What is the Hall Effect?
The 19th century was a time of great scientific leaps. It was back in that era when the Hall effect was first discovered; a way to measure a magnetic field. It may seem like a long time ago and discoveries of that age can seem relatively primordial compared to newer scientific concepts like human genome sequencing and quantum computing. But just because it’s long in the tooth doesn’t mean it isn’t useful.
If you’ve read through our blog “What is a Thermocouple?” you will be familiar with the Seebeck effect, an 1800s discovery of measuring the voltage of metals to help tell temperature; an instrument that is in widespread use still to this day. The Hall effect follows a similar path but what exactly is the Hall effect and how does it work? Who uses Hall sensors or Hall devices, and in what way? Read on to find out the basics of this impressive, and now prolific, discovery.
Figure 1: A magnet, one of the most basic forces
What is the Hall Effect?
Electromagnetism is one of the four fundamental forces; gravity, weak nuclear force, and strong nuclear force being the others. These topics are far too broad for this humble blog, but keep this mind; there is a connection between electricity and magnetism. What we perceive as electricity is basically the movement of free electrons through a conducting wire. Since electrons are negatively charged and can be pushed or pulled by a magnetic field. Therefore, with the use of a magnet, one could push electrons through a wire and create a current. No, this isn’t some magic trick, it’s called induction. This relationship between a magnet and an electrical current is what the Hall effect is based on, namely the effect of a magnetic field on electrons that make up a current.
Hall effect devices are one of the most common ways to measure magnetic fields. With this type of sensor, the path of a flowing electrical current within a semiconductor is changed by a nearby magnet. This alteration of the current can be measured as a voltage since one side of the semiconductor will have more electrons and be negatively charged. Whereas the other side, will have less electrons and, therefore, a more positive charge. The size of the voltage is proportional compared to the affecting magnetic field; if it is strong then there will be a large potential difference, and if it is weak there will one be a small voltage. Let’s look at a diagram of a basic Hall effect sensor to better grasp this idea.
Figure 2: A basic Hall Effect sensor
A Hall effect sensor is a simple wafer of semi-conductive material that is part of a circuit. As you can see in Figure 2, at the moment nothing terribly exciting is happening; electrons of an electrical current are flowing through our simple circuit. We’re going to put a magnet near this device.
Figure 3: Magnet nearing the Hall sensor
When a magnet approaches, the electrons of the current are deflected. Hence, as the magnet gets closer the electrons are deflected more. Therefore producing a larger and larger measurable voltage as the magnet nears ever closer. Alternatively, Instead of moving the magnet closer you could make the magnet stronger. This would create a larger magnetic field and cause a bigger increase in voltage that way.
Figure 4: A magnet very close to the Hall sensor making for a larger voltage
Note the different readings on the multimeters throughout Figures 2, 3, and 4. With an increase in the strength of a magnetic field, there is an equal increase in the voltage. Even with strong magnetic fields the potential difference produced is incredibly small. A complete Hall effect device may include some sort of amplifier to boost the signal in order to produce voltage at a magnitude that we can use.
Hall Effect discovery
Although the Hall effect was discovered in the late 19th century. It wasn’t until 70 years later that this discovery was first used in practical applications. A microwave power switch was the first implementation of the device. With semiconductors becoming cheaper to manufacture the employment potentials of Hall effect sensors grew. Now you can find them in everything from aircraft, cell phones, and even in dishwashers.
Let’s put aside theory for a moment and brainstorm some practical uses of this principle. The most simple would be to employ the Hall effect as a sort of proximity sensor. If a magnetic field can cause a change in the path of an electrical current then we could embed a magnet in an object and quantify the number of times it neared another object just by sensing the voltage change when it nears an electrical current.
Were we to place our rudimentary proximity sensor onto the blade of a windmill generator, for example, we could sense each time the blade made a full revolution. This sensing of rotation could let a windmill operator measure all sorts of things like the outdoor wind speed or the potential power that could be generated. The information could even be used as a safety switch that could trigger an alarm if the revolving blades were turning at too high a speed.
Hall Effect sensor application
Enough hypothetical, let’s take a quick look at one modern device, the automobile, and see how prevalent Hall sensors really are. If you have enjoyed the convenience of powered windows or side mirror controls then you’ve been using a Hall sensor. Be it a gas guzzler or economy class, your car is getting better fuel mileage from a fuel injection system and crankshaft performance monitoring, both possible due to Hall sensors. You might be familiar with the term “pump the brakes” but if not that just means you are lucky enough to have grown up in the era of antilock braking systems.
In this invention a Hall sensor device keeps your tires rotating at an optimal pace and, instead of skidding to a halt, it actually brings your car to a stop faster when you slam on the brakes. Indeed, you’ll find Hall sensors everywhere, in washing machines, on computer boards, and within industrial machinery. The use of this device has allowed for a proliferation of inventions; that assist us in everything from safety to efficiency, and even just modern conveniences that we take for granted.
Figure 5: Automobile features that use Hall sensors
Another large use of Hall effect sensors is their employment as current transducers. Since the Hall effect measures a magnetic field, a product of a process wire carrying current, it is a great type of sensor to use in some types of current transducers. When a process current runs through the aperture of our transducer it creates a proportional secondary magnetic field within the instrument that acts on a Hall sensor. Current transducers are a fascinating process control device and we go over them in much more detail at “What is a Current Transducer?” Here at Enercorp, we supply current transducers to all sorts of industry professionals. Have a look at our Current section where you can check out some real-world examples and see the wide range of abilities and characteristics of this instrument.
Figure 6: A pair of Enercorp current transducers
Since a magnetic field is the key variable in causing Hall effect sensors to react. Anything you can affix a magnet to could become some sort of Hall device. Now, with process controls and automation becoming such an important part of operations; the need for custom and capable sensors is an unrelenting requirement. It would be impossible, without Hall effect sensors, for modern-day machines to run with the sort of features and standard efficiency that we have come to expect. Everything from transportation, manufacturing, healthcare, building automation, and other industries, would be exponentially different than how we know them today.