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I have never heard of such thing. But it would really suit my needs.

I'm inventing a device and during it's operation, it should be able to catch an iron object any time (with a permanent magnet). The device should be mobile, so using inductor would really reduce the time of operation.

But the device must also be able to turn the field off, so that the objects can drop down.

I only could think of one concept: Put inductor behind a permanent magnet in a way that when the inductor is turned on, it's field will negate the permanent field of the magnet.

magnet that can be turned off

As you see it's my attempt to draw a coil connected to permanent magnet (isolator is in-between). When the coil goes on, it should produce field of oposite direction to the field created by magnet.

Is this possible? Is there a better trick?

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  • \$\begingroup\$ Why aren't you using the inductor as an electromagnet to catch iron objects? Then just shut it off when you want the iron object to fall. If anything, you could have it sense that an iron object is incoming and then turn it on saving power. \$\endgroup\$ – horta Sep 25 '14 at 20:47
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    \$\begingroup\$ As I pointed out, that would reduce the time of operation of a mobile device, because when electromagnet is on, it constantly drains battery even when doing nothing. Permanent magnet can pick up stuff and drains no power at all. \$\endgroup\$ – Tomáš Zato Sep 25 '14 at 20:49
  • \$\begingroup\$ You only turn the inductor on when you sense an iron object is nearby. At any rate, your solution would also work, but I don't know why you would need an isolator. Just have your inductor's magnetic field swamp out the permanant magnet's field. \$\endgroup\$ – horta Sep 25 '14 at 20:50
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    \$\begingroup\$ Oh and one other disadvantage of your solution may be that the lifetime of the permanent magnet would be severely degraded because you'd basically be trying to demagnetize it every time you turn the permanent magnet "off". \$\endgroup\$ – horta Sep 25 '14 at 20:53
  • \$\begingroup\$ @horta The isolator is there partialy because I was playing with gradients in Inkscape. You're still telling me to do the more power-consuming thing without any reasoning behind it. Then you go hating on my pretty isolator. I think this is not necessary. I posted this before that. \$\endgroup\$ – Tomáš Zato Sep 25 '14 at 20:53
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Your need is well served by an "Electro permanent magnet"

This Wikipedia link has a good summary description but no useful material, but a web search under that term will find a large amount of related material.

Wikipedia text at end of this post.

Images link below gives many examples.

Useful explanation of a variant

This Kickstarter project gives a good practical idea.


This ROBOT Pebbles page uses EPMs to hold the pebbles together. Construction details given. They say -

  • The top view of an electropermanent (EP) magnet used for latching, communication, and power transfer. Each EP magnet assembly is composed of two pole pieces (a,b) which sandwich cylindrical Alnico (c) and NdFeB (d) magnets. The entire assembly is wrapped with 80 turns of #40 AWG wire (e) and held together using epoxy (f) (which makes the Alnico magnet appear larger than its NdFeB counterpart).

enter image description here


DIY Drones EPM cargo gripper.

One version:

  • The EPM688-5 is designed to hold a cargo of 1kg securely and switch on and off effectively with low power consumption. 800mA at 5V for 3S or 3.4J per cycle.

  • Under the capacitor discharge condition pull either S_on or S_off high Via RC channel 7 or 8 for 3 seconds and then low. The capacitor is fully charged and the device is ready to cycle. Pulling either S_on or S_off high via Via RC channal 7 or 8, causes the magnetic field to move either in the on or off state.

    • The complete device will have a mass of about 50 grams, and is capable of holding a mass of 7kg in optimal conditions, in practice it should securely hold 1kg of cargo on a quad.

These linked images provide a large number of ideas.

Wikipedia says:

  • An electropermanent magnet is a type of magnet which consists of both an electromagnet and a dual material permanent magnet, in which the magnetic field produced by the electromagnet is used to change the magnetization of the permanent magnet. The permanent magnet consists of magnetically hard and soft materials, of which only the soft material can have its magnetization changed. When the magnetically soft and hard materials have opposite magnetizations the magnet has no net field, and when they are aligned the magnet displays magnetic behaviour.

