Inductance meter

Objective

A traditional multimeter accessible to the hobbyist can measure all essential electric properties, including capacitance. The only property that still remains difficult to measure without extra investments (which can become rather essential) is inductance. Solenoids/coils are a significant part of any analogue circuit and, as opposed to all other discrete components, are often homemade. There are formulae helping to calculate parameters like number of windings, cross-section of the core etc, but one critically needs to verify the inductance of the final product. This article describes how one can build a rather accurate L-meter at home for probably less than $10. Here you will find the schematic, PCB layout, micro-controller source code; basically all you need to DIY.

Description

The L-meter presented here is based on a rather popular solution using the LM311 comparator (see e.g. [1]). It is stripped from the capacitance measuring capability due to micro-controller (MCU) source code size limitation. This limitation is dictated by the compiler I have used: MikroC PRO for PIC, ver. 6.0.0. The free license of the compiler allows up to 2k of program words (2066 to be exact, and the present code is 2065 program words).

Other solutions omitting the LM311 and using built-in the MCU comparators can also be found (see e.g. [2]).

The MCU used in this project is PIC 16F88. Obviously, the code given below is also for this MCU.

Specs

The L-meter presented here has a lower limit of ~1 µH and an upper limit of ~1MH (Mega Henry).

Some sources (e.g. [3]) claim lower limits as low as 10 nH, but I personally do not see how this can be achieved with the current solution. For more details see section "Theory and Schematic".

Theory and Schematic

L-meter schematic

The working principle of the schematic is as follows:

1. A tank circuit consisting of the capacitor C3 and the unknown coil will vibrate at a resonance frequency: