The relationship between the sensed magnetic flux density and the magnet-to-sensor distance along with the B_OP and B_RP determine the magnet-to-sensor distance at which the switch changes state. In many cases, the magnet travels in a direct linear path of travel, so distance is often expressed by the direct magnet-to-sensor distance. In Figure 1-1, however, distance is specified by the angle of opening because the refrigerator door opens on a hinge that causes the magnet to move nonlinearly. For Figure 1-1, the magnet-to-sensor distance increases as the angle of opening increases.

Figure 1-3 shows an example graph of how the sensed magnetic flux density varies as the angle of the opened refrigerator door 1 varies. The magnet-to-sensor distances at which the switch changes state depends on the B_OP and B_RP. If you want to determine the typical distance at which the sensor switches state, find the locations on the Figure 1-3 plot where the sensed magnetic flux density values are equal to the B_OP,TYP and B_RP,TYP specifications listed in the device data sheet. Alternatively, when determining the worst-case device to device variation for output switching, refer to the B_OP,Max and B_RP,Min specifications. B_OP,Max and B_RP,Min account for process variation, temperature and voltage. Designing the system within these bounds will result with consistent operation despite these variables.

As an example, let’s say there are two Hall-effect switches with the following specs:

• Switch 1

  • B_RP,MIN =5.12 mT

  • B_RP,TYP =6.87 mT

  • B_OP,TYP =7.87 mT

  • B_OP,MAX =9.62 mT

• Switch 2

  • B_RP,MIN =0.73 mT

  • B_RP,TYP =1.58 mT

  • B_OP,TYP =2.58 mT

  • B_OP,MAX =3.43 mT

If switch 1 is used in refrigerator 1, the output of switch 1 will be low when the angle of opening for door 1 is ≤1.8°, regardless of variations in process variation, temperature, and voltage. The angle of opening for door 1 is ≤1.8° when the sensed magnetic flux density is greater than the 9.62 mT B_OP,MAX. However, the typical distance where the output of switch 1 is asserted low would be at 2°, which is when the sensed magnetic flux density equals the device’s B_OP,TYP value of 7.87 mT. The output of switch 1 will be high when angle of opening for door 1 is ≥2.5°, which is when the sensed magnetic flux density is less than the 5.12 mT B_RP,MIN. The typical distance where the output of switch 1 is asserted high, however, would be at 2.2°.

If switch 2 is used in refrigerator 2, the typical distance at which the output of switch 2 is asserted low would be at 3.5°. The output would be low at an angle ≤3.1° regardless of variations in process variation, temperature, and voltage. Additionally, the output of switch 2 would be asserted high at 4.4° typically. The output would be asserted high at an angle ≥6.0° regardless of variations. The different switching distances of switches 1 and 2 shows how switching distance is dependent on the B_OP and B_RP specs of a Hall sensor.