Source for small electromagnets?

Thomas Fletcher

I have googled and googled, but I can't find a site to purchase small electromagnets.

I am wanting to hold a drop door closed (maybe 5 lbs). It is something I want to hang from a kite. I
thought about micro servos and latches, but I was thinking an electromagnet(s) might save a little weight.

Does anybody have a source?

bill190

All relays have electromagnets in them.

Other names for relays are solenoid and contactor.

Then in the security system business, they use electromagnets for door latches. Enter an access code or whatever and the door unlatches.

And in the fire alarm business, they use electromagnets to hold doors open in businesses, hospitals, buildings, etc. These are always on holding the interior doors open. When the fire alarm is activated, the power is removed from the electromagnets, then the doors close. This would contain and block the spread of a fire.

Then search google.com for the words...
magnetic locks
...and you will find all sorts of stuff.
·

kwinn

I have removed and used the coil from a relay a couple of times for something similar. If the core will not come off easily you can use a steel bolt of the appropriate size. Even better, if you can use a rod magnet for the core it can hold the door closed and you only need to power the coil when you want to drop it.

ElectricAye

Thomas Fletcher said...
.... I was thinking an electromagnet(s) might save a little weight.

....

If the electromagnet must be ON all the time in order to work, then it will be consuming a lot of power, and power will require batteries that will add weight. You will want a configuration in which the electromagnet needs to be powered ONLY when you want it to perform a function, like pull on a pin, etc. I'm sure you already knew this, but I thought I'd mention it just in case.

Phil Pilgrim (PhiPi)

I'd go the micro servo route. In the end, it will be simpler and probably a lot lighter, considering the force advantage you get from gearing, which can be provided by a minimal-weight motor. Compare this with an electromagnet, which has to provide the same amount of force in a single action without the mechanical advantage of gears.

For the absolute least amount of weight, though, memory wire is probably the way to go. But it's fussy to work with. You can't solder memory wire: it has to be crimped; and, despite its high force, displacement is small, so precision in the actuator design is paramount.

-Phil

erco

@Phil: That Nitinol/Flexinol memory wire has forever been a solution in search of the right problem. You remember the Spacewings kit, the Boris "robot". Takes a lot of current to heat that stuff up. The best example I ever saw: 20 years ago, some vendor (?) gave us samples of a tightly-wound tension spring with hooks on the end. At room temperature, it was as soft as solder; you could easily stretch it out several inches and it would stay that length. Pass a current through it and it would heat up and try to snap back to its shorter length. Very handy. Wish I had a drawer full of those now. I'm sure I could conquer the world.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
·"If you build it, they will come."

Phil Pilgrim (PhiPi)

erco said...
Takes a lot of current to heat that stuff up.

True enough. But for a one-shot release mechanism, it doesn't matter. Attached are some photos of a shutter release I designed years ago for a kite cam (that I never finished ). The shutter release worked great, and it doesn't get any lighter weight than than some thin wire and a small spring!

-Phil

erco

Yep, that's how I used those springs, for one-shots.

For cheap,·lightweight servos, have a look at ·http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180399101188


▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
·"If you build it, they will come."