Latching Hall
A latching (bipolar) Hall-effect sensor sets its output high on one magnetic pole and holds it high until the opposite pole passes. This makes it ideal for brushless motor commutation feedback and rotational pulse counting with alternating-pole ring magnets.
Two threshold comparators respond to opposite polarities of the perpendicular flux. When the positive-threshold is crossed, the output latches in one state; when the negative-threshold is crossed, it latches in the other. Internal hysteresis between the two thresholds prevents chatter at the pole boundaries.
In plain terms
A set-reset flip-flop controlled by magnetism: a north pole sets it, the output stays set until a south pole resets it — the output tracks which pole was last seen, with no power needed to hold the state.
Why designers use it
- Provide rotor position feedback for brushless motor commutation.
- Count rotations of a multi-pole ring magnet in flow meters and odometers.
- Build magnetic-pole-edge detectors for quadrature encoding.
Best for
- BLDC commutation
- Flow meters
- Magnetic encoding
Key specifications
- Operate point Bop: 1.5 mT – 30 mT
- Release point Brp: −1.5 mT – −30 mT (Opposite pole)
- Hysteresis: 3 – 60 mT (Bop − Brp)
- Supply: 2.7 V – 24 V
- Output: Open-drain
When not to use it
- When you don't control magnet orientation — use an omnipolar sensor so either pole triggers.
- When absolute angle is needed — use an AS5047 Hall-encoder IC.
Common mistakes
- Using a latching Hall with a single-pole magnet — the sensor sets on approach but never resets because the opposite pole never comes.
- Mounting the sensor at the geometric centre of a ring magnet where flux is zero and switching is unreliable.
Where you will find it
- An electric bicycle's hub motor uses three latching Hall sensors spaced 120° apart around the stator to commutate the BLDC winding: as the rotor's alternating north-south poles pass each sensor, the sensors' output pattern tells the controller which winding phase to energise next.
- A residential water meter uses a latching Hall sensor to detect the alternating poles of an impeller-mounted magnet: each north→south transition counts as one unit of flow, and the latch means even a very slow flow accumulates count pulses without missed transitions due to bounce.
- A 3D printer's filament run-out sensor uses a latching Hall and a notched magnet wheel on the feed roller: the alternating poles produce a pulse train proportional to filament movement; if the pulses stop while the extruder motor is running, the firmware pauses and alerts the user.
A short history
Latching Hall-effect sensors (also called Hall latches) set their output to one logic level when a south magnetic field exceeds the operate threshold and hold that state until a north field exceeds the release threshold—providing inherent memory of the last magnet position even when the magnet is removed. This behaviour, first implemented in integrated Hall ICs by Sprague Electric in the late 1960s, is ideal for brushless DC motor commutation where the sensor must 'remember' rotor position between pole transitions. Allegro MicroSystems and Infineon are major current producers of latching Hall ICs. They are found in BLDC motor drives, door-open detection, and speed sensing in automotive and industrial equipment.
Good to know
- Latching Hall sensors require a north pole to set, and a south pole to reset — the basis of every BLDC motor's commutation feedback and every contact-less rotary encoder.
- Three latching Hall sensors spaced 120° around a magnetic rotor produce a 6-state quadrature waveform that an FOC controller can interpret as rotor position.
- The original 1968 Sprague UGN-3020 latching Hall is still in production — it's the ancestor of every brushless-DC motor's rotor-position sensor.