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I was wondering if anyone can help me with choosing a right inductor. My aim is to create circuit, which would start oscillation of a steel-core string. This would be achieved by oscillator (tuned to the fundamental frequency of the string), amplifier and inductor (which would transfer the oscillations to magnetic field). The only thing I am not sure about is the inductor - can you please help me with some formulas / tips maybe? I've hear an advice to just start wiring coil until it works, but I am not sure about the diameter of the wire also.

Thank you very much for any tips/help!

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You don't want a inductor, you want a electromagnet. Inductors are designed for their electrical properties with the external magnetic field being a byproduct. In fact, some inductors are designed to minimize the external electric field because this can cause interference and stray pickup in some circuits.

Electromagnets are designed to deliberately produce a external magnetic field. This is what you want because you want to interact with a magnetic guitar string.

I haven't seen a lot of deliberate electromagnets available as individual parts. Fortunately, these really are easy to make yourself. You wind some thin wire around a core. The more turns, the stronger the magnetic field for the same current, but also the higher the DC resistance.

For quick testing you can use a small iron or steel (not stainless steel) rod, like a nail. However, a conductive core also acts like the secondary of a transformer and essentially shorts out the transformer at AC. If you just want to make a controllable magnet that will be on long periods of time compared to the switch on time, then a iron core is fine. It will only add extra load when the magnetic field is changing. However, in your case you want to change the magnetic field at audio rates, so a conductive core is not a good idea.

What you want is a ferrite core. Ferrite is a material that does not conduct electricity but is still magnetic. Plain ferrite cores can be bought off the shelf in a variety of sizes. Fair-Rite is one company that comes to mind, and I know Mouser sells at least some of their stuff. A small ferrite rod is what you want.

Once you have the rod, wrap a few layers of tightly space magnet wire around it. This is around #30 wire with thin enamel insulation intended for exactly this kind of application.

To drive this, use a ordinary audio power amplifier intended for driving a loudspeaker. The impedance of your eletromagnet may be lower than the 8 Ω the power amp is expecting, so it might be a good idea to put a resistor in series with the coil, at least for starters to see how things go. A 4 Ω 2 W resistor should do it.

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  • \$\begingroup\$ Good morning Olin. How's that: a fresh question with no answers yet, just for you! :-) \$\endgroup\$ – stevenvh May 30 '12 at 11:44
  • \$\begingroup\$ @stevenvh: I was surprised both you and Russell missed this one. \$\endgroup\$ – Olin Lathrop May 30 '12 at 12:13
  • \$\begingroup\$ The community for this Q&A is so small... \$\endgroup\$ – kevlar1818 May 30 '12 at 12:20
  • \$\begingroup\$ Well, we thought, let's leave this to Olin. It's not yet his birthday, but whatever ;-) \$\endgroup\$ – stevenvh May 30 '12 at 12:40

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