Has anyone seen or heard of a small (<1/4 inch in diameter) electromagnet that is SMD or through hole mount directly to a PCB? I need some really small ones for a wearable electronics project but wasn't sure if that was a thing or not. The closest I have found are inductors, but I'm not sure those are what I need or what H value would be right, as they are not designed with the intention of generating a useful field.
Hi,
Inductors in SMD from probably won't help, the ferrite material they are made of is designed to keep the magnetic field inside the assembly.
You might have a strength problem too, with SMD sized electromagnets, not enough area or windings space to make a feasible field strength.
Try Google;
smd solenoid
Tom... 
So what is your spec for this device, in terms of resistance, magnetic field, current etc? What direction do you want the field relative to the PCB plane?
There are SMD and thro' hole inductors available but very often they are axial so the coil will be parallel to the PCB whilst I would guess you want it perpendicular. Something like this?
That's a Murata 10mH type, comes in a wide range of values up to 68mH. Any use? I would guess from the shape that it has a bobbin-shaped ferrite core with a coil wound on it. It supports up to 6A DC so not I think a closed magnetic circuit. Look at any electronics distributor's catalogue. This came from RS. Diameter is 9.5mm.
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Thanks, I did find a bunch of small solenoids like that but I need just the magnet not the spring loaded plunger part. I need a tiny amount of field so small is okay.
If you can remove the spring, you may be able to glue the armature inside the coil, the armature will concentrate the field as the field emerges from the end of the coil.
Power needed will be a factor as well, you may have to experiment.
Tom...
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definitely have to try it, I don't have any hard numbers but I would like to run the coil at 3-ish V and 250-500 mA. I know that's really low power but I don't need much holding strength. Basically I have a rotor with a small permanent magnet and I want to EM to energize and provide a bit of "detent" to the rotor as it is spun by the user.
This form factor is exactly what I am looking for but I'm not sure how H relates to field strength for use as an EM
Well if these devices have a core, which they must do to get that much inductance in such a small package, I doubt if you would need any polarising current, there would be quite a lot of torque between your magnet and the core. Unless you also need to pick up an induced signal from the coils, why use EMs at all? You could just glue ferrite beads to the PCB.
How many detent "teeth" do you need? One option could be to use one of the cheap 48 or 64 step stepper motors, which would give you some detent and pickup coils. (Obviously not driven as a motor.)
I like this idea, however I need the detents to be able to turn off when not needed so that's why I want an EM. I will have 24 detents around the rotor.
Ah, well that gives you a problem! To avoid any detent when turned off you probably can't have a magnetic core because there would inevitably be some attraction between that and your permanent magnet. That in turn means that the EM current would need to be quite large. I suggest you need to do some experiments, making some small coils without cores and measure what current you need to get the detent you want. You can of course use a very powerful neodymium type rotating magnet.
An interesting challenge!
gotchya, I wondered how it would work too because I am essentially building a motor/generator so I will need to deal with the reverse current from a moving rotor over a coil weather it's energized or not.
I don't think that would be a big problem TBH, I doubt the induced voltage would be that high. I guess you would connect all the coils in series so they share the current and connect them to the supply through a MOSFET, with a current limiting resistor is needed. One clamp diode on the MOSFET output, or two for belt and braces, should do the job.
Hi,
Can you please tell us your application?
Tom....
It is strictly a conceptual project at this point but the idea is for a watch for vision impaired users. you rotate the bezel and the bezel will snap to the current time which you can feel from the detent. The EM's need to be able to turn on and off to indicate the rotor (bezel) to the correct spot.
Just thinking out loud here but would the effect be the same if the rotor had a magnet on its circumference that passed over a series of Hall effect switches and when the switch(s) corresponding to the correct time was in line a haptic feedback occurred?
Remember, magnetic circuits need to be completed, just like electrical circuits. If you do not provide any material for the magnetic field to follow it will randomly find it's own way to the other pole. Consider a metal shield over your EM to limit the area of the magnetic field and supply a known connection for the magnetic field.
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This should work, my system works by allowing the rotor to spin freely, but when a button is pressed the coil for the corresponding time is energized so that the rotor can be spun and run into that energized coil.
Would a flat stepper motor work for this?
Not having to power electromagnets would extend battery life. Depending on where you placed the haptic vibrator(s) you could simulate detection of hours and minutes. You could even pulse them to indicate a count if you wanted.
Anyway, just a thought.
possibly, but it would need to have the correct number of poles, and feedback so it knows where to stop in its rotation which might make it too complex. in a way the construction i am imagining is very similar to a stepper motor