parallel to Cv), the Cv axis must be recalibrated with corresponding standard coils.

List of required components:

  • R1 = 12 K
  • R2 = 68 K
  • R3 = 3.6 K
  • R4 = 510 K
  • R5, R6 = 150 K
  • R7 = 3.9 K
  • C1 = 47 pF
  • C2, C3 = 10 nF
  • C4 = 510 pF
  • C5 = 2 nF
  • C6 = 4.3 nF
  • C7 = 10 uF / 16V
  • D1 = 1N4148
  • Q1 = 6.5 - 8 Mhz (or between 1 - 3 Mhz for measuring larger inductors)
  • T1 = BC107 (BC547)
  • Cv - variable capacitor = 500 - 1000 pF
  • microammeter (or measuring instrument with a scale of 100 uA)
  • 2 x terminal contacts (for supply and for measured coil)
  • K = simple switch


Tehnium Supplement Magazine 1991 (Laboratory)

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