# Making iron tube resonate with inductor

Hey all, some help would be greatly appreciated!

I have an iron tube which i want to make resonate using a self-wound inductor (inductor placed diametrically on the end of the tube, with some 3 mm gap between it) I already made it happen by connecting it to an old audio amplifier and playing a sine wave with the correct frequency.

Resistance including the wires was 1.5 Ohm. I already thought my audio amp wouldn’t like this, but it did work. Only when the tube suddenly HIT the coil, i blew up the poor old amp :( (could this be some back EMF which made this happen?) The inductor has a resistance of 0.5 Ohm. I measured that i needed about 5V AC across it to make the tube resonate. For larger tubes with lower frequencies, i would probably need some more...

Anyway, now i want to do this via an arduino. Idea: change a PWM signal of a certain frequency to a high power AC signal of the same frequency. It doesn’t have to be an exact sine wave, a square would also to the trick. I’m pondering whether it would work to use a power transistor in combination with a capacitor?

Any help would be greatly appreciated. I'm looking for the most simple solution. And i’m also wondering if there’s any danger with what i’m doing?

Thanks a lot, Thomas

By the way, i meant a self-wired electromagnet, of course. And to make more clear of what i was thinking of: amplyfying the PWM signal with a power transistor, then putting a capacitor in series to remove the DC bias

For proper circuit design it is essential to know the frequency and the impedance of the inductor at that frequency.

In any case, the project seems to call for a rather high power amplifier, which is not trivial to design.

Drive an inductor with a voltage square wave and the current will be a triangular wave.

You'll still get resonance, but you'll also be able to resonate on odd harmonics.

Drive an LC circuit and you'll get much better resonance and power transmission - however you'll have to keep the coil fixed relative to the tube to keep the inductance constant.

You blew up your amp by overcurrent - bringing the coil closer to the iron allowed the magnetic circuit to saturate, so the current jumped dramatically. Until then the inductance would have limited the current nicely I suspect. Ferromagnetic materials like iron have a maximum flux density they can respond to, above that the material is fully magnetized and can no longer increase its magnetisation, leading to a dramatic drop in inductance above the threshold, and rapid increases in current in the driving coil.

Thanks for your comments. I will dive into this a bit more.

any chance you’re trying to build an induction heater?