Overheating Coils with Audio Signal


Old 03-05-14, 09:36 AM
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Overheating Coils with Audio Signal

Hello Forum,

I'm not sure this post belongs here but I'm looking for help anywhere I can.

I'm working on a project that requires controling the frequency and amplitude of multiple string vibrators and the easiest way for me to do this is using audio signals controlled with a MAX/MSP patch.
I bought this device String Vibrator - WA-9857 : PASCO, it consists of a copper coil, a rare earth magnet and an aluminium reed, I already made my home imitation (foto). Since this device is made to work with a 5V AC power adapter at 60hz I have the following problem:

- If I use an amplified audio signal the coil overheats.
- If I use a sine wave generator I'm unable to control the signal's frequency and amplitude from my computer, not to mention that I would need multiple generators to control each vibrator.

I've also got Economy Wave Driver - WA-9854 : PASCO, which is basically a loudspeaker with no cone and a reed to insert the string. This one does not overheat but it sounds loud and my project requires no sound or a soft sound. This would be my best solution but I need a way to get rid of the sound.

I am a musician and really don't know much about electricity or amplification, I would love to hire someone to help me build a device that meets my requirements or I wonder if any of you has any ideas or advice for me.

Thank you very much in advance
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Old 03-05-14, 11:13 AM
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"I would love to hire someone to help me build a device that meets my requirements..." You never stated your requirements or what you are trying to build. It would help if you said what you are trying to accomplish.
Old 03-06-14, 05:29 AM
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If an audio signal is overheating the coil you're using too much power and/or the frequencies are too high. Try turning down the amplifier and put a low-pass filter on the signal. Are you using more than one frequency? If so the overtones may be coming into play to heat the coil.

If you're using this to teach a class about wave propagation, consider using water in a tank. It almost perfectly emulates sound wave patterns that develop in a reflective room.
Old 03-06-14, 07:23 AM
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@Pilot: I require a device to help me vibrate strings for several hours (12+ hours a day) at low frequencies (20hz - 120hz) producing the least amount of sound possible while at it.

@Rick: This is for an Art project, and I'm using sine waves at low frequencies since with them is no only easier to witness the effect but also they have a much higher amplitude. I am indeed using sometimes simultaneous overtones (e.g 20hz + 60hz).
From what I've been told, the impedance gets lower the lower the frequency, so from my understanding the fact that I'm working with low frequencies is what is causing me trouble, but the thing is that these are the frequencies I require for my project.

Thanks again for your help guys, most appreciated.
Old 03-06-14, 09:02 AM
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Looks like Pasco is a little dodgy in providing specifications for their products...

Making some assumptions I imagine the string vibrator is internally like the cone-less loudspeaker, only much smaller--and weaker. The input jack says 10V max. 20 volts peak-to-peak equals 7VRMS and if the device impedance is 8 ohms (an assumption) it's consuming about 6 watts (E^2/R)--which seems reasonable given the warning not to exceed 1A (7V/8ohms = ~1A).

It's quite likely you are overdriving the device if you're using a typical audio power amp. Your ear's sensitivity drops with lower frequencies so it takes more power to maintain the same perceived volume. The larger transducer looks to be a better choice if the movement has to do any real work by moving more than just a string.
What is the "art project" at the other end of the string? Is it large enough that when excited by the vibrating string it produces a lot of sound intensity? Are you trying to get paint or particles to "dance" on a "canvas" and capture the image created by the interference standing waves?
Old 03-07-14, 12:22 AM
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12+ hours a day No wonder it's overheating.

I'm a little lost.... art project and musician.
I've seen these used for demo.... just not that long of a duration.
Old 03-10-14, 08:35 AM
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Thank you so much for your answer. As you imagined, the string vibrator is very similar to a speaker, but in this case the magnet is inside of the coil, attached to a reed, heres a picture of one I made at home: https://imagizer.imageshack.us/v2/47...0/542/zzao.jpg

Regarding your questions, the only reason I'm using an audio signal is because its a lot easier to manipulate from a computer application, I may also work with several vibrators at the same time, so it would be the easiest way to handle multiple signals simultaneously as well.

Regarding the goal, the actual result I am expecting is merely visual, since the sound I can get is very unpleasant, I would like to get rid of it, this is where the string vibrator works better than the economy wave driver, it produces a quiet sound.

The economy Wave driver is a 5, 1/4" sub woofer with a post to attach the string to, do you have any ideas as to how could I get rid of the sound (or most of it)?

Thank you very much for your help.
Old 03-13-14, 08:52 AM
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first and only time I will bump, to give it a second chance

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