The Hall effect is the production of a voltage across an electrical conductor transverse to the flow of a current, when in a magnetic field that is perpendicular to the current. This is best understood by the looking at a diagram like that of Figure 1. Here a rectangular conductor has a current flowing along it. The conductor is subject to a magnetic field B which is perpendicular to the direction of current flow.
The Hall effect occurs when a current (I) flows in a conductor perpendicular to a magnetic field (B). A Lorentz force on the moving electrons forces them to one side of the conductor (red arrow) causing a transverse electric filed and corresponding voltage across the conductor.
Lorenz’s law tells us that the moving electrons that make up the current will be subject to a force which will push them to the left side of the conductor (red arrow). This means there will be a positive charge on the right-hand side of the conductor and a negative one on left. This potential difference is measurable as a voltage across the conductor. This voltage – the Hall voltage – is proportional to the current that flows and the magnetic field density.
If the circuit is completed as shown in the diagram, a current will flow which, in equilibrium, will offset the transverse field, returning the electron path to the straight one shown bu the dotted arrow.
In practical terms a Hall effect sensor is any device that uses the Hall effect to measure a magnetic field. Such sensors are extremely useful in sensing magnetic field strength, proximity, speed or current.
Let’s look at a typical example of one of the simplest – the TMAG5124 magnetic field sensor from TI. This is a two-pin device that incorporates a current source that switches between a high current (14.5mA) and a low current (3.5mA) when a specific magnetic field density (BOP) is reached. Figure 2 shows the block diagram, output characteristic and typical application circuit extracted from the data sheet.
The TI TMAG5124 magnetic sensor is a two-wire device which sinks a high current until the magnetic field reaches a certain threshold BOP at which point it switches to sinking a low current. This can be sensed remotely across a resistor as shown. There is some hysteresis in the switching thresholds to ensure clean transitions.
This type of sensor is often used to detect the proximity of a permanent magnet attached to a moving component such as a motor shaft or rotor to detect position or speed. Figure 3 shows a typical example of a brushless DC motor stator where you can just make out three Hall sensors positioned at the ends of the poles to detect the position of the permanent magnet rotor.
A typical application for a Hall effect switch is a brushless DC motor such as this which uses three Hall effect sensors (just visible near the ends of the stator poles) to control the current in the windings dependent on the position of the rotor. The rotor, with its permanent magnets, is not shown here.
Another classic application of the Hall effect is in non-contact current sensors. These use a Hall effect device to measure the magnetic field produced by the current to be measured. The big advantage of this type of sensor is that the measuring circuit can be electrically isolated from the measured current as it is with a current transformer but unlike them, works down to DC.
A good example is the Allegro ACS758x family. These use a Hall effect transducer to measure full-scale current in the ±50 to ±200A (depending on the model) and with DC to 120kHz bandwidth. Figure 4 shows how the device looks – it is not much bigger than a TO-220 package. The two high-current pins are isolated from the rest of the circuitry up to 4800VDC and can withstand a momentary overcurrent up to 1200A.
The Allegro ACS758x sensors can measure currents up to ±200A over a DC to 140kHz bandwidth. The high current terminals are isolated from the power and output terminals.
The output is a DC voltage, centred on half of the nominal 3.3V to 5V supply voltage, that is proportional to the input current. Internally there is a sophisticated linearisation, trimming and temperature compensation circuit that results in an overall error of less that ±2% over a wide temperature range. Figure 5 gives a hint of the complexity of this internal signal processing. The Hall effect was discovered by Edwin Hall in 1879, only a decade after Maxwell quantified the interaction between magnetic and electrical fields. This old idea, combined with modern integrated circuit technology has led to a wide range of interesting and useful devices we can employ in our designs.
The internals of the ACS758x sensor include a Hall effect element (marked X) that measures the magnetic field produced by the current to be sensed. The output from the Hall sensor is amplified and conditioned to produce an output voltage proportional to the current with an accuracy of better than ±2% over a wide temperature range.
Bibliography
“Hall Effect Sensor.” In Wikipedia, June 17, 2024. https://en.wikipedia.org/w/index.php?title=Hall_effect_sensor&oldid=1229639726.
“TMAG5124 Data Sheet, Product Information and Support | TI.Com.” Accessed July 16, 2024. https://www.ti.com/product/TMAG5124.
“Current Sensor IC with 100 µΩ Current Conductor – ACS758 | Allegro MicroSystems.” Accessed July 16, 2024. https://www.allegromicro.com/en/products/sense/current-sensor-ics/fifty-to-two-hundred-amp-integrated-conductor-sensor-ics/acs758.
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Andrew Levido (andrew.levido@gmail.com) earned a bachelor’s degree in Electrical Engineering in Sydney, Australia, in 1986. He worked for several years in R&D for power electronics and telecommunication companies before moving into management roles. Andrew has maintained a hands-on interest in electronics, particularly embedded systems, power electronics, and control theory in his free time. Over the years he has written a number of articles for various electronics publications and occasionally provides consulting services as time allows.
