Portable Electromagnets

Would an electromagnet be relevant, useful and required in a bike lock? Would a rare earth magnet or permanent magnet be preferable? What are the pro's, con's and costs associated to all?

First thing - what is a magnet and what is an electro magnet!?

These videos are just perfect - short, informative and aimed at my level of knowledge! I now know just enough to inform the next stage of research.

"•Does the number of coils that is wound to the metal bar affect
the strength of the electromagnet?


The higher the number of windings the more
"resistance" there will be within the coil. You actually want this.
This will slow down the drain on the battery while allowing the magnet to
remain strong: more electrically eco. (It may cost more in the short term).

•Does the wire gauge (20, 24, 28, 30, 34, 36 wire gauge)
make a difference is the strength of the magnet?

Yes. (Electric economy again)

This can be understood better with the idea of filling a
pipe to a certain water pressure (voltage = water pressure).

The wider your pipe, the more water it will take to fill it.
This matter more if you are turning the power on and off, but still makes a
difference while on due to the impact of the current (same as water current). Putting
a resistor will also help to slow down the amp impact.

•Does the voltage I supply to the coil effect the


Voltage is the electrical equal of water pressure. The more
pressure = the more power. The whole idea is to get very high voltage with very
low amps.

•What type of battery is best for an Electromagnet (AA, AAA,
C, D), and what voltage (1,5V, 3V, 4,5V, 6V, 9V)???

•Depends. Personally, I'd use rechargeable 9V batteries
because they can be easily linked together. They also cost less in the long
run, but bite in price at the start.

Another tip for making it stronger is to use small wire and
rap it in a single direction. By this I mean that you make a primary direction
of your wire. You rap it going in one direction (left to right, for example),
then when you reach the end, you bring it all the way back and continue the
next level till done. (This is patented by the way, so don't try manufacturing
this idea, but there shouldn't be any problem for what you seem to want).

An added benefit to this method is it allows you to make
(using DC current) an electro magnet which can repel a permanent one." (78, 2009)

"For safety purposes, electric locks and electric strikes can be designed to operate in one of two modes:

Fail-safe – to protect people: The lock is released if power cuts off.
Fail-secure (also known as non fail safe) – to protect property: The lock remains closed if power cuts off. This type of lock has a cylinder, similar to those found in conventional locks. The cylinder makes it possible for the lock to remain secure, even if the power supply is cut off". (Electromagnetic lock, 2016)

"Pro's -
quick operation
no interconnecting parts
unlock instantly when the power is cut - quick release
There are no moving parts in an electromagnetic lock to break.

Con's -
Requires a constant power source in order to be secure
Can de-energize in the event of a power outage, disabling security
Expensive in comparison to mechanical locks" (Electromagnetic lock, 2016)

Remaining Questions -
How long will a 9v battery last?
To be fail secure will this lock need a cylinder too?
Can an electromagnet reverse or 'turn-off' a permanent magenet (PM)?

Electropermanent magnet

"An electropermanent magnet (EPM) 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 (low coercivity) 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 behavior.[1][2]

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]"(Electropermanent magnet, 2016)

The EPM tecnology is being developed currently for use on drones as an open grab.

(Pledge $1 or more about $1.00 USD, 2012)

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.



78, T. (2009) Electromagnet help! How to make a small but very strong electromagnet??? Available at: http://www.instructables.com/answers/Electromagnet-Help-How-to-Make-a-Small-but-very-S/ (Accessed: 24 April 2016).

Electromagnetic lock (2016) in Wikipedia. Available at: https://en.wikipedia.org/wiki/Electromagnetic_lock (Accessed: 24 April 2016).

kire Eng (2012) Permanent magnet as electromagnet - magnetic engineering. Available at: http://www.eng-tips.com/viewthread.cfm?qid=327731 (Accessed: 24 April 2016).

Electropermanent magnet (2016) in Wikipedia. Available at: https://en.wikipedia.org/wiki/Electropermanent_magnet (Accessed: 24 April 2016).