Reed Switch and Hall Effect Magnets

An Introduction to Reed Switch and Hall Effect Magnets

Almost any permanent magnet could be used to activate a Reed Switch or Hall Effect Device.

A Reed Switch is activated by being in the presence of a magnetic field strength of high enough magnitude. It is usually unimportant whether a North or South Pole is facing the Reed Switch. The Reed Switch, in simplest terms, is two very thin nickel-iron plates (“reeds”) overlapped but with a small gap between them. The reeds are sealed within a glass casing. When a magnetic field is present, the reeds “carry” the magnetic field (by virtue of having a high magnetic permeability) and the overlapping sections of the reeds are magnetically attracted to each other until the field strength is large enough to cause mechanical contact (when the magnetic pull between the reeds exceeds the spring force of the reeds). When mechanical contact is made, if the ends of the Reed Switch are part of an active electrical circuit, the Reed Switch conducts electricity. When the magnetic field is reduced, the spring force exceeds the magnetic pull force and the contacts open and the Reed Switch can no longer conduct electricity. This simple version of a Reed Switch is termed Single Pole Single Throw Normally Open Reed Switch (SPST-NO). Other varieties exist (allowing different applications and uses).

A Hall Effect Device is sensitive to the magnitude and direction of the magnetic field being applied to it. The Hall Effect Device is a semiconductor in which a current is flowing through it and, when a magnetic field is perpendicular to the active element, a voltage is created (the Hall Voltage) which is proportional to the magnitude of the perpendicular magnetic field. As such, the output from the Hall Effect Device (a voltage output) is a function of the magnetic field strength – but the “sense” of the field is important (some Hall Effect devices only work with a South Pole of a magnet facing it; others only work with a North Pole), unlike Reed Switches which activate regardless of magnet polarity. The Hall Voltage is created because the magnetic field causes the charge carriers, electrons and holes in the semiconductor to move to one side of the semiconductor, creating a Potential Difference across the semiconductor which is measured as the Hall Voltage. Ideally the voltage output is linearly proportional to the applied field strength. The voltage is also linked to the perpendicularity of the applied field - a Sine (angle) relationship exists e.g. maximum output when the field is 90 degrees to the semiconductor “active element”. Generally, Hall Effect Devices have zero voltage output when no magnetic field is present. Some electronic chips have multiple Hall Effect Devices to allow measurement of North and South Magnetic fields. Some Hall Effect chips contain more than one device and allow applications such as 3D field mapping to be possible. Other chip variants also exist e.g. some Hall Effect Devices allow temperature correction.

Almost any magnet can be used to activate a Reed Switch or a Hall Effect device. The criteria is to deliver a field strength of large enough magnitude at the distance required.

Some Reed Switch and Hall Effect device manufacturers give an Ampere-turn rating for trigger levels instead of a Gauss or Tesla reading. Reed Switch manufacturers use a standard coil (with an Ampere-turns rating) which creates a magnetic field within that coil and the Reed Switch is positioned inside the central region of the coil.

It is possible to calculate the magnetic field strength in the region of the coil where the Reed Switch is placed. When converting to using a magnet, it is generally accepted that the field strength from the magnet should be about 20% higher than the calculated value. So the required field strength is 0.4 times pi (approximately 3.142) times the Ampere-turns rating of the coil divided by the coil length (in centimetres) times a safety factor of 1.2 (thus taking the 20% into account). It is therefore possible to calculate the distance at which a Reed Switch or Hall Effect Device will activate and de-activate.

B(Gauss)=0.4 x pi x NI(Ampere Turns) / L(cm) x 1.2(safety factor)

Reed Switch manufacturers may use a standard coil known as a Narm 1 coil. It has a length, L, of 1.039 cm. When the coil length is not known, using L=1.039 is a good approximation.

As will become apparent, different sized magnets, grades and materials give different magnetic field strengths with distance. This allows the user to use magnets of shapes and sizes other than any which a Reed Switch or Hall Effect Device manufacturer specifies to extend the number of applications. The specification for a Reed Switch or a Hall Effect Device, the actual application and the environment of the application can be combined to specify the magnet to use.

