How Hall effect sensors and probes work (2024)

by Chris Woodford. Last updated: June 7, 2024.

Measuring electricity is really easy—we'reall familiar with electrical units like volts, amps, and watts (and most of us have seenmoving-coil metersin one form or another). Measuring magnetism is a little bit harder. Ask mostpeople how to measure the strength of a magnetic field (the invisiblearea of magnetism extending out around a magnet) or the units inwhich field strength is measured (webers or teslas, depending on howyou're measuring) and they wouldn't have a clue.

But there's a simple way to measure magnetism with a devicecalled a Hall-effect sensor or probe, which uses a clever bit ofscience discovered in 1879 by American physicistEdwin H. Hall(1855–1938). Hall's work was ingenious and years ahead of its time—20 yearsbefore the discovery of the electron—and no-one really knew what to do with it until decades later when semiconducting materials such as silicon became better understood. These days, Edwin Hall would be delightedto find sensors named for him are being used in allkinds of interesting ways. Let's take a closer look!

Photo: A Hall-effect sensor (indicated by the white arrow) helps to measure the rotational position of this old floppy disk motor. More about this in a moment...

What is the Hall effect?

Working together, electricity and magnetism can make things move:electric motors, loudspeakers, andheadphones are just a few of the indispensablemodern gadgets that function this way. Send a fluctuating electriccurrent through a coil of copper wire and (although you can't see ithappening) you'll produce a temporary magnetic field around the coiltoo. Put the coil near to a big, permanent magnet and the temporarymagnetic field the coil produces will either attract or repel themagnetic field from the permanent magnet. If the coil is free tomove, it will do so—either toward or away from the permanent magnet. In anelectric motor, the coil is set up so it can spin around on the spotand turn a wheel; in loudspeakers andheadphones, the coil is gluedto a piece ofpaper, plastic, orfabric that moves back and forth topump out sound.

Photo: You can't see a magnetic field, but you can measure it with the Hall effect. Photo bycourtesy of Wikimedia Commons.

What if you place a piece of current-carrying wire in a magnetic field and the wirecan't move? What we describe as electricity is generally a flow ofcharged particles through crystalline (regular, solid) materials (either negatively charged electrons, from inside atoms, or sometimes positively charged "holes"—gaps where electrons should be).Broadly speaking, if you hook a slab of a conducting material up to a battery,electrons will march through the slab in a straight line. As moving electric charges,they'll also produce a magnetic field. If you place the slab betweenthe poles of a permanent magnet, the electrons will deflect into acurved path as they move through the material because their ownmagnetic field will be interacting with the permanent magnet's field.(For the record, the thing that makes them deflect is called theLorentz force, but we don't need to go into all the details here.)That means one side of the material will see more electrons than theother, so a potential difference (voltage) will appear across thematerial at right angles to both the magnetic field from thepermanent magnet and the flow of current. This is what physicists call the Hall effect.The bigger the magnetic field, the more the electrons are deflected; the bigger the current,the more electrons there are to deflect. Either way, the bigger thepotential difference (known as the Hall voltage) will be. In otherwords, the Hall voltage is proportional in size to both the electriccurrent and the magnetic field. All this makes more sense inour little animation, below.

Using the Hall effect

You can detect and measure all kinds of things with the Hall-effect using what's knownas a Hall-effect sensor or probe. These terms are sometimes usedinterchangeably but, strictly speaking, refer to different things:

• Hall-effect sensors are simple, inexpensive,electronic chips that are usedin all sorts of widely available gadgets and products.
• Hall-effect probes are more expensive and sophisticated instruments usedin scientific laboratories for things like measuring magnetic field strength with very high precision.

Photo: 1) A typical silicon Hall-effect sensor. It looksvery much like a transistor—hardly surprising since it's made in a similar way.Photo by explainthatstuff.com. 2) A Hall-effect probe used by NASA in the mid-1960s. Photo by courtesy ofNASA Glenn Research Center (NASA-GRC).

Typically made from semiconductors (materials such as silicon and germanium), Hall-effectsensors work by measuring the Hall voltage across two of their faceswhen you place them in a magnetic field. Some Hall sensors arepackaged into convenient chips with control circuitry and can beplugged directly into bigger electronic circuits. The simplest way ofusing one of these devices is to detect something's position. Forexample, you could place a Hall sensor on a door frame and a magneton the door, so the sensor detects whether the door is open or closedfrom the presence of the magnetic field. A device like this is calleda proximity sensor. Of course, you can do the same job just as easilywith a magnetic reed switch(there is no general rule as to whetherold-style reed switches or modern, Hall-effect sensors are better—itdepends on the application). Unlike reed switches, which are mechanical and rely on contactsmoving in a magnetic field, Hall sensors are entirely electronic and have no moving parts, so(theoretically, at least) they should be more reliable. One thing you can't do with a reed switch is detect degrees of "on-ness"—the strength of the magnetism—because a reed switch is either on or off. That's what makes a Hall-effect sensor so useful.

What are Hall-effect sensors used for?

Hall-effect sensors are cheap, robust and reliable, tiny, and easy to use,so you'll find them in lots of different machines and everyday devices,from car ignitions to computer keyboards and factory robots to exercise bikes.

Here's one very common example you might be using in your computer right now. In a brushless DC motor (used in such things as hard- and floppy-disk drives), you need to be able to sense exactly where the motor is positioned at any time. A Hall-effect sensorstationed near the rotor (rotating part of the motor) will be able todetect its orientation very precisely by measuring variations in themagnetic field. Sensors like this can also be used to measure speed(for example, to count how fast a wheel or car enginecam or crankshaft is rotating). You'll often findthem in electronic speedometersand anemometers (wind-speed meters), where they can be usedin a similar way to reed switches.

