I have recently been working on tuning circuits, which typically consist of an inductor and a capacitor (i.e. an LC circuit). For those not familiar with inductors, they are passive components that store energy in a magnetic field. They oppose changes in current, meaning that they pass direct current (DC) and block alternating current (AC).
While there are many types of inductors, ferrite core inductors are useful because the high magnetic permeability of ferrite leads to a significant increase in induction. One common use case for ferrite core inductors is ferrite loopstick antennas in AM radios, where the inductor acts as both an antenna that interacts with the magnetic component of the carrier wave, and a component of the tuning circuit. Even the earliest transistor AM radios, such as the Regency TR-1 and Sony TR-55, used ferrite loopstick antennas.
(original Sony TR-55 internals image from u/1Davide on Reddit, original Regency TR-1 internals image from Nuts & Volts)
While inductors can be difficult to work with, they are fairly easy to make with basic supplies. Doing so allows you to manipulate the design of the inductor to fit the necessary attributes for your application.
An Air Core Inductor Link to heading
In order to observe ferrite’s impact on inductance, we can start by creating an inductor consisting of only a wire coil, or an “air core”. To make it easier to reuse in the future, we can create the coil by wrapping it around a ferrite rod, then slide it off. I typically will use a piece of paper (such as a sticky note) to make it easier to remove the coil, or move it to a different position on the core material.
We’ll start, somewhat arbitrarily, with 30 turns of 28 AWG magnet wire around the core. Starting by creating a small loop and feeding one end of the wire through it helps to keep the coil from unraveling.
We can also use tape to secure the core to the paper to keep it intact as we move and test it. There are a variety of strategies that can be leveraged to measure inductance, but we’ll use an LCR (inductance, capacitance, resistance) meter for convenience. I have the DE-5000, which is an affordable, decent quality option. Because the magnet wire is enameled, we need to strip the ends before we can measure with the LCR meter. A box cutter, craft knife, or sandpaper can be used. You should be able to see the exposed copper at the ends after the enamel is removed.
After stripping, the LCR meter alligator clips attachment can be used to clamp onto each of the ends of the coil.
As expected, the measured inductance is quite low ( 5.3 uH ).
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