We can provide the Tools and Technical Support to assist in calculating the magnetic field strengths from almost any size and shape of magnet and can supply the magnets in small to very high volumes.

The choice and suitability of magnet is affected by the application and includes factors such as environmental conditions (e.g. temperature), distance from magnet and locations of other magnets and ferromagnetic materials. We will work with you to identify the magnet(s) which will meet your requirements and can supply made-to-measure magnets. We can produce the magnets in a wide range of grades and coatings.

Therefore, if you need help with magnet selections for Reed Switches and Hall Effect Devices, please get in touch.

As a starting point, we provide below (as a guide) a list of our NdFeB and SmCo magnets and have included the Gauss (Ga) readings that could be seen at ambient on the surface (0mm), at 5mm away from the surface and at 10mm away from the surface (for the ring magnets, in the middle of the central air hole, equivalent to on the surface (0mm), at 5mm away from the surface and at 10mm away from the surface); the values taken along the central axis. The values are affected by temperature, actual shape, direction of magnetisation, etc. We can provide the below in electronic format upon request (feel free to call us on 0114 276 2264 and ask for a member of our technical sales team or alternatively send an e-mail to sales@e-magnetsuk.com). If you wish to buy any of the magnets you see below then please go to the relevant sections on our web site (as given by the section names).

