Bipolar Hall
Senses perpendicular magnetic flux density via the Hall effect; outputs digital ON when South-pole flux exceeds BOP, switches OFF only when North-pole flux falls below BRP — the wide hysteresis envelope eliminating output chatter from vibration.
A current-biased silicon Hall element produces a transverse voltage proportional to flux × bias current. An on-chip chopper-stabilised amp removes offset drift, then a window comparator with separate operate and release thresholds outputs an open-drain or push-pull digital signal. Bipolar's distinguishing feature: BOP > 0 (S pole) and BRP < 0 (N pole), so the switch latches state until reverse polarity arrives.
In plain terms
Like a turnstile that opens for green-shirts going east but only relocks when a red-shirt walks west — it remembers which polarity passed last and ignores fluctuations between.
Why designers use it
- BLDC commutation — three bipolar Hall switches around a rotor's magnet ring tell the controller which phase to energise, in millions of cordless drills, drone motors, and HVAC fans.
- Bidirectional position — door-open / door-closed sensing where a single magnet passes north-then-south as the door swings.
- No moving contact — replaces a reed switch's millions-of-cycle wear life with effectively unlimited solid-state lifetime.
- Wide voltage / temp — automotive-rated parts run from 3 to 24 V across −40 to +150 °C with no derating.
Best for
- BLDC motors
- Door sense
- Wheel speed
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- Where a single trip threshold is enough (one-way detection) — a unipolar Hall is cheaper.
- For analog magnetic measurement — pull a linear Hall sensor; bipolar latches output, hiding the underlying field strength.
Common mistakes
- Forgetting the magnet's polarity orientation when designing the rotor — install a south-up magnet next to a chip expecting north and the BLDC commutation runs backwards.
- Skipping the bypass cap on VCC — the chip's chopper internally injects switching noise back into the supply that re-couples to the Hall element.
Where you will find it
- A DJI Mavic 3 brushless motor stator embeds three Honeywell SS361CT bipolar Hall switches at 120° spacing: each switch's output flips as the rotor's NdFeB magnets sweep past, telling the BLDC controller's commutation state machine which winding to energise next at 8000 RPM.
- A Tesla Model 3 door handle's auto-presenting motor uses an Allegro A1153 bipolar Hall switch to sense end-of-travel: a magnet on the handle's gear box sweeps past the chip when the handle reaches its presented position, telling the motor controller to stop driving.
- A Honeywell HMC1043 wheel-speed Hall sensor on a Cummins ISX engine's flywheel watches a 60-tooth pulser ring: each tooth gives a North-then-South transition, the bipolar comparator's hysteresis suppresses bounce, and the ECU reads engine RPM from the pulse rate.
A short history
Hall-effect sensors exploit the Hall effect discovered by Edwin Hall at Johns Hopkins University in 1879: a magnetic field perpendicular to a current-carrying conductor deflects charge carriers, producing a transverse voltage. The first integrated Hall-effect sensor ICs were developed by Sprague Electric Company and introduced around 1969. Bipolar Hall switches have a defined operate point (South pole) and a separate, lower release point (North pole), providing hysteresis to prevent output chatter. They replaced reed switches in brushless DC motor commutation circuits, fluid-level detectors, and seat-position sensors in automobiles, offering millisecond response with no moving parts.