Magnet catch-release help please

Sorry, nothing to do with fishing.

I have an idea for a release mech for an rc airplane.

A small super-magnet surrounded by a coil. The magnet holds whatever is to be held. The coil makes a magnetic field that cancels the magnet and whatever was attached falls away.

First, can I make a coil that will do that - shape, power required? Magnet weight would be 5grams or less.

Second, the coil only has to be energized for a fraction of a second. Every gram counts - how much can I reduce the coil wire guage because it won't have enough time to heat up?

Third, if enough not current is available from the battery to run the motors/electronics/etc AND fire the release coil what/where's a simple circuit to charge a cap and then dump it? I'll have power, gnd, and a signal wire that will either be 0 or whatever the single cell lipo is (3.7-4.2).

Reply to
unk
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Yes you can do that, but there are so many details not given that no one can be of much help.

Dan

Reply to
dcaster

I have done some more digging.

Looks like the magnet will have a strength of >1000 Gauss and be around

5mm by 2mm. 10 Amps through a coil of 100 turns that would surround it makes >5000 Gauss. The coil thus needs a resistance (for 3.7v) of less than 1/2 ohm.

Looks like the coil will be about 150 cm of wire, so somewhere around 30 guage copper. That will weigh about half a gram, so that isn't a problem.

Ampacity rating of 30 gauge is less than 1 amp ("Chassis wiring"; table says "rule of thumb").

20 gauge wire will carry 11 amps but is ten times heavier, starting to get into the range where weight reduction is important. Would a 30 gauge coil make smoke in 1 sec at 10 amps?

To me it looks do-able; am I missing anything?

10 Amps is more than 1/2 the typical max load on the battery, so a circuit to dump a cap required (is that the best way?)
Reply to
unk

You might try a scaled down magswitch and a small motor as an actuator.

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Reply to
krw

Their lightest is 133grams. It would have to be scaled down a LOT.

Plus simpler is better - that thing has moving parts and a motor and so on. A 10cm disc with a coil around a magnet is likely small than whatever I could make, plus making it would be easier.

Total under 10 grams is almost a necessity.

Reply to
unk

Well, these things are intended to hold woodworking fences, featherboards, and such. You can do the same thing, yourself, with a couple of small rare-earth magnets.

Use the coil to turn one of the magnets. It will take a *lot* less energy to rotate one of the magnets than necessary to double its magnetic field.

Reply to
krw

Rather than focusing on making the coil light, I would suggest you find ways of making the magnet smaller and lighter which will also allow a lighter coil.

Using a lever arrangement will allow an object to be held securely with a lot less force from the magnet. There is no need for the magnet to directly hold weight of the thing being held.

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Rick
Reply to
rickman

Some soap dispensers in dishwashers have a magnet catch that gets released when a small AC current is run around a soft iron pole piece. The AC demagnetizes the iron, which falls away from the permanent magnet.

But, if you want low weight, consider a hot wire and matchhead, melting a bit of nylon strap or string.

Reply to
whit3rd

In fact, there is no need at all for a magnet to be used. You can design a latch which holds the item securely by the force of gravity. The electromagnet can release the latch and allow the object to separate. You may want a very small magnet to hold the latch securely in case something happens that causes your load to go zero g.

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Rick
Reply to
rickman

Yes, that sounds like a good idea.

A similar idea: I have heard of people using a melting fuse wire to release things that fall (to allow props to be deployed during a theatrical production). Provided the battery can supply a few amps for a few hundred milliseconds, that should be the lightest option. Obviously keep the fuse wire short enough so that the battery voltage is sufficient, bearing in mind the resistance of hot fuse wire.

to the OP: Out of curiosity, what is being released?

Chris

Reply to
Chris Jones

I don't know what you're trying to do, but getting a coil strong enough to overwhelm a neodymium magnet is going to be a challenge.

I know you're averse to mechanical solutions, but pulling the magnet away from the surface with a servo should work very well, and should be pretty darned reliable. It takes as much force as you're holding, but only long enough to get the magnet out of the way.

If you can slide the holding magnet sideways without just having the outside piece follow it then you can get away with much less force.

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Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

Whaaat? Really, the force required to move a magnet is dependent on the direction traveled? Oh, you are talking about the equivalent of a lever.

Better would be a spring released by a latch. Lots of energy stored in a spring with a very small activation by the battery. Heck, why even use a magnet?

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Rick
Reply to
rickman

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