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Magnets & Magnetics

Why would magnetic mapping be done?

In reed sensor applications it is important to understand the exact pull-in and drop-out fields.  This information then allows one to properly position the magnet and sensor well within appropriate guard bands and avoid any tolerance issues.  Most users have no idea what the actual magnetic field looks like. Presenting the fields in a three dimensional view gives the user a better chance to have an optimized design.  This will help define adequate operate and deactivate points. Additionally, this will insure operation takes place well within the magnetic envelopes to avoid tolerance issues.  We can insure acceptable hysteresis between the operate and deactivate points.  We can optmize the sensor and magnet costs.

What is magnetically mapping?

Magnetic mapping is the method of incrementally measuring the pull-in and drop-out points by either holding the sensor stationary while moving the magnet or vice versa.  The movement must be carried out in all three dimensions.  Software is then used to bridge all the points allowing the magnetic field to be visualized in three dimensions.

What is an electromagnet?

An electromagnet is a magnetic field generated when a coil of wire is formed in a cylindrical shape.  The magnetic field will be uniform through the entire length of its inner opening.

What does ferromagnetic mean?

Ferromagnetic is the property of a material that allows it to become magnetized permanently or temporarily when in the influence of a magnetic field generated by a permanent magnet or an electromagnet.

Will the magnetic properties of a magnet change if the magnet is dropped on a hard surface?

No nothing will happen.  There were rumors abound which indicated that the magnetic strength would be affected, but this is just not true.

Why use different types of magnets?

Using different magnets allows you to select the characteristic which best fits the application:

  • Ferrites are low cost
  • Alnicos are the most stable over a wide temperature range
  • Rare earth offer the strongest magnetic fields
How do you make a uniform magnetic field and why would you?

A uniform magnetic field can be made by making a relatively long cylindrical coil. Once current is flowing through the coil a uniform magnetic field will exist all along the inside of the coil. This is not true at the very ends of the coil.  Helmholtz coils can be bought for the very reason of supplying a uniform magnetic field.  In either case, the uniform magnetic fields allow for calibration.

Is there such a thing as an ideal size magnet?

Yes there is and it clearly depends on the type of magnet.  The length to diameter is the key ratio.

  • Alnico 5 is 5:1
  • Alnico 8 is 3:1
  • Ferrites are 1:1
  • Rare earth is 1:1
Why use a Helmholtz coil?

Using a Helmholtz coil makes it very easy to calibrate magnetic fields in either ampere turns (AT) or milliTesla (mT).

What is a Helmholtz coil?

A Helmholtz coil is actually two concentric coils mounted parallel to each other and when energized by passing a current through them they will produce a uniform magnetic field between the two coils.

What is the Curie effect?

The Curie effect is when a magnet reaches a certain temperature, its magnetic properties will be eliminated.  Once the temperature drops below the Curie temperature the magnetic effects of the material will return.

How is a magnetic field produced?

Generally winding fine copper wire in a cylindrical configuration will create a magnetic field internal to the cylinder, when a current is passed through the copper wire.

How is an artificial magnet made?

Artificial magnets can be created by doping iron, nickel, and/or cobalt with other elements.  Doping with rare earth materials has been particularly successful, producing very strong magnets.

Where does magnetic energy come from?

The magnetic force is generated at the subatomic level and the energy comes from heat.  Any temperature above absolute 0 (-273°C)

What is a dipole?

A dipole is the basic building block of a magnetic field.  A dipole is the magnetic effect from a single atom.  When taken many million times over, one has a magnetic field being generated from a magnet.

What are the different types of magnets?

There are mainly three different types of permanent magnets:

  1. Alnico -most stable and best temperature effects
  2. Rare earth -strongest magnets
  3. Ferrite -lowest cost
What is a magnet?

A magnet is composed of ferromagnetic material which means it must contain at least one of the following: nickel, iron or cobalt. It must also be able to sustain magnetism.

What is magnetic and where does it come from?

Magnetic is a force produced at the subatomic level.  It is caused by electrons spinning and also rotating around the nucleus of the atom.

What is annealing?

When metal is subjected to a very high temperature bath, that process is called annealing.  The temperature is slowly increased to a max temperature where it is stabilized for a period of time, and then the temperature is slowly reduced back to room temperature.  This process will leave the metal in its softest state.  For a reed switch this is very important because this point is also where the nickel/iron leads have near zero magnetic retentivity.  This means when the reed switch contacts are subjected to a magnetic field and then the magnetic field is removed, there will be no residual magnetism on the leads.

Is there any net effect if a reed switch is subjected to an extremely high magnetic field?

No.  There is no net effect on the reed switch, once the magnetic field saturates the reed switch contacts it no longer has any effect.

Can a magnet packaged with a reed switch be turned into a temperature sensor?

A magnet and reed switch can be turned into a temperature sensor by using a magnet that has a certain curie temperature for the temperature you want to sense.  When that curie temperature is reached the magnet loses its magnetic properties whereby the reed switch contacts open.  When the temperature drops below the curie temperature, the reed contacts will close.

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