# Do you guys think this will work?

## Main Question or Discussion Point

I am making a locking mechanism with a very powerful magnet. The electromagnet in the diagram is held in place. Basically when the electromagnet turns on it will push the permanent magnet out. Here is a picture of what I want to do.
http://www.mypicx.com/05202009/fas/

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berkeman
Mentor
I am making a locking mechanism with a very powerful magnet. The electromagnet in the diagram is held in place. Basically when the electromagnet turns on it will push the permanent magnet out. Here is a picture of what I want to do.
http://www.mypicx.com/05202009/fas/
Welcome to the PF. The traditional way to do that function is with a solenoid, which attracts a ferrous bolt into a soltnoid with the current flows through the solenoid (Quiz Question -- why does it attract the bolt?).

The problem with using a permanent magnet to be repelled by the coil field, is that the coil field will tend to demagnetize the permanent magnet over time. If you could use two opposing electromagnets, you would not have that problem. But the traditional solenoid arrangement is usually the most cost-effective arrangement for generating linear forces with a current input.

I see. So what you're saying is that I should build a solenoid of some sort and maybe place an iron bolt inside that is spring loaded? That is very interesting. Do they sell these things or do you think it's better for me to make it myself?

edit: lol nvm, you don't have to answer that. Just found out what latching solenoids are. heh thx man!

Another question. Why will the coil field demagnetize the permanent magnet?

Last edited:
berkeman
Mentor
Another question. Why will the coil field demagnetize the permanent magnet?
Just as a magnetic field in one direction is used to magnetize the magnet, a magnetic field in the opposite direction will serve to demagnetize it (or even magnetize it in the opposite direction, if the field is stong enough).

A permanent magnet is just a magnetic material with magnetization at one end of its hysteresis loop:

http://en.wikipedia.org/wiki/Magnetic_hysteresis#Magnetic_hysteresis

.

Danger
Gold Member
One thing to consider while designing this thing is whether you want it 'fail safe' or 'fail secure'. Ie: if the electromagnet loses power, do you want the door unlocked or locked?

russ_watters
Mentor
Ie, many commercial buildings use purely magnetic locks (a magnet literally holds the door closed). They release when power is lost, which can be advantageous for an emergency.

berkeman
Mentor
Ie, many commercial buildings use purely magnetic locks (a magnet literally holds the door closed). They release when power is lost, which can be advantageous for an emergency.
Those are internal doors, obviously. Our elevators are also designed to drop to Floor 1 and open their doors in emergencies. We test those things pretty regularly.

russ_watters
Mentor
Those are internal doors, obviously. Our elevators are also designed to drop to Floor 1 and open their doors in emergencies. We test those things pretty regularly.
Yes. And ironically, the opposite also exists (didn't think of it when I wrote that last post). Fire doors can be propped open normally if they are held open by a magnet, closing (but not locking) in the event of a fire to stop it from spreading.