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# Slow-discharge circuit for gradually diminishing electromagnet field? Answered

As I understand electromagnets, you wrap a wire coil around an iron rod and run current through the coil. As long as there's current going through the coil, you have a magnet... But when you cut the current, no more magnetism...

I want to create a circuit in which a hand crank charges up one or more batteries or capacitors, which in turn discharge slowly through an electromagnetic coil (such that the magnetic field of the electromagnet gradually diminishes) over the course of an hour or so.

The overall idea is that you crank the crank for a short while, creating a magnetic field so that BBs cling to the electromagnet. But little by little, over the course of maybe 20 or 30 minutes (longer would be OK), the BBs drop off one by one (or a few at a time).

Can anyone suggest what such a circuit might look like? What variables will determine how long the crank has to be turned, how strong the magnetic field will be and how slowly it will discharge? And will the BBs only cling to the ends of the electromagnet? Or will they cling along the length of it as well?

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## 10 Replies

Downunder35m (author)2014-08-04

It won't work that way:

The BB's will magnetise and stick together.

There is no way of controlling how they "drop".

Much better solution would be a "free feeder".

Nice funnel filled with BB's, neck is getting smaller in diameter until only a single BB will fit through.

The magnet pulls or pushes a mechanism that grabs one BB at a time to release it further down.

Basically just an off-center hole with a blocking plate - one BB fits in the top and is stopped by the plate, maget pulls/pushes the mechanism so the hole aligns with the entering hole into the firing mechanism - BB drops out.

Magnet releases and the next BB is filled.

To get a better idea of the working simply check how the dispenser for those sugar free coffee sweeterens work.

But would actually help to know what the intended purpose is otherwise it is hard to give a specific solution to the problem ;)

Toga_Dan (author)2014-08-10

I've tried permanently magnetizing BBs. I can't make it happen even when I try. It is easy to magnetize a nail, a needle, a screwdriver, but for some reason, it's tough to magnetize a bb. I'd like toknow why.

Downunder35m (author)2014-08-10

There is a way but it is not the easiest:
Take two thin sheets of wood.

Use a hammer, tweezers (for your fingers ;) ) and a BB to make nice dints into the wood so you can place the BB's quite close to fill the board.

I used pieces about 20cm long and 2cm wide as the 2cm perfectly matched the big magnet I had.

Once you got the dints place the other sheet on top and drill holes through each corner for some screws.

Fill the dints with BB's and secure with the top plate and screws.

Now your BB'S are wedged in and can't move, so when you magnetise them the field builds up in always the same direction.

Most BB's are made from carbon steel and barely get enough magnetized to stick to metal, when vertical they simply roll down but overhead they should hold on.

Downunder35m (author)2014-08-04

It won't work that way:

The BB's will magnetise and stick together.

There is no way of controlling how they "drop".

Much better solution would be a "free feeder".

Nice funnel filled with BB's, neck is getting smaller in diameter until only a single BB will fit through.

The magnet pulls or pushes a mechanism that grabs one BB at a time to release it further down.

Basically just an off-center hole with a blocking plate - one BB fits in the top and is stopped by the plate, maget pulls/pushes the mechanism so the hole aligns with the entering hole into the firing mechanism - BB drops out.

Magnet releases and the next BB is filled.

To get a better idea of the working simply check how the dispenser for those sugar free coffee sweeterens work.

But would actually help to know what the intended purpose is otherwise it is hard to give a specific solution to the problem ;)

Toga_Dan (author)2014-08-10

(duplicate post, ^ downunder. i've done that before, too. tried to flag duplicate, just to clean stuff up. easier, tho, perhaps to delete the extra yerself.)

Qcks (author)2014-08-03

Actually... you're asking about inductors, and this is something that is somewhat complex.
First, a bit about magnetic fields. The BB's are going to be preferentially held in the mid point of the magnetic field. That is the point that the magnetic field is going to try to push the bb's to, and, if you want the magnetic field to persist in a predictable manner, the place that you should start the bb's off at.

As for how long and strong the magnetic field is and needs to be, you need to know the mass of the bb's. At a minimum, a rough estimation of average mass is needed. Then you need to use the formula for the acceleration of an object due to gravity. That tells you force which the magnetic field is going to be resisting.

(force = mass * acceleration)

After you have the force exerted by gravity, you have the minimum force that needs to be applied to the bb via the electromagnet.

The force of the magnet is probably going to be most easily figured via the math of a solenoid. You'll need to convert the magnetic flux density into a Force, preferably in the same units as the force due to gravity.

Magnetic field = ((Permability of free space) * (magnetic permability of iron core)*(number of loops in electromagnetic coil)*(amps)) / (Diameter of a single loop)

The best description of the math used is here: http://en.wikipedia.org/wiki/Solenoid

At any rate, you have the minimum magnetic field needed, and the requisite amps to power it, that tells you how much power you need to have stored. Now, I would actually suggest using a capacitor, rather then a battery. Batteries are heavy and actually do not charge quickly. You need to have the ability to quickly charge the power source, in addition to increased duration of discharge.

Toga_Dan (author)2014-08-10

Athough I sometimes crunch numbers before building somethin, this project sounds like one to just make somethin with the bits n pieces yoou have on hand. Whats the worst that can happen?

Donny Bahama (author)2014-08-08

Thanks for the replies, everyone! Downunder35m, you're absolutely right - I should have been more clear about the intended purpose.

I'm trying a new take on the "rain box". For those who aren't familiar, a rain box is a decorative wooden box with chimes inside. Mounted to the underside of the lid are paper discs coated with the adhesive used on Post-It notes. You turn the box upside down, shake it around gently, and the BBs cling to the Post-It note discs. Then you turn it right side up and the BBs fall off, a few at a time, striking the chimes on the way down. The sound is very random and sounds kind of like rain. When the rain box is new, the sound continues for an hour or more - but over time, the adhesive on the Post-It note discs gets weaker and weaker. I'm trying to address that by replacing the adhesive discs with slowly discharging electromagnets.

Since my original post, I've come to the conclusion that it would probably be better if the BBs rested on top of the magnet structure (which would be tilted slightly) rather than clinging to the bottom. Seems to me the electromagnet wouldn't have to work so hard that way. Also, rather than a hand crank to charge up the batteries/capacitors that power the electromagnet, I'm thinking it might be better to use batteries and a momentary switch to quickly charge one or more capacitors which would then supply the current for the electromagnet.

Downunder35m (author)2014-08-08

Ahh, so no precision feeding required :)
Did you consider a spinning disc?

If you have a concave disc filled with your BB's you can let it spin and the centrifugal force will drop the BB's off the disc.

With a timed feeder from above you can get the dropping pretty random as some BB's will stay in the center of the disc and spin with it until touched by others disrupting them. So you never know if one or more fall or when they fall.

The "ECO" version could be just a long glass filled with peas and water - water makes the lower peas expand and the top ones are pushed out, nice to hide somewhere at night on an oven tray ;)
I think the problem with your glue is quite simply dust, so if the basic priciple with the glue works for you, maybe re-designing the box so the inside is sealed and the opening of the chimes oeas out could make the glue last longer, sometimes a good heat up helps too with such glues to make them sticky again.

But it would not be hard to use electronics an a tiny hammer to get random noise from the chimes without any BB's or glue.

Kiteman (author)2014-08-03

Try simply discharging a small lead-acid battery through the coil (you'll probably need a resistor in series as well).