NdFeB (Neodymium Iron Boron) Disc Magnets:-
EP305 (1mm dia x 1mmA , N42 - NiCuNi plated): 5813.8Ga, 9.8Ga, 1.4Ga.
EP303 (2mm dia x 1mmA , N42 - NiCuNi plated): 4596.2Ga, 37.8Ga, 5.6Ga.
EP400 (3mm dia x 1mmA , N42 - NiCuNi plated): 3605.6Ga, 80.1Ga, 12.3Ga.
EP392 (3mm dia x 2mmA , N42 - NiCuNi plated): 5200Ga, 129.8Ga, 21.7Ga.
EP342 (4mm dia x 0.5mmA , N42 - NiCuNi plated): 1576.5Ga, 73.6Ga, 11.4Ga.
EP343 (4mm dia x 1mm A , N42 - NiCuNi Plated): 2906.9Ga, 131.3Ga, 21.4Ga.
EP393 (4mm dia x 3mmA , N42 - NiCuNi Plated): 5408.3Ga, 270.8Ga, 50.6Ga.
EP401 (5mm dia x 1mmA , N42 - NiCuNi plated): 2414Ga, 186.2Ga, 32.4Ga.
EP304 (5mm dia x 3mm A , N42 - NiCuNi plated): 4993.4Ga, 390.3Ga, 77.1Ga.
EP641 (5mm dia x 5mmA , N38 - NiCuNi plated): 5523.1Ga, 467.5Ga, 100.4Ga.
EP306 (6mm dia x 1mmA , N42 - NiCuNi plated): 2055.5Ga, 240.1Ga, 45.1Ga.
EP390 (6mm dia x 2mmA , N42 - NiCuNi plated): 3605.6Ga, 400.7Ga, 80.1Ga.
EP643 (6mm dia x 3mmA , N42 - NiCuNi plated): 4596.2Ga, 512.4Ga, 107.7Ga.
EP657 (6mm dia x 4mmA , N42 - NiCuNi plated): 5200Ga, 592.7Ga, 129.8Ga.
EP658 (6mm dia x 5mmA , N42 - NiCuNi plated): 5573.7Ga, 652.2Ga, 147.9Ga.
EP644 (6mm dia x 6mmA , N38 - NiCuNi plated): 5523.1Ga, 662.4Ga, 154.7Ga.
EP322 (8mm dia x 1mmA , N42 - NiCuNi plated): 1576.5Ga, 332.7Ga, 73.6Ga.
EP323 (8mm dia x 2mmA , N42 - NiCuNi plated): 2906.9Ga, 567.9Ga, 131.3Ga.
EP380 (8mm dia x 4mmA , N42 - NiCuNi plated): 4596.2Ga, 864.1Ga, 214.8Ga.
EP324 (8mm dia x 5mmA , N42 - NiCuNi plated): 5075.6Ga, 959.5Ga, 245.4Ga.
EP327 (9mm dia x 3mmA , N42 - NiCuNi plated): 3605.6Ga, 833.8Ga, 214.9Ga.
EP349 (10mm dia x 0.4mmA , N42 - NiCuNi plated): 518.3Ga, 173.3Ga, 44.4Ga.
EP307 (10mm dia x 1mmA , N42 - NiCuNi plated): 1274.8Ga, 397.2Ga, 103.6Ga.
EP399 (10mm dia x 1.5mmA , N42 - NiCuNi plated): 1867.8Ga, 555.9Ga, 147.2Ga.
EP403N (10mm dia x 2mmA , N35 - ZINC plated): 2219.1Ga, 637.1Ga, 171.2Ga.
EP659 (10mm dia x 3mmA , N42 - NiCuNi plated): 3344.2Ga, 915.8Ga, 253Ga.
EP654 (10mm dia x 4mmA , N42 - NiCuNi plated): 4060.5Ga, 1085.8Ga, 307.5Ga.
EP645 (10mm dia x 5mmA , N42 - NiCuNi Plated): 4596.2Ga, 1217.6Ga, 352.7Ga.
EP308 (12mm dia x 1mmA , N42 - NiCuNi Plated): 1068.6Ga, 435Ga, 132.6Ga.
EP370 (12mm dia x 2mmA , N42- NiCuNi plated): 2055.5Ga, 774Ga, 240.1Ga.
EP656 (12mm dia x 3mmA , N38 - NiCuNi plated): 2761.5Ga, 986.9Ga, 311.6Ga.
EP404 (12mm dia x 3mmA , N50 - NiCuNi plated): 3164Ga, 1130.7Ga, 357.1Ga.
EP359 (14mm dia x 4mmA , N42 - NiCuNi plated): 3224.9Ga, 1352.7Ga, 488.8Ga.
EP290 (15mm dia x 0.5mmA , N42 - NiCuNi plated): 432.4Ga, 238.3Ga, 89.3Ga.
EP309 (15mm dia x 1mmA , N42 - NiCuNi plated): 859.1Ga, 455Ga, 170.5Ga.
EP310 (15mm dia x 2mmA , N42 - NiCuNi plated): 1674.8Ga, 829.5Ga, 312Ga.
EP528 (15mm dia x 4mmA , N42 - NiCuNi plated): 3058.8Ga, 1387.9Ga, 529.6Ga.
EP371 (17mm dia x 2mmA , N42 - NiCuNi plated): 1488.8Ga, 836.5Ga, 351.6Ga.
EP372 (18mm dia x 3mmA , N42 - NiCuNi plated): 2055.5Ga, 1161.7Ga, 512.8Ga.
EP655 (19mm dia x 5mmA , N42 - NiCuNi plated): 3027.4Ga, 1685.1Ga, 778.8Ga.
EP559 (20mm dia x 0.5mmA , N42-NiCuNi plated): 324.6Ga, 225.6Ga, 110.7Ga.
EP407 (20mm dia x 1mmA , N42 - NiCuNi plated): 646.8Ga, 437.3Ga, 213.4Ga.
EP406 (20mm dia x 3mmA , N42 - NiCuNi plated): 1867.8Ga, 1153.6Ga, 555.9Ga.
EP336 (20mm dia x 5mmA , N42 - NiCuNi plated): 2906.9Ga, 1689.3Ga, 812.1Ga.
EP646 (20mm dia x 10mmA , N42 - NiCuNi plated): 4596.2Ga, 2501.4Ga, 1217.6Ga.
EP543 (22mm dia x 8mmA , N42 - NiCuNi plated): 3823.1Ga, 2272.3Ga, 1174Ga.
EP355 (22mm dia x 10mmA , N42 - NiCuNi plated): 4372.4Ga, 2551.9Ga, 1323Ga.
EP311 (25mm dia x 2mmA , N42 - NiCuNi plated): 1026.9Ga, 761.9Ga, 440.9Ga.
EP358 (25mm dia x 10mmA , N42 - NiCuNi plated): 4060.5Ga, 2579.4Ga, 1451.5Ga.
EP360 (40mm dia x 5mmA , N42 - NiCuNi plated): 1576.5Ga, 1330.4Ga, 993.1Ga.


