Electromagnetic locks

[ncait]

I've seen a ton of posts on here using electro magnets as a release and locking method. All of them use door holder electromagnets though. I figured I would post my most recent diy, an electromagnet I made from stuff I had lying around my garage.

I began by taking a iron bolt about 3 inches long. I then wrapped it in about 60ft of 18ga wire I had sitting around. It is important to keep the wraps tight and all going in the same direction. You can just wrap up and down the bolt asany times as you want, I did 4 layers. The more layers the stronger the hold. I then used a doorbell transformer to step 110vac to 10vdc 10amps. If there is more amps that also increases strength. I plan to use a timer plug to control the power, ensuring I'm stuck until time runs out.

This method can also run off a battery pack, but 9volt batteries would be best for the source to ensure strength.

[Sergio]

You don't say whether you use the magnet to directly restrain you or to hold the keys out of reach. If the latter then be careful because using plain iron (or steel) there will be residual magnetism when the power is removed after some time and it may be strong enough to keep hold of the keys.

[ncait]

I am planning on using it to hold a latch closed on a set of stocks I'm building. With this home made low voltage, the residual magnetic force is negligible, in my tests, the residual after 4 hours powered has a holding force of about 1 gram so stuck keys are not an issue. Compared to the 80lb holding force of when it is powered, the drop off surprised me. I think it has to so with the fact I am using 18ga nylon coated wire. The fields are not strong enough to maintain after the power is removed

[Blacky]

If used regularly and with the same polarity each time the iron you use as core material will probably build up a certain permanent magnetism which will become stronger with time.
You might want to make sure to switch the polarity of the coil frequently to avoid that static magnetism.
Another potential issue might be heat. If you actually run 10 A through an 18ga wire it could become rather warm. You might want to keep an eye on that, too.

Other than that it is a nice and cheap solution. Thanks for sharing!

[TicleMeEndless]

Forget about 9V batteries. It haves too small capacity an can't provide a big amperage. More, it can be too less even for arduino power supply (sic!). May be replace it for block of 6x1,5v batteries or 3x18650 akkus. Also, 10A is a so big amperage and can overheat a power cells and wires of coil. Be sure to measure and watch it at testing. Also, check the resistance and amperage on coil: it may be a short connection.

[Shannon SteelSlave]

You could series chain enough 9 volts to power probably a Prius. (Except that runs on 12 volts) 500 mah X the number of batteries = series mah.
I would definitely bench test this set up prior to actual use due to the wire temperature. Make sure you're not smoking the insulation.

[lj]

I'll just put in a few bits of electrical information.

Magnetic force is related to the current and the number of turns of wire.

A 9volt PP3 radio battery is perfectly capable of generating enough magnetic force in an electromagnet to retain a proper iron bar strongly enough to stop a normal man from separating the two for several hours. (I have one, it's a standard door lock, the running current was about 30mA so the battery was good for about 3 hours -approx 90mAH - an excellent secondary safety release!)

The limitation when manufacturing a solenoid/electromagnet is that the physical size of the magnet is determined by the size and type of core and the number of turns of wire - you can have lots of turns of fine wire and a low current, or fewer turns of wire with a higher current. You then have a limit provided by the ability of the structure to dissipate the heat caused by the electric current passing through the resistance of the wire - thin wire will have a higher resistance than thick wire. Then you have the quality of the core to determine the flux density generated by the current passing through the wire.

A 10Amp current is a serious figure, you really don't need that high a current to retain a lock, just a better and more efficient electromagnet design.

Oh and you might add the duty cycle - you can over-power an electromagnet for short periods. I designed a piece of kit years ago, using a bought-in solenoid, and needed far more power than the unit could provide continuously, but only for a couple of seconds every 30 seconds, so I used 5 times the "normal" current", well below the fuse limit of the wire in the solenoid but giving 5 times the "normal" power.

re. the Prius, the propulsion battery is actually around 350v (95 x 3.7v 25AH lithium batteries) - the 12v supply is for the usual accessories, lighting etc

re. residual (remanent) magnetism, Blacky is quite right. Continued current flow in the same direction will gradually increase the residual magnetism, so just make sure the force holding the two items is always exceeded by something, eg weight, trying to separate them! or reverse polarity each time you use the magnet.

[Shannon SteelSlave]

re. the Prius, the propulsion battery is actually around 350v (95 x 3.7v 25AH lithium batteries) - the 12v supply is for the usual accessories, lighting etc

Yeah, I guess I did forget to mention that. I just needed the name of a car that began with P so it would roll off the tongue. No one knows what a Pontiac is anymore. Maybe next time a Porsche, Peugot, Panoz.

[bound_jenny]

re. the Prius, the propulsion battery is actually around 350v (95 x 3.7v 25AH lithium batteries) - the 12v supply is for the usual accessories, lighting etc

Yeah, I guess I did forget to mention that. I just needed the name of a car that began with P so it would roll off the tongue. No one knows what a Pontiac is anymore. Maybe next time a Porsche, Peugot, Panoz.

How about Packard?

Plymouth?

Pierce-Arrow?

I'll take the Packard over a Porsche or a Prius any day. At least there's some solid metal in there. No recycled beer cans.

Jenny.