What Should you Know About Hall Effect Sensors?

What Should you Know About Hall Effect Sensors

Hall effect sensors provide a linear or digital output. The output signal of the linear (analog) sensor is directly taken from the output of the operational amplifier, and the output voltage is proportional to the magnetic field passing through the Hall sensor. Linear or analog sensors provide a continuous voltage output, which increases with a strong magnetic field and decreases with a weak magnetic field. In linear output Hall-effect sensors, as the magnetic field strength increases, the output signal of the amplifier also increases until it starts to saturate the limits imposed by the power supply. Any additional magnetic field increase will not affect the output, but will make it more saturated. When the magnetic flux passing through the Hall sensor exceeds a preset value, the output of the device quickly switches to the "on" state between its "off" states without any type of contact bounce.

When the sensor moves in and out of the magnetic field, this built-in hysteresis eliminates any oscillations in the output signal. Then the digital output sensor has only two states, "ON" and "OFF". There are two basic types of digital Hall effect sensors, bipolar and unipolar. Bipolar sensors need a positive magnetic field (south pole) to operate them and a negative magnetic field (north pole) to release them, while unipolar sensors only need a magnetic south pole to operate and release them in and out of the magnetic field. Most Hall-effect devices cannot directly switch large electrical loads because their output drive capability is very small, about 10 to 20 mA. For high current loads, add an open collector (current sink) NPN transistor to the output. The transistor works as an NPN absorption switch in its saturation region. As long as the applied magnetic flux density is higher than the “ON” preset point, the output terminal will be shorted to ground. The output switch transistor can be an open emitter transistor, an open collector transistor configuration or both provide a push-pull output configuration, which can draw enough current to directly drive many loads, including relays, motors, LEDs and lights.