NdFeB (Neodymium Iron Boron) Rod Magnets:-
EP345 (2mm dia x 8mmA , N42 - NiCuNi plated): 6449.8Ga, 107.1Ga, 22.3Ga.
EP652 (3mm dia x 4mmA , N38 - NiCuNi plated): 5781.8Ga, 176.4Ga, 33.2Ga.
EP346 (3mm dia x 13mmA , N42 - NiCuNi plated): 6457.2Ga, 251.7Ga, 58.1Ga.
EP347 (3mm dia x 20mmA , N42 - NiCuNi plated): 6481.8Ga, 262.5Ga, 63.8Ga.
EP642 (5mm dia x 10mmA , N42 - NiCuNi plated): 6305.9Ga, 597.8Ga, 143.9Ga.
EP344 (5mm dia x 20mmA , N42 - NiCuNi plated): 6449.8Ga, 654Ga, 171.6Ga.
EP653 (5mm dia x 30mmA , N42 - NiCuNi plated): 6477.5Ga, 669.7Ga, 181.4Ga.
EP375 (8mm dia x 20mmA , N42 - NiCuNi plated): 6373.8Ga, 1342.7Ga, 407.9Ga.
EP325 (8mm dia x 30mmA , N42 - NiCuNi plated): 6120.8Ga, 1313.2Ga, 411Ga.
EP348 (10mm dia x 10mmA , N42 - NiCuNi plated): 5813.8Ga, 1570.2Ga, 492.1Ga.
EP376 (10mm dia x 20mmA , N42 - NiCuNi plated): 6305.9Ga, 1777.6Ga, 597.8Ga.
EP377 (10mm dia x 30mmA , N42 - NiCuNi plated): 6411.6Ga, 1838.5Ga, 636Ga.


NdFeB (Neodymium Iron Boron) Rectangular Block Magnets:-
EP316 (3mm x 3mm x 3mmA , N42 - NiCuNi plated): 5666.8Ga, 201.5Ga, 36.7Ga.
EP553 (3mm x 3mm x 8mmA , N42 - NiCuNi plated): 6359.4Ga, 287.7Ga, 62.5Ga.
EP317 (5mm x 5mm x 2mmA , N42 - NiCuNi plated): 3786.5Ga, 364.1Ga, 71.4Ga.
EP318 (5mm x 10mm x 2mmA , N42 - NiCuNi plated): 3143.8Ga, 517.2Ga, 124.8Ga.
EP333 (10mm x 3.5mm x 2.25mmA , N45 - NiCuNi plated): 4196.5Ga, 439Ga, 101.8Ga.
EP200 (10mm x 10mm x 5mmA , N42 - NiCuNi plated): 4333.3Ga, 1333.4Ga, 418.7Ga.
EP332 (12.5mm x 5mm x 1mmA , N45 - NiCuNi plated): 1765.1Ga, 324.8Ga, 82.5Ga.
EP331 (15mm x 5mm x 1mmA , N45 - NiCuNi plated): 1726.9Ga, 327.6Ga, 89.7Ga.
EP330 (20mm x 6mm x 1.5mmA , N45 - NiCuNi plated): 2085.5Ga, 512.1Ga, 164.2Ga.
EP293 (20mm x 10mm x 5mmA , N42 - NiCuNi plated): 3666.6Ga, 1502Ga, 601.8Ga.
EP279 (22mm x 7mm x 2mmA , N42 - NiCuNi plated): 2262.3Ga, 672Ga, 234.2Ga.
EP280 (22mm x 11mm x 2mmA , N42 - NiCuNi plated): 1616.8Ga, 789.5Ga, 327.8Ga.
EP319 (25mm x 5mm x 5mmA , N42 - NiCuNi plated): 4728.1Ga, 983.4Ga, 352.1Ga.
EP647 (25mm x 10mm x 3mmA , N42 -NiCuNi plated): 2420Ga, 1048.9Ga, 435.7Ga.
EP283 (25mm x 22mm x 0.4mm , N42 - NiCuNi plated): 200.3Ga, 159.3Ga, 96.6Ga.
EP352 (40mm x 20mm x 10mmA , N42 - NiCuNi plated): 3666.6Ga, 2444.8Ga, 1502Ga.
EP329 (42mm x 8mm x 10mmA , N45 - NiCuNi plated): 5280.6Ga, 1899.1Ga, 857Ga.
EP181 (50mm x 12mm x 25.4mmA , N38 - NiCuNi plated): 5538.3Ga, 2868.8Ga, 1578.3Ga.
EP648 (50mm x 25mm x 10mmA , N42 - NiCuNi plated): 3143.8Ga, 2330.7Ga, 1590.5Ga.
EP320 (50mm x 50mm x 5mmA , N42 - NiCuNi plated): 1151.4Ga, 1047.8Ga, 882.4Ga.
EP335 (50mm x 50mm x 25mmA , N42 - NiCuNi plated): 4333.3Ga, 3601.2Ga, 2874.8Ga.
EP321 (100mm x 20mm x 4mmA , N42 - NiCuNi plated): 1607.2Ga, 1145.2Ga, 712.3Ga.
EP340 (100mm x 50mm x 10mmA , N50 - NiCuNi plated): 1920.7Ga, 1739.6Ga, 1501.2Ga.


