FAQ – Magnets for Sensors

How do I convert an Amp-Turn rating to Gauss?

In the SI system; for coils longer than 5 times their mean diameter, the formula to convert Amp-Turns to Tesla is:

B = (µ0N * I) / coil length

where N = number of turns in the coil, I is the current in Amperes and coil length is in meters. As a close approximation, 80 Amp-Turns per meter of coil length develops 0.0001 Tesla of magnetizing force, or a one Tesla field density, in an air core coil.


How do I select a magnet to use with a reed switch rated in ampere-turns?

Reed switches are rated, in their as-manufactured geometry, for the value of the ampere-turn product required for “pull in” and “drop out” in “standard” test coils. These standard test coils are described in reed switch manufacturers’ literature; geometries and turn counts vary with manufacturer, but some use the EIA/NARM-421-A standard coils. The magnet selected must produce the same flux density at the position of the reed switch as the coil when the rated ampere-turn value is applied. If the switch leads are shortened or bent, the Amp-Turn values will increase, so a stronger magnet would be required. The reason for this is that the altered reed switch has a lower magnetic permeance.

How does a reed switch work? Dexter Magnet Reed Switch Application

Flux density produced in a test coil, or produced at the reed switch position by a magnet, becomes the coercive force, “H”, to create magnetic induction, “Bi”, in the permeable reeds. The field induced in the reeds adds to the magnetizing field strength (coercive force) to develop field intensity in the gap between the reeds. As the gap flux density increases, the magnetic attraction between the reeds causes them to bend and close. An open reed switch has two short, low permeance parts, and when the switch closes, the reeds join to become one high permeance part. This is why the “pull-in” rating (required flux density), is greater than the “drop-out” rating.

What are “standard” reed switch test coils?

It is best to consult the reed switch manufacturer for this information as some use their own version.

EIA/NARM RS-421-A Standard Test Coils:- (second value is approx. millitesla for 10 mA)

Coil I : 10.4mm long, 4.0mm ID, 5,000 turns of Ø .040mm:- 60.5

Coil II : 19.1mm long, 4.3mm ID, 10,000 turns of Ø .032mm:- 66.0

Coil III: 25.4mm long, 7.6mm ID, 10,000 turns of Ø .071mm:- 49.5

Coil IV: 50.8mm long, 7.6mm ID, 10,000 turns of Ø .090mm:- 24.7

Hamlin uses the coils listed in the Moskowitz book, all are wound on a nylon coil form 6.4mm dia. x 50.8mm long. (Second value is approx. Gauss for 10 mA)

Coil 1: 3,000 turns of Ø .180mm:- 7.4

Coil 2: 3,600 turns of Ø .160mm:- 8.9

Coil 3: 4,400 turns of Ø .143mm:- 10.9

Coil 4: 5,000 turns of Ø .127mm:- 12.4

Coil 5: 6,000 turns of Ø .113mm:- 14.8

Coil 6: 7,000 turns of Ø .101mm:- 17.3

Coil 7: 9,000 turns of Ø .090mm:- 22.3

Coil 8: 13,000 turns of Ø .080mm:- 32.2

Coil 9: 15,000 turns of Ø .063mm:- 37.1

Coil 10: 25,000 turns of Ø .050mm:- 61.8

Hamlin specific coils

L-4988: 2.145″ long, .230″ ID, 10,000 turns Ø .101mm:- 23.1

L-4989 .875″ long, .137″ ID, 5,000 turns Ø .080mm:- 28.3

What is a reed switch?

Reed switches are made by sealing overlapping strips of magnetic material (frequently Kovar) in a small glass tube. Kovar is used because it has an expansion coefficient that matches the glass. The strip size and overlap is designed to produce specific switch ratings.

Just like any other electrical switch, it can perform any desired electrical switching function. A popular application is in security systems where the field of a permanent magnet holds the switch closed when a window closed. When the window is opened the magnet moves away, and when the switch opens an alarm is set off. The “Permanent Magnet Design and Applications Handbook” by Lester R. Moskowitz presents reed switch information and sketches in a manner that is easy to read and understand.

What is the best magnet to use in a Hall sensor application?

Hall effect switches and sensors usually do not contain magnetic materials, and they respond to some specified magnetic field intensity at a point. Any magnet that produces the required field is useable; cost and end application usually determine the magnet to be used. Some clever magnetic circuits can make Hall effect device function more precise, efficient and effective. Most of these devices are polarity sensitive.