These devices are everywhere, but how do they work?
By Sam Brown, MST/Appliantology.org
Whether you were aware of them or not, Hall effect sensors are everywhere. Any time you see a motor on an appliance with any kind of RPM feedback or speed signal, there’s a Hall sensor on that motor.
As with any ubiquitous appliance technology, it’s important for us techs to know how they work, since a deeper understanding of the technology means a deeper understanding of how to troubleshoot it.
So, what is a Hall sensor, and how does it work?
Named after the American physicist Edwin Hall, these sensors are semiconductor devices, so as with most electronics, it’s not important to get too far into the weeds. We just want a functional understanding of them as far as it affects our troubleshooting. To that end, here’s a little animation to show you a Hall sensor in action:
Those red discs are permanent magnets, similar to the kind you might stick on your refrigerator. The blue box is the actual Hall sensor. Notice how every time the spinning white disc (that’s the rotor of a motor) takes a magnet past the Hall sensor, the sensor lights up. This is all Hall sensors do: They detect the proximity of a magnet. It’s that simple.
How do they do this? By using an electromagnetic principle known as the Hall effect (hence the formal name for the sensors). The Hall effect states that if you have a current flowing through a conductor, and you then apply a magnetic field to that conductor, it will induce a voltage difference across that conductor perpendicular to the current flow. This induced voltage difference is often called the Hall voltage.
Here’s an image demonstrating the Hall effect in action in a Hall sensor:
Again, it’s not important that you completely wrap your head around everything that’s going on in that picture. What’s important is that you understand what inputs a Hall sensor needs and what it outputs. That’s the kind of information you need for troubleshooting.
Below is what a Hall sensor looks like physically. Note that it has three terminals, and in this photo, each terminal is labeled with what that terminal carries:
It’s super simple: You merely have a 5 VDC power supply, a DC ground, and then the Signal, which is that very same DC voltage — the Hall voltage! — induced from the magnet that we talked about before. The 5 VDC and the DC ground are there to produce the constant current through the sensor, across which the Hall voltage is induced.
All three of these wires would connect back to a control board. This board supplies the DC voltage and ground, and it receives the sensor’s signal as an input. The logic on the board then interprets that signal to make calculations about whether the motor is running and how quickly it is turning.
So now that you know how they work, how do you test a Hall sensor? It’s actually very easy, and you might even have guessed it at this point. All you need to do is manually spin the motor that the sensor is attached to while you have your voltmeter measuring the signal line with respect to the DC ground. If you see some fluctuating voltage appear while the motor is spinning, you know that the sensor is doing its job.
And that’s all there is to it! Hall sensors are very simple, but also very useful and versatile devices. We see them all over the place, and now you know what’s going on inside that tiny black rectangle.
Want to really understand the technologies present in all appliances and learn how to apply that knowledge to your troubleshooting for maximum profits? Check out our online Core Appliance Repair Training course over at Master Samurai Tech, and be sure to use your BrandSource discount coupon for 15 percent off your enrollment.
Sam Brown is “Professor of Appliance Repair Mastery” at BrandSource partner MasterSamuraiTech.com (MST), an online training academy for appliance repair personnel, and administrator of its sister tech support site, Appliantology.org. For more information, email Sam at email@example.com or call (603) 290-5516.