Photo: This small brushless DC motor from an old floppy-disk drive has three Hall-effect sensors(indicated by red circles) positioned around its edge, which detect the motion of the motor's rotor (a rotating permanent magnet) above them (not shown on this photo). The sensors are not much to look at, as you can see from the closeup photo on the right!

It took a few decades for Edwin Hall's revolutionary discovery to catch on, but now it'sused in all kinds of places—even in electromagnetic space rocket engines.It's no exaggeration to say that Hall's groundbreaking work has had quite an effect!

Artwork: How a typical Hall sensor is packaged. Magnetic fields can be very small, so we need our detectors to be as sensitive as possible, and here's one way to achieve that. The Hall chip itself (green, 17) is mounted on an iron carrier plate (gray, 16) sandwiched inside two molded plastic sections (gray, 11, 12). The chip is wired by leads (19) to terminal pins (blue) by which it can be connected into a circuit. But the really important parts are two soft iron "flux concentrators" (orange, 15, 21), which make the device very much more sensitive. When you place a magnet (22) near the sensor, these concentrators allow the magnetic flux (the "density" of magnetism produced by the magnetic field) to flow around a continuous loop through the Hall chip, producing either a positive or negative voltage. If the magnet slides over to the other side of the sensor, it produces the opposite voltage. Artwork from US Patent 3,845,445: Modular Hall Effect Device by Roland Braun et al, IBM Corporation, October 29, 1974, courtesy of US Patent and Trademark Office.

Find out more

You might also like these articles...

Articles

• Researchers Simplify Switching for Quantum Electronics by Edd Gent. IEEE Spectrum, October 30, 2023. What is the quantum Hall effect... and what use is it?
• Graphene Magnetic Sensor Hundred Times More Sensitive Than Silicon by Dexter Johnson. IEEE Spectrum, June 26, 2015. German researchers develop magnetic Hall effect sensors based on graphene.
• How Do You Measure the Magnetic Field? by Rhett Allain. Wired, January 21, 2014. Comparing traditional compasses with Hall-effect sensors found in a smartphone.
• The Nobel Prize in Physics 1998: The Fractional Quantum Hall Effect: A simplified description of the physics that won a prize for Horst Störmer, Robert Laughlin, and Daniel Tsui. For a more detailed description, try The Quantized Hall Effect by H. L. Stormer and D. C. Tsui, Science, 17 Jun 1983.

History

• [PDF] The discovery of the Hall effect by G.S. Leadstone, Physics Education, Volume 14, 1979. How Hall discovered his effect and figured out what it meant by challenging some of the earlier work by James Clerk Maxwell.

More technical

• The Vertical Hall-Effect Device by R. S. Popovic, IEEE Electron Device Letters, Vol.5, No.9, pp.357–358, Sept. 1984, doi: 10.1109/EDL.1984.25945.

Papers by Edwin Hall

• On a New Action of the Magnet on Electric Currents byEdwin H. Hall, American Journal of Mathematics, Vol. 2, No. 3 (Sep., 1879), pp. 287–292. Hall's original paper.
• An Explanation of Hall's Phenomenon by Edwin H. Hall,Science, Vol. 3, No. 60 (Mar. 28, 1884), pp. 386–387. Hall's own description and explanation of his original experiment.
• A Theory of the Hall Effect and the Related Effect for Several Metals by Edwin H. Hall, PNAS USA, Vol. 9, No. 2 (Feb. 15, 1923), pp. 41–46. One of Hall's later papers.

Books

• Hall-Effect Sensors: Theory and Applications by Edward Ramsden. Newnes, 2006. Covers the physics behind Hall-effect sensors and how to incorporate them into practical circuits. Includes coverage of proximity sensors, current-sensors, and speed-and-timing sensors. Also has a handy glossary and list of suppliers.
• Hall-Effect Devices by R. S. Popović. Institute of Physics, 2004. A somewhat bigger and more detailed book, but covering similar ground with a mixture of theory, practical circuits, and everyday applications.
• The Hall Effect and Its Applications by C. Chien (ed). Plenum Press, 1980/Springer, 2013. A reissue of the proceedings of a 1979 symposium at Johns Hopkins University, on November 13, 1979 to commemorate the 100th anniversary of Hall's discovery.
• The Hall Effect in Metals and Alloys by Colin Hurd. Springer 1972/2012. A modern reissue of a 1970s introduction.

Practical projects

• Door Activated LED Lighting using Hall Effect Sensors: Woody1189 wires up his closet with a Hall-effect sensor so it lights up automatically when he opens the door!
• Electric Bike Hub Motor—How to Replace a Hall-effect Sensor: Jeremy Nash explains what a Hall-effect sensor does in a brushless motor—and how to replace the sensor when it fails.

Videos

• How to make a magnet polarity detection circuit: Thomas Kim shows us how to make a magnet detector based on a Hall-effect sensor extracted from a laptop cooler fan.

Patents

A few more technical examples of Hall detectors and their uses:

• US Patent 3,845,445A: Modular hall effect device by R. Braun et al, IBM, October 29, 1974. The concentrating, modular Hall effect device illustrated above.
• US Patent 3,845,445A: Hall element device with depletion region protection barrier by R. Popovic, Siemens, May 29, 1990. A Hall element that can be incorporated into an integrated circuit that's designed to be stable over a long lifetime.