Electronics > Beginners
Did I get screwed on these electromagnets?
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stj:
they are sold as "holding magnets",
these are a specific type of magnet inteded to do things like keeping doors open,
they often have no noticeable pull but are very strong once a steel plate is against them.

i have worked with rectangular ones that had so much hold that you couldnt seperate the plate - used for security doors that unlock in a fire situation., yet the "pull" was un-noticable to the point i actually wondered if the coil power was routed via the casing so it only energised when it was in contact.
T3sl4co1l:

--- Quote from: TakeItUpANotch on July 24, 2018, 09:33:18 am ---That's interesting and is actually relevant to my project. I'm using the magnets to vibrate strings (hence the 100-500Hz range) and ideally I want the strings to start vibrating as quickly as possible when pulsed, so I might have to look into spiking the voltage for the first few hundred milliseconds or so. I'm controlling the magnets with an ardunio and L293D H bridges that are fed a constant 12V from a fairly hefty power supply (so no noticeable voltage drop under load). Do you have any suggestions for how I would go about spiking the driving voltage for the initial pulses?

--- End quote ---

Oh, so not even 12V, but 10V...

L293 is pretty shit, it's old and best forgotten.  A MOSFET bridge (with logic level inputs) is available almost as cheaply, while capable of more voltage and current, faster, and with less voltage drop.  (I don't have a number handy.)

That said, the contemporary solution would be L297.

You might as well get an integrated coil driver -- yes, there are ready-made ICs for this!

So, that said, to "pluck" the string as fast as possible: consider this is a linear system, from coil voltage/current, to string wobbling.  You're fine, it's fairly instantaneous even at a tiny voltage.  As long as you don't mind having a small deflection.  If this is an electric guitar, just turn up the gain.  SNR may be worse, feedback may be worse, but there's probably enough dB inbetween to get away with it.

Now, if you know how much deflection you need,  then you simply need that much coil drive times the mechanical gain of the magnet.

Note that mechanical gain depends wholly on how strong the field is.  Is this an air cored bobbin, to be fit onto something?  Does this have pole pieces?  Surely you need to extend the pole pieces with a nose, so that they pluck just one string, not "most" of them?  If there are no pole pieces, it's just about hopeless, the field from an air core coil is very weak indeed.  You will gain far more from adding pole pieces, than going from a AA battery to anything that has been discussed here!

Tim
TakeItUpANotch:

--- Quote from: T3sl4co1l on July 24, 2018, 05:10:52 pm ---
--- Quote from: TakeItUpANotch on July 24, 2018, 09:33:18 am ---That's interesting and is actually relevant to my project. I'm using the magnets to vibrate strings (hence the 100-500Hz range) and ideally I want the strings to start vibrating as quickly as possible when pulsed, so I might have to look into spiking the voltage for the first few hundred milliseconds or so. I'm controlling the magnets with an ardunio and L293D H bridges that are fed a constant 12V from a fairly hefty power supply (so no noticeable voltage drop under load). Do you have any suggestions for how I would go about spiking the driving voltage for the initial pulses?

--- End quote ---

Oh, so not even 12V, but 10V...

L293 is pretty shit, it's old and best forgotten.  A MOSFET bridge (with logic level inputs) is available almost as cheaply, while capable of more voltage and current, faster, and with less voltage drop.  (I don't have a number handy.)

That said, the contemporary solution would be L297.

You might as well get an integrated coil driver -- yes, there are ready-made ICs for this!

So, that said, to "pluck" the string as fast as possible: consider this is a linear system, from coil voltage/current, to string wobbling.  You're fine, it's fairly instantaneous even at a tiny voltage.  As long as you don't mind having a small deflection.  If this is an electric guitar, just turn up the gain.  SNR may be worse, feedback may be worse, but there's probably enough dB inbetween to get away with it.

Now, if you know how much deflection you need,  then you simply need that much coil drive times the mechanical gain of the magnet.

Note that mechanical gain depends wholly on how strong the field is.  Is this an air cored bobbin, to be fit onto something?  Does this have pole pieces?  Surely you need to extend the pole pieces with a nose, so that they pluck just one string, not "most" of them?  If there are no pole pieces, it's just about hopeless, the field from an air core coil is very weak indeed.  You will gain far more from adding pole pieces, than going from a AA battery to anything that has been discussed here!

Tim

--- End quote ---

Do you have any recommendations on what MOSFET bridge I should get? The good thing about the L293D is it allows me to drive four magnets off of a single H-bridge making it a fairly inexpensive solution. But the voltage drop is a concern for sure.

I was thinking I could just simply drive the magnets with a simple transistor and put a flyback diode to protect the rest of the circuitry. Any reason why I shouldn't?

What I mean by having an instantaneous string reaction is overcoming the initial inertia of the string, so I get a quick sonic response out of it. The strings will vibrate an acoustic box and I don't plan on amplifying the strings at all by using guitar pickups or something like that. So the more they vibrate/deflect, the better really (within reason of course).

For reference, I'm building my own version of this instrument:

http://aaron-sherwood.com/works/magnetophone/

I'm admittedly not that familiar with magnetic field theory, but is it possible to focus the magnetic field of by adding a contraption that narrows the flux? A quick google gives me this

If a certain number of lines of magnetic flux is flowing in a piece of iron, and the shape of the iron changes so that its cross sectional area becomes smaller but the same number of lines remains the same, the flux density will increase. This is a way to increase B without having to increase Φ.

I was thinking of achieving this by simply adding a chopped of screw and nut and attaching it to the core piece (making sure the nut and screw sits flush against the core piece) like this:



Do you think that could work? I'm hoping that this area reduction in the magnetic field combined with a doubling of driving voltage (24V instead of 12V) will give me plenty magnetic strength to vibrate the strings effectively.

I appreciate the help and feedback btw. I'm new here but this forum really is a great place to ask these questions.
T3sl4co1l:
Why not rip open a small (say 10-50W) loudspeaker, and arrange a linkage from voice coil to wire support, the support being flexible so it can be pushed, while eliminating tensile strain from the loudspeaker?

It's ready made, the pole pieces are as good as they can get, and it's magnet based so you don't have to worry about attracting a string (square law, it attracts from either polarity), it's linear (pulls one way, pushes the other).  You'll also get enough efficiency that you can drive the strings with whatever waveform you like, you can "pluck" them very hard, drive fundamental and harmonics, whatever you like.  It's a loudspeaker with a weird notchy filter on it (the wire and attachments resonate at whatever frequencies).

Tim
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