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AC powered Led Zeppelin electromagnet

  1. Aug 29, 2007 #1
    I've posted on here a few times concerning the electrical problems I continue to encounter for my video installations.

    And yet, here's another one.

    If anyone remembers, I posted a month back or so concerning the use of an audio signal from a home audio receiver to open and close a small electromagnetic valve. I tried a few methods and was unsuccessful, but after blowing the fuse in my multimeter I'm back at it again.

    I threw the receiver on Phono, and started up my LP of Led Zeppelin I. Metering the the speaker wire on the ACV setting, I believe Robert Plant managed to get around 30 volts before my mom demanded to know "Just what I thought I was doing". The receiver I'm using still had about 10 percent more until maximum volume, so I'm pretty confident I could get around 40 volts. I think it was the loudest I've had my speakers, and boy did it sound good.

    So, my question is this: Can 30 AC volts provide enough power for an electromagnet? From what I've read up on, it's largely dependent upon how many/how tightly wrapped the coils are. Store-bought electromagnets like this one are often powered by 6 DCV, is that something that can be powered with AC as well?


    If so, then would I just connect the speaker wire directly to the electromagnet? Or will I need to get an AC to DC converter?

    I realize that these are probably all basic electrical knowledge questions, and really appreciate anyone who helps me out. If I need to clarify any part of the setup or a diagram would help, please let me know.
  2. jcsd
  3. Aug 29, 2007 #2


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    Staff: Mentor

    Your speakers are themselves electromagnets:


    You can use AC or DC to energize electromagnets. You should at least try to make your electromagnet look like a speaker (in terms of AC impedance and DC resistance), so that you don't hurt your audio power amp by connecting it to your electromagnet experiments.
  4. Aug 29, 2007 #3
    I did the math [correctly, i hope] and plugged in 8 ohms for resistance as that is what my receiver states as the minimum. That gives me:

    30 volts, 3.75 amps, 8 ohms, and 112 watts at peak power.

    If I understand electromagnets, the current is what determines the strength. So if I can create an electromagnet which has 8 ohms of resistance, then the amperage is going to be anywhere from zero to maybe 4.5 amps. The windings of the coil will build up to 8 ohms i'm pretty sure, but i could always add a small resistor to the connector.

    Referring to "making the electromagnet look like a speaker", if I have the receiver outputting up to 30 volts, and the electromagnet's resistance at 8 ohms, do I need to worry about any other factor?
  5. Aug 29, 2007 #4


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    Staff: Mentor

    The speaker input impedance is a combination of the resistance of the wire in the speaker coil, and the inductive impedance of the wire coils sitting around the mu of the stationary magnet. And here, I need to plead ignorance and bail -- sorry. I honestly don't know how those two components contribute to the impedance seen by the power amp. Does the wikipedia.org link talk about that at all?
  6. Aug 30, 2007 #5
    It didn't mention anything that I could see; but if the 8 ohms of a normal speaker is a nominal resistance that varies, then making a magnet with a constant resistance of 8 ohms would be seen as "the perfect speaker" by the amplifier. Right?

    I'll study up on it some more tomorrow, thanks for all your help!
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