Electronics: Home made air core inductor.

This is my first stab at a home made inductor of a large-ish value.

Mister G. at work got me interested in building a home made guitar effects pedal. He has an inductor-less Colorsound Wah pedal that he brought in for me to check over. whilst checking the pedal potentiometer and watching the output on my oscilloscope, I wondered if the earlier 'large inductor' version of the Wah pedal would sound nicer (the inductor-less version is apparently not as effective at resonating at the lower end of the frequency range of a guitar.

According to all the available schematics, the Inductor used was around 500mH (milliHenrys). That is a flipping big inductor. I read around the guitar forums and understand the ones used in effects pedals need a fairly hefty ferrite core to the inductor to get the inductance values up high enough to work at the low guitar frequencies.

I found this great video on YouTube that demonstrates the differences quite nicely. https://youtu.be/4nl7xmcRLBU

The inductor works with a suitable capacitor to form an 'L-C tank' circuit). the bigger the values, the lower the resonant frequency. In a capacitor, the current leads the voltage by 90 degrees in an AC circuit. In an inductor, the current lags the voltage by 90 degrees. because the two are effectively 180 degrees out of phase, one is 'empty' while the other is 'full', and vice-versa. at the 'right' frequency, called resonance, this effect is at it's most efficient and has the least energy losses. The electrons move back and forth between the inductor (generating a magnetic field) and the capacitor (generating an electrostatic charge). the larger the sizes of these components the longer it takes for the charge to move and therefore the lower the resonant frequency.

Until I can source some ferrite core kits, I thought I'd see how big an inductor I could make without a ferrite core, this is called an air core (though I did use a steel bolt to secure it, which may have had some effect).

I went to Maplin and bought the thinnest enamelled copper wire I could find. in stock they had 40 SWG (Standard Wire Gauge) which equates to 0.125mm diameter.

I then went to Hobbycraft and bought some plastic sewing machine bobbins, 3 for £1.50.

I put a 6mm steel bolt through the bobbin (which *may* help increase the inductance slightly), put it in a cordless drill, then threaded the copper wire through the centre of the bobbin and wound her up until the bobbin was full.

as this was completely un-scientific, I didn't count how many turns I made. I kept going until the bobbin was nearly full. this was in the principle that it would never be as good as a ferrite core inductor, and I wanted to know how large a value I might obtain from a cheap plastic bobbin and some thin-ish wire. the spool of wire doesnt look any smaller, so I guess it'll be enough for all the inductors I'll ever attempt to make.

As it was winding on, I tried to move the copper wire back and forth across the width of the bobbin as evenly as I could to create even layers, as I've read that this helps create greater inductance.

Once the bobbin was full, I used a hot-melt glue gun to secure the copper wire.

I then drilled a 6mm hole in an off-cut of strip board (Veroboard) and bolted the bobbin to it.

I fed both ends of the wire through the outermost strips in the board to avoid them short-circuiting through the bolt.

Soldering is straight forward. Reasonable heat will melt the enamel on the copper wire. I tested continuity with a meter to be sure.

Finally I put a couple of PCB pins on the ends of the strip so that the whole thing can be pushed into a breadboard to experiment on.

Once complete, I hooked up my PEAK LCR45 meter to the inductor and measured the value. it came out at a shade over 42mH. It's quite big for an air core inductor, but only a tenth of the size I need.

My next step is to purchase some ferrite pot inductor kits from eBay. There a few about from eastern European sellers, but understanding the quality and efficiency of the ferrite core is currently beyond me. As they're only a few quid for a bag full, I'll give them a go and see what difference it makes.

Below are some pictures of the finished inductor.