I built this inductance meter adapter a long time ago for use with a digital multimeter capable of frequency measurement. The basic idea came from an article published in Funkchau issue 1/1981. I slightly modified the original circuit and designed a new PCB. Of course, I no longer use this device for measuring coils today, but back then it served me well.
The instrument can measure inductance values from 0.1 μH to 1 H with acceptable accuracy (1–2%). Its drawback is that the connected multimeter (or frequency counter) does not display the result directly in henries; instead, you need to calculate the inductance from the displayed frequency using the following formula derived from Thomson’s equation:
L [mH] = 2533 / f² [kHz]
The schematic makes the operation easy to understand. On the right side, the power supply built with transistors Q4 and T1 provides a constant voltage to the device regardless of the battery condition. Its power consumption is low, about 6–8 mA. The precision LC oscillator built with transistors Q1 and Q2 has a critical component: capacitor C1 (10 nF). This must be a high-quality, low-tolerance precision capacitor. The coil to be measured is connected in parallel with C1 at points X1 and X2. Q1 operates in a grounded-base configuration (its base is bypassed by C4 and C5), with the resonant circuit located in the collector circuit. The signal taken from the collector through C6 is amplified by Q2, whose emitter is connected to Q1’s emitter via R2 and C3. This provides positive feedback, which sustains oscillation. The bias of Q2 is set with R6, and the amount of feedback is controlled by potentiometer R8 to ensure stable oscillation. The oscillator’s output signal is fed through capacitors C7 and C8 to Q3, an FET source follower that isolates the output. The D1-D2 rectifier generates an auxiliary negative voltage, which is fed to Q1’s base to stabilize the oscillation amplitude. When properly adjusted, the output delivers a 2–3 Vpp signal with a frequency corresponding to the measured inductance. For a 0.1 μH coil, the frequency is 5.033 MHz; for 1 H, it is 1.592 kHz.
The PCB contains all components and can be built into a small instrument case with a standard battery holder:
The original German article also included a nomogram for determining inductance values. Since a pocket calculator is practically always at hand, this is unnecessary. However, I created a small chart for approximate readings, printed it, and attached it to the front panel.
| Part | Value | Device | Package | Library | Sheet |
|---|---|---|---|---|---|
| C1 | 10 nF | C-EU050-075X075 | C050-075X075 | rcl | 1 |
| C2 | 56 pF | C-EU025-025X050 | C025-025X050 | rcl | 1 |
| C3 | 4.7 nF | C-EU025-025X050 | C025-025X050 | rcl | 1 |
| C4 | 100 nF | C-EU025-025X050 | C025-025X050 | rcl | 1 |
| C5 | 4.7 µF | CPOL-EUE3.5-8 | E3,5-8 | rcl | 1 |
| C6 | 220 nF | C-EU025-025X050 | C025-025X050 | rcl | 1 |
| C7 | 4.7 µF | CPOL-EUE3.5-8 | E3,5-8 | rcl | 1 |
| C8 | 100 nF | C-EU025-025X050 | C025-025X050 | rcl | 1 |
| C9 | 4.7 µF | CPOL-EUE3.5-8 | E3,5-8 | rcl | 1 |
| C10 | 470 µF | CPOL-EUE3.5-10 | E3,5-10 | rcl | 1 |
| C11 | 4.7 µF | CPOL-EUE2.5-5 | E2,5-5 | rcl | 1 |
| C12 | 220 µF | CPOL-EUE2.5-6 | E2,5-6 | rcl | 1 |
| D1 | 1N4148 | 1N4148DO35-7 | DO35-7 | diode | 1 |
| D2 | 1N4148 | 1N4148DO35-7 | DO35-7 | diode | 1 |
| D3 | ZPD12 | ZPD | DO35Z10 | diode | 1 |
| D4 | 1N5819-T | 1N5819-T | DO41-7.6 | diode | 1 |
| L1 | R1405VB9 | B9 | — | inductor-nkl | 1 |
| Q1 | 2N2222A | 2N2222A | TO18 | transistor-npn | 1 |
| Q2 | 2N2222A | 2N2222A | TO18 | transistor-npn | 1 |
| Q3 | BF245 | BF245 | TO92 | transistor-fet | 1 |
| Q4 | BD139 | BD139 | TO126AV | transistor-power | 1 |
| R1 | 620 Ω | R-EU_0207/10 | 0207/10 | rcl | 1 |
| R2 | 470 Ω | R-EU_0207/10 | 0207/10 | rcl | 1 |
| R3 | 33 kΩ | R-EU_0207/10 | 0207/10 | rcl | 1 |
| R4 | 100 kΩ | R-EU_0207/10 | 0207/10 | rcl | 1 |
| R5 | 180 kΩ | R-EU_0207/10 | 0207/10 | rcl | 1 |
| R6 | 25 kΩ | TRIM_EU-LI10 | LI10 | pot | 1 |
| R7 | 4.7 kΩ | R-EU_0207/10 | 0207/10 | rcl | 1 |
| R8 | 100 Ω | TRIM_EU-LI10 | LI10 | pot | 1 |
| R9 | 10 kΩ | TRIM_EU-LI10 | LI10 | pot | 1 |
| R10 | 22 kΩ | R-EU_0207/10 | 0207/10 | rcl | 1 |
| R11 | 100 kΩ | R-EU_0207/10 | 0207/10 | rcl | 1 |
| R12 | 1.2 kΩ | R-EU_0207/10 | 0207/10 | rcl | 1 |
| R13 | 100 Ω | R-EU_0207/10 | 0207/10 | rcl_ |