Frequency/inductance calculator

Oscilloscope Inductance Meter

About

The Oscilloscope Inductance Meter is a simple inductance (L) meter adaptor for oscilloscopes, driven off a single 9V alkaline battery. When the inductor under test is attached to the two crocodile clips, it becomes part of a two-transistor pulse oscillator circuit. The frequency output is almost completely linear on a log/log-scale, and measures to, after conversion, within ±1% of the actual inductor value (as measured by several very fancy and expensive LCR meters).

So why make this device if great LCR meters exist? Well, this device costs about a dollar in components, and easily interfaces to a scope that is much cheaper than any of the very precise LCR meters. Additionally, many very cheap LCR meters have issues at the lower and high ranges; this device measures from at least 500 nanohenries to 1 henry; that's 51/2 decades of measurement! No mean feat, at all.

You will need a 50MHz or better oscilloscope to use it with very small values; I use a Rigol DS1102Z 100MHz/1GSS oscilloscope without any issues at the extreme low inductance range.

Circuit diagram, including power supply


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Circuit, explained

Power supply

The power supply section uses the simple, inexpensive MC/TS/AZ34063 switching converter in a buck configuration, converting the nominal 9V value from the battery to a fixed 2V ±1% supply for the meter circuit. Inexpensive PET metalized film capacitors are used for C1 and C2; all resistors are ±1% 1/2W metal film resistors, and the inductor L1 is an inexpensive, 1W, resistor-sized axial part. The largest component, C3, is a 16V 2200µF electrolytic capacitor; the large value is chosen to minimize voltage ripple on the output.

The power supply is dimensioned for up to 150mA output at 2V; the actual draw is typically <30mA. The green LED, LED1 indicates that the test device is on; it does not have a resistor, because its forward voltage at 20mA is more than the 2V of our power rail; therefore, the low voltage will limit its current.

L/F converter ("the meter")

The actual conversion circuit is a simple, two-NPN transistor astable multivibrator circuit. It is a variant of a similar, simple, two-capacitor/two-transistor oscillator circuit, but rearranged to instead use a single inductor.

The device-under-test inductor charges slowly up, until there is a voltage difference of about 40mV across it. At that point, one of the two transistors (can be either) turns off, since its VBE falls below ~620mV - below the threshold for turning on the transistor. This causes the inductor to rapidly discharge its magnetic field back into the turned-off transistor, causing the voltage to fall on the other side; this process repeats at a fixed frequency.

The speed with which the inductor charges depends both on R11/R12 and the input voltage; therefore, the input supply voltage is fixed to 2V in the device. Reducing the size of the two resistors decreases the frequency and the measurement range; the values were chosen to fit regular 50MHz oscilloscopes for most applications.

The secondary, optional output stage (R13 and on) simply conditions the signal by amplifying it to improve the rise/fall time, and then AC couples it to OUT2 so the waveform's maximum and minimum is ±1V instead of ~1.8V to Gnd. Which output you prefer is a matter of taste.

If the circuit fails to start oscillating, add a 1K resistor in parallel with R11 or R12 to help the inductor-under-test to develop the desired potential difference; once oscillating, this can be removed. With the values chosen, I have not experienced a failed start with values from .5µH to 1H. Finally, it is important to keep leads and connectors as short as possible; every conductor is an inductor, and by minimizing the parasitic inductance, the accuracy improves.

Prototype


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The prototype was made on a simple 5x10cm piece of TriPad protoboard with cheap components and two S9018 NPN transistors. The secondary (optional) output stage was omitted for clarity.

Measurements

With the chosen values, you can calculate the inductance of your tested device from the oscilloscope-measured frequency using this calculator I developed.

Inductance calculator

PCB

I am working on a simple PCB, most likely using two CR2025 cells, as well as surface-mount devices instead of the current, prototype through-hole parts. Replacement transistors will have to be found that match the 1.0 GHz transition frequency of the SS9018 transistors without breaking the bank.

The PCB gerber files will be made available here, as well as a simple, inexpensive kit, making this the absolutely cheapest inductance meter you can find anywhere. And it's pretty damn precise, too.

About the author

David Christensen is an electronics fanatic, especially in the audio and low-frequency areas. The author can be contacted at m e (at) davidh.info.
Copyright © David H. Christensen, 2017. All hardware available is Open Hardware.