NdFeB (Neodymium Iron Boron) Ring Magnets:-
EP300 (6mm dia x 5mmA x 2mm dia hole , N42 - NiCuNi plated): -3747.8Ga, -800.1Ga, 558.2Ga, 130Ga.
EP387 (10mm dia x 2mmA x 5mm hole , N42- NiCuNi plated): -2278.6Ga, -1646.5Ga, 385.5Ga, 128.8Ga.
EP388 (15mm dia x 3mmA x 8mm hole , N42 - NiCuNi plated): -2015.1Ga, -1486Ga, 398.3Ga, 252.7Ga.
EP389 (20mm dia x 3mmA x 15mm hole , N42 - NiCuNi plated): -621.1Ga, -546.3Ga, 17.2Ga, 125.7Ga.
EP391G (24mm dia x 8mmA x 12mm dia hole , N42 - Gold plated): -3100.1Ga, -1594.4Ga, 535.7Ga, 654.4Ga.
EP391N (24mm dia x 8mmA x 12mm dia hole , N42 - NiCuNi plated): -3100.1Ga, -1594.4Ga, 535.7Ga, 654.4Ga.


NdFeB (Neodymium Iron Boron) Adhesive Backed Disc Magnets:-
EP306A-ADH (6mm dia x 1mm NORTH, N42 - NiCuNi plated): 2055.5Ga, 240.1Ga, 45.1Ga.
EP306B-ADH (6mm dia x 1mm SOUTH, N42 - NiCuNi plated): -2055.5Ga, -240.1Ga, -45.1Ga.
EP322A-ADH (8mm dia x 1mm NORTH, N42 - NiCuNi plated): 1576.5Ga, 332.7Ga, 73.6Ga.
EP322B-ADH (8mm dia x 1mm SOUTH, N42 - NiCuNi plated): -1576.5Ga, -332.7Ga, -73.6Ga.
EP307A-ADH (10mm dia x 1mm NORTH, N42 - NiCuNi plated): 1274.8Ga, 397.2Ga, 103.6Ga.
EP307B-ADH (10mm dia x 1mm SOUTH, N42 - NiCuNi plated): -1274.8Ga, -397.2Ga, -103.6Ga.
EP308A-ADH (12mm dia x 1mm NORTH, N42 - NiCuNi plated): 1068.6Ga, 435Ga, 132.6Ga.
EP308B-ADH (12mm dia x 1mm SOUTH, N42 - NiCuNi plated): -1068.6Ga, -435Ga, -132.6Ga.
EP309A-ADH (15mm dia x 1mm NORTH, N42 - NiCuNi plated): 859.1Ga, 455Ga, 170.5Ga.
EP309B-ADH (15mm dia x 1mm SOUTH, N42 - NiCuNi plated): -859.1Ga, -455Ga, -170.5Ga.
EP323A-ADH (8mm dia x 2mm NORTH, N42 - NiCuNi plated): 2906.9Ga, 567.9Ga, 131.3Ga.
EP323B-ADH (8mm dia x 2mm SOUTH, N42 - NiCuNi plated): -2906.9Ga, -567.9Ga, -131.3Ga.
EP403A-ADH (10mm dia x 2mm NORTH, N42 - NiCuNi plated): 2414Ga, 693.1Ga, 186.2Ga.
EP403B-ADH (10mm dia x 2mm SOUTH, N42 - NiCuNi plated): -2414Ga, -693.1Ga, -186.2Ga.


