2026-08-23
A Hall effect sensor exploits a subtle quantum-adjacent phenomenon: when current flows through a thin conductor and a magnetic field passes perpendicular to that current, the Lorentz force pushes charge carriers to one side of the strip. The resulting sideways voltage — the Hall voltage — is proportional to the field strength. Discovered by Edwin Hall in 1879, it stayed a physics curiosity until semiconductors made the effect large enough to be useful.
Modern Hall sensors are integrated circuits: a Hall element plus an amplifier, temperature compensation, and often a Schmitt trigger or linear output stage, all in a TO-92 or SOT-23 package for under a dollar.
Three flavors you'll actually use:
Rule of thumb for current sensing: a Hall-effect current sensor (like the Allegro ACS712) outputs about 66 mV per amp for the 30A version, with the current-carrying conductor passing through the chip's built-in loop. No shunt resistor in the current path means no insertion loss and full galvanic isolation — the sensor never touches the measured circuit.
Concrete example — BLDC motor commutation: A typical brushless DC hobby motor or hard-drive spindle uses three latching Hall sensors spaced 120° around the stator. As the rotor spins, its permanent magnets pass over each sensor in turn, producing a 3-bit Gray-code sequence (like 001 → 011 → 010 → 110 → 100 → 101). The controller reads these three bits and knows exactly which of six commutation states to energize next. This is why cheap BLDCs have five wires from the driver (three phases plus a Hall sensor cable of five: +5V, GND, and three signals).
Watch out for: temperature drift (linear sensors drift ~0.05%/°C — bad for precision current sensing without calibration), stray field pickup from nearby wires (a 100A conductor 5 cm away creates ~4 gauss — enough to fool a sensitive linear sensor), and mechanical alignment (moving the magnet 1 mm can halve the field strength since field falls off with the cube of distance for a dipole).