Hall-effect sensors

Last updated: July 29, 2009.

Measuring electricity is really easy—we're all familiar with electrical units like volts, amps, and watts (and most of us have seen moving-coil meters in one form or another). Measuring magnetism is a little bit harder. Ask most people how to measure the strength of a magnetic field (the invisible area of magnetism extending out around a magnet) or the units in which field strength is measured (webers or teslas, depending on how you're measuring) and they wouldn't have a clue.

But there's a simple way to measure magnetism with a device called a Hall-effect sensor or probe, which uses a clever bit of science discovered in 1879 by American physicist Edwin H. Hall (1855–1938). Hall's work was ingenious and years ahead of its time: 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 delighted to find sensors named for him are being used in all kinds of interesting ways. Let's take a closer look!

Photo: Magnetic test equipment used for studying the Hall effect. Photo by courtesy of Brookhaven National Laboratory and US Department of Energy (DOE).

What is the Hall effect?

Working together, electricity and magnetism can make things move: electric motors, loudspeakers, and headphones are just a few of the indispensable modern gadgets that function this way. Send a fluctuating electric current through a coil of copper wire and (although you can't see it happening) you'll produce a temporary magnetic field around the coil too. Put the coil near to a big, permanent magnet and the temporary magnetic field the coil produces will either attract or repel the magnetic field from the permanent magnet. If the coil is free to move, it will do so—either toward or away from the permanent magnet. In an electric motor, the coil is set up so it can spin around on the spot and turn a wheel; in loudspeakers and headphones, the coil is glued to a piece of paper, plastic, or fabric that moves back and forth to pump out sound.

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

What if you place a piece of current-carrying wire in a magnetic field and the wire can't move? What we describe as electricity is generally a flow of charged 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 between the poles of a permanent magnet, the electrons will deflect into a curved path as they move through the material because their own magnetic field will be interacting with the permanent magnet's field. (For the record, the thing that makes them deflect is called the Lorentz 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 the other, so a potential difference (voltage) will appear across the material at right angles to both the magnetic field from the permanent magnet and the flow of current. 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 the potential difference (known as the Hall voltage) will be. In other words, the Hall voltage is proportional in size to both the electric current and the magnetic field. All this makes more sense in our little animation, below.

Using the Hall effect

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

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

Photo: Left: A typical silicon Hall-effect sensor. It looks very much like a transistor—hardly surprising since it's made in a similar way. Photo by Explainthatstuff.com. Right: A Hall-effect probe used by NASA in the mid-1960s. Photo by courtesy of NASA Glenn Research Center (NASA-GRC).

Typically made from semiconductors (materials such as silicon and germanium), Hall-effect sensors work by measuring the Hall voltage across two of their faces when you place them in a magnetic field. Some Hall sensors are packaged into convenient chips with control circuitry and can be plugged directly into bigger electronic circuits. The simplest way of using one of these devices is to detect something's position. For example, you could place a Hall sensor on a door frame and a magnet on the door, so the sensor detects whether the door is open or closed from the presence of the magnetic field. A device like this is called a proximity sensor. Of course, you can do the same job just as easily with a magnetic reed switch (there is no general rule as to whether old-style reed switches or modern, Hall-effect sensors are better—it depends on the application). But what 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.

How else are Hall-effect sensors used? In a brushless DC motor (used in such things as floppy-disk drives), you need to be able to sense exactly where the motor is positioned at any time. A Hall-effect sensor stationed near the rotor (rotating part of the motor) will be able to detect its orientation very precisely by measuring variations in the magnetic field. Sensors like this can also be used to measure speed (for example, to count how fast a wheel or car engine cam or crankshaft is rotating).

Photo: This small brushless DC motor from a 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!

It took a few decades for Edwin Hall's revolutionary discovery to catch on, but now it's used 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!