SmCo (Samarium Cobalt) Disc Magnets:-
EP305SM (1mm dia x 1mmA , SmCo26): 4628.7Ga, 7.8Ga, 1.1Ga.
EP303SM (2mm dia x 1mmA , SmCo26): 3659.3Ga, 30.1Ga, 4.4Ga.
EP400SM (3mm dia x 1mmA , SmCo26): 2870.6Ga, 63.7Ga, 9.8Ga.
EP100SM (4mm dia x 5mmA , SmCo26): 4804.9Ga, 269.6Ga, 55.1Ga.
EP306SM (6mm dia x 1mmA , SmCo26): 1636.5Ga, 191.1Ga, 35.9Ga.
EP643SM (6mm dia x 3mmA , SmCo26): 3659.3Ga, 408Ga, 85.7Ga.
EP644SM (6mm dia x 6mmA , SmCo26): 4628.7Ga, 555.1Ga, 129.6Ga.
EP323SM (8mm dia x 2mmA , SmCo26): 2314.3Ga, 452.2Ga, 104.6Ga.
EP307SM (10mm dia x 1mmA , SmCo26): 1014.9Ga, 316.3Ga, 82.5Ga.
EP403SM (10mm dia x 2mmA , SmCo26): 1921.9Ga, 551.8Ga, 148.3Ga.
EP645SM (10mm dia x 5mmA , SmCo26): 3659.3Ga, 969.4Ga, 280.8Ga.
EP348SM (10mm dia x 10mmA , SmCo26): 4628.7Ga, 1250.2Ga, 391.8Ga.
EP656SM (12mm dia x 3mmA , SmCo26): 2314.3Ga, 827Ga, 261.2Ga.
EP406SM (20mm dia x 3mmA , SmCo26): 1487Ga, 918.5Ga, 442.6Ga.


SmCo (Samarium Cobalt) Rectangular Block Magnets:-
EP316SM (3mm x 3mm x 3mmA , SmCo26):-4511.6Ga, 160.4Ga, 29.2Ga.
EP317SM (5mm x 5mm x 2mmA , SmCo26):-3014.6Ga, 289.9Ga, 56.8Ga.
EP318SM (5mm x 10mm x 2mmA , SmCo26):-2503Ga, 411.8Ga, 99.3Ga.
EP200SM (10mm x 10mm x 5mmA , SmCo26):-3450Ga, 1061.6Ga, 333.4Ga.
EP332SM (12.5mm x 5mm x 1mmA , SmCo26):-1353.3Ga, 249Ga, 63.2Ga.
EP330SM (20mm x 6mm x 1.5mmA , SmCo26):-1598.8Ga, 392.6Ga, 125.9Ga.
EP281SM (24mm x 12mm x 6mmA , SmCo26):-2919.2Ga, 1408.4Ga, 639.4Ga.
EP319SM (25mm x 5mm x 5mmA , SmCo26):-3764.3Ga, 783Ga, 280.4Ga.
EP647SM (25mm x 10mm x 3mmA , SmCo26):-1926.7Ga, 835.1Ga, 346.8Ga.
EP510SM (60mm x 12mm x 2.5mmA , SmCo26):-1327.7Ga, 687.9Ga, 326.1Ga.

You can purchase any of the above listed magnets through this web site.

If you need to know the Gauss levels at other distances, please let us know and we can send you the required calculation tools. Please feel free to call us on 0114 276 2264 and ask for a member of our technical sales team or alternatively send an e-mail to sales@e-magnetsuk.com