  • They allow creating controllable permanent magnets where the magnetic effect can be maintained without requiring a continuous supply of electrical energy. For these reasons, electropermanent magnets are essential components of the research studies aiming to build programmable magnets that can give rise to self-building structures.[2][3]

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  • \$\begingroup\$ There is no need of an EPM for his application. He only needs an electromagnet combined with a permanent one. It is generally sold as a self-holding, degauss or latching electromagnet. \$\endgroup\$ – gstorto Apr 15 '18 at 13:27
  • \$\begingroup\$ @gstorto Maybe so. MAybe not. Without more details than you provide thatis exactly what he proposed initially and asked about alternatives to it. His (and your) solution MAY be the best one.And maybe not. Which is what he was asking. That said - tell us more. More information is always useful. Also read all the comments where alternatives are discussed. \$\endgroup\$ – Russell McMahon Apr 16 '18 at 2:22
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It's possible to "turn a magnet off" without requiring continuous power in either position- simply shunt the field lines away from the open end.

Here is an example of a magnet with an on-off switch:

https://www.rockfordsystems.com/online/photos/on-off-magnetic-base_new.jpg

And here is how it works:

http://1.bp.blogspot.com/-giIynxvM34g/Tb0qg4wYPVI/AAAAAAAAAuU/5a1I3bJ600k/s1600/mag_base_operation.jpg

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  • \$\begingroup\$ Few seconds before this answer a found the wikipedia article. I think it's the best idea so far, because this setup doesn't degrade the magnet, as horta has pointed out about my setup. \$\endgroup\$ – Tomáš Zato Sep 25 '14 at 21:00
  • \$\begingroup\$ But on the second thought - when I put magnet to iron, it remains magnetic. Are there types of iron that don't do this? \$\endgroup\$ – Tomáš Zato Sep 25 '14 at 21:01
  • \$\begingroup\$ I use these things all the time- there's surprisingly little attractive force in the 'off' position. Very effective and cheap. \$\endgroup\$ – Spehro Pefhany Sep 25 '14 at 21:02
  • \$\begingroup\$ Interesting solution. Not electronic but maybe that's even better. Is the shape of the bottom the main reason why flux appears to travel through more the bottom than the top? Does the top of it not really attract anything just due to the shape? \$\endgroup\$ – horta Sep 25 '14 at 21:02
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    \$\begingroup\$ There are pneumatically operated versions of this device that are suitable for automation. With a solenoid valve you can obtain electrical control over it for milliwatts. \$\endgroup\$ – Doug McClean Sep 26 '14 at 14:40
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Edit: Take this with a grain of salt because comments suggest I may have it completely wrong.

This may not be a practical design for your application, but an interesting option is this: use a permanent magnet and a heating element. With the magnet at room temperature, it attracts objects. To let them drop, energize the heating element so that the magnet gets hot and exceeds its Curie temperature - it becomes paramagnetic and ceases to attract the objects. When it cools, it goes back to normal.

I have a rice cooker that uses this principle as a thermal switch: a contact is held closed by a magnet with a Curie temperature of a little over 100 C. When the water on the rice has all boiled off, the rice's temperature rises above 100 C and the magnet no longer holds the contact closed, so the cooker switches off. (The other side of the contact is on a lever which drops several cm when the magnet demagnetizes, so by the time the magnet cools, it is too far away for the magnet to close it again.)

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  • \$\begingroup\$ Weller Magnastat Soldering Irons work in a similar way, though at a higher temperature. \$\endgroup\$ – gbulmer Sep 26 '14 at 0:50
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    \$\begingroup\$ I think that after you heat a magnet to above the Curie point and allow it to cool it will no longer be a magnet, except to the extent there was some field existing at the time it passed through the Curie temperature on the way down. My old Weller WTCP iron tips (the part that loses magnetism- since it's got a fixed temperature setting) are not permanent magnets, they're ferromagnetic. The magnet is in the handle. \$\endgroup\$ – Spehro Pefhany Sep 26 '14 at 1:04

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