Thanks to the growth of online retailers such as Mouser and Electronic Goldmine, it is easy to search, compare and purchase thousands of electronic components at afforable prices. Even so, certain components are relatively uncommon and difficult to find. Such is the case with variable inductors.
Inductors are of particular interest because of their ability to operate in two completely different energy domains. Electric current is a physical thing, readily sensed by human touch. But pass that current through an inductor, and something happens in the space around it. A stressed condition occurs, which is intangible and yet capable of extracting energy from the circuit and storing it in a magnetic field. If the electric current is reversed or shut it off abruptly, the stored energy is released back into the circuit, with greatly increased intensity. The voltage potential resulting from inductive discharge can be many times greater than that of the orginal electric current. In other words, fast switching of current in an inductor causes electricity to raise its own potential. This principle, known as “Method of Conversion”, was discovered by Tesla in 1887, is used throughout modern industry in the design of high efficiency power supplies.
Inductors are measured and rated according to their inductance. Variable inductors are designed with a moving part that can be used to make fine adjustments. They are generally used in the tuning of resonant circuits. There is quite a lot to say about oscillators and resonant circuits. See for example, Chapter 25 in Benjamin Crowell’s book Light and Matter.
Variable inductors fall into two basic categories: inductors for signal applications (sometimes called chokes) and power inductors. Most variable inductors available online are of the signal variety. My interest lies in power inductors, and these are much less common and therefore harder to find.
Fortunately, it is not very difficult to make your own variable power inductor. The basic components include a cylindrical ferrite core (such as Mouser part # 623-2631102002), and a roll of stiff, 18 gauge insulated wire. I found some plastic coated steel wire at a hardware store, and a ferrite core in my spare parts box, left over from a previous project.
The first thing we need to do is wind the coil. We’re using steel wire, which is relatively stiff and will hold its shape well. But the downside is, it takes some work to wind it into the desired shape. To make this easy, we’ll build a jig out of the ferrite core we plan to use, some hardwood disks from the craft store, and a few bits of hardware to put it all together. No dimension is critical, and anything you can build from the odd bits on your workbench should work fine. The disks I used are about 8 cm in diameter, which was a confortable size to handle. Smaller disks would require a tighter grip and more force to wind the stiff wire.
Before you assemble the winding jig, drill an extra hole in one disk for a wood screw to hold the free end of the wire. The wood screw holds one end of the coil in place, allowing you to wind the rest securely. It is important to keep the jig in tension once you start winding, or else the wires will tend to spring back and loosen a bit.
The first coil I made had only a single winding layer. But the inductance was too low, so I made a second coil with three layers. Inductance increases with each additional layer, although the energy conversion efficiency decreases slightly. The three layer coil is working great in my circuit.
By using an odd number of layers in the winding, the two free ends of the coil can be used to mount the coil to a wooden base. Once the winding is finished, use a pair of wire cutters to cut the wire, leaving about 8 cm extra. Dis-assemble the jig, and then reshape and trim the two free ends as needed. Scrape enough of the plastic insulation away, so that some of the bare wire can be pushed directly into the wooden base, leaving another section exposed. These become connection terminals for jumper wires that can be attached with alligator-style clips.
With the coil finished, it’s time to put together the wooden base and side support. I used some pre-cut hardwood strips from the craft store. The ferrite core is mounted onto a large bolt, using a few turns of electrical tape to make a tight fit. Drill a hole through the upright support, and enlarge the hole as needed with a file, until a nut can be pressed into it. The fit should be snug enough to hold the bolt firmly, but not so tight that tapping the nut into place with a hammer causes the wood to split. It took me a few extra minutes, working with a round file, to get it right. Once the nut is pressed into place, attach the upright to the base using two small wood screws from the underside.
Trim the coil ends as needed, to position the coil directly in front of the ferrite/bolt assembly. Find the right position to mount the coil, by twisting the bolt to extend or withdraw the ferrite core. There should be enough travel in the ferrite/bolt assembly, to move the ferrite core completely into and out of the coil. When you have it placed, mark the base for two small mounting holes.
I discovered while assembling the project, that the two free ends of the coil fit too loosely into the mounting holes. I had used the smallest drill bit available, but the 18 gauge wire did not fit snugly enough to hold the coil in position.
Fortunately, the remedy was simple. Using a pair of needle-nose pliers, I created a little zig-zag in each wire end. To do this, simply create two slight bends, a short distance apart, in opposite directions. With the zig-zags in place, the wire required some force to press back into the mounting holes. Working with steel wire at this point is quite convenient, since it is stiff enough to hold its shape.
Once the project was fully assembled, I pulled out a test meter to calibrate the variable inductor. Using jumper wires, I connected an Extech LCR meter to the coil terminals, and selected the “L” setting to measure inductance. I found this unit to vary between 41 and 112 micro-henries (uH), depending on the where the ferrite coil was positioned.
The free end of the bolt can be turned quite easily, to position the coil and set the desired inductance. If the variable inductor is used in conjuction with a resonant circuit, then the adjustment can be made based on observation of circuit operation.