# How to design a contactless mechanical shaker [closed]

I am trying to design a circuit to make an inductor oscillate with high-current (around 50A) at a low frequency. The inductor will act as a high-powered electromagnet.

Most designs I've worked/messed with for oscillators were used for higher frequencies, in MHz or KHz, like the Hartley or Colpitts oscillators. But I only want the inductor to resonate at 80Hz.

Edit: To be more specific, what would be the "best" (simplest, cheapest, more efficient, any combination of thereof) way to drive an inductor (1-10mH, this is the kind of coil I work out to obtain the electromagnetic forces required for my application) with a 80 Hz periodic signal (doesn't have to be sine, square would be better) and high current.

Mechanical shakers are what we have used in the past for this. However, here I need to shake an object inside a closed vessel without moving the vessel itself, only the "shaker" therein, which would be a permanent magnet. The vessel has to be non-moving so that it can be connected to rigid fittings, so fluids can be pumped in and out of it. And the fluid system can have no penetration to the outside, because no contamination must occur (biomedical application). Thus, the force must be applied at a distance with no direct contact, which is why I am looking at electromagnetism.

## closed as too broad by Dave Tweed♦Jun 24 at 11:34

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• How big is your inductor? If you want an electromagnet, wouldn't it be easier to weigh the current versus the number of turns? Perhaps a lower current, but higher number of turns would work better, and allow you to use a smaller capacitance too. – Joren Vaes Jun 24 at 6:29
• Serious question: Have you considered using a generator instead? – Marcus Müller Jun 24 at 6:31
• What are your requirements in frequency accuracy and stability and amplitude fidelity? Are we talking about percents of tolerance, or parts per billion? – Marcus Müller Jun 24 at 6:37
• "High-powered electromagnet" implies that you want to extract energy from the inductor's magnetic field. Doing so will both change its inductance and make it extremely "lossy" -- not good if you're trying to also use it as frequency-control element in an oscillator. What is the problem you're actually trying to solve? – Dave Tweed Jun 24 at 11:38
• 1/2 cycle @ 80Hz = 6ms @ LdI/dt= 10mH 50A /6ms = 83V ignoring losses and 12.5 Joules and loss, saturation issues unless huge ~ 2Hp linear motor. It's possible if you have one. – Sunnyskyguy EE75 Jun 24 at 14:50

A high-powered electromagnet excited with some sinusoidal signal?

Sounds like a PA speaker / subwoofer to me!

So, instead of building a high-powered oscillator, I'd simply use a soundcard, or a low-powered one, and feed it to a cheap power amplifier for speakers.

You'll mostly find amplifiers for 4Ω impedance loads, so to achieve 50A, you'd need P=I²/R=2500 A²/4Ω = 625 W of speaker power. (Often, stereo amps come with the ability to drive twice as much output power with the outputs bridged to just one speaker, or in your case, electromagnet.)

You can buy stage-grade PAs for that at reputable sources (Thomann etc), or you could build your own class-D amp (considering you really don't need to switch overly fast to drive something at 80 Hz).

You could of course also just build/get a class-AB amplifier, and add negative, phase-delaying feedback just so that you get your 80 Hz oscillation. That's cool, because it's then an oscillator on its own, but seriously, feeding in an external 80 Hz reference is both easier and more stable, precise and reliable.

• Marcus, thanks for the post. So, I have little experience with audio systems so bear with me, but I think I understand what you mean, in theory. I'd have a soundcard which acts as a function generator and sends a sinusoidal signal to the power amplifier. The PA takes in the signal and DC power, and outputs the signal but can supply it at a higher current. Plugs into the coil and bob's your uncle. If all that is right, I mostly have a practical question left: what kind of soundcard? Would the one I install in my desktop computer work for this, or do you mean another kind of device? – Xylord Jun 24 at 15:11
• You could use pretty much any microcontroller (e.g. Arduino) to generate a low frequency sine from its PWM analog output. – Transistor Jun 24 at 20:24
• @Xylord any soundcard would be suitable. The one built into your desktop PC. The one in your smartphone. the one in your laptop. A USB one. – Marcus Müller Jun 24 at 21:08