Digital Hall-effect switches nearly always have an "open-collector" output. This is an integrated npn transistor switch. Digital switches simply turn on or off with the presence or absence of a magnet, vs. a linear output Hall-effect sensor IC, whose output changes in proportion to the magnetic field present.

Q3: What is the "face" of a Hall-effect sensor IC?

The "face" is the branded or printed side of the sensor IC

Q4: What is meant by the term "digital latch"?

Like digital switches, digital Hall-effect latches nearly always have an "open-collector" output. Digital latches turn on with the presence of a magnet, (usually the South Pole) and stay "latched" on until the opposite magnetic field (usually the North Pole) is presented to the face of the sensor IC.

Q5: How is the sensor IC biased?

The normal biasing scheme places a positive dc bias on the sensor IC's supply (Vcc) pin and the negative bias is connected to the sensor IC's ground pin. The output pin is connected to the positive bias through a pull-up resistor.

Q6: What value pull-up resistor should be used?

At a minimum, the value must be large enough to limit the current flowing in the output to less than the data sheet maximum output current specification. 10,000 ohms is a very common value.

Q7: What happens when the output switches?

Most Hall-effect switches provide an output which is normally off (no magnetic field). In the off state, the output will be high, at full supply voltage. In the on state, the output will be low, typically 100 mV.

Q8: As all magnets have two poles (minimum), which pole activates the sensor IC?

Nearly all Hall-effect switches are configured to switch on with a positive magnetic field. The positive field is associated with a magnetic south pole.

Q9: How is the magnet oriented in respect to the sensor IC?

With few exceptions, the south pole must face the sensor IC such that the magnetic lines of flux pass directly through the Hall IC. Regardless of package type, (leaded or surface mount) the south pole must face the branded face of the sensor IC. (the surface of the sensor IC on which the manufacturer has stamped its logo, part number, and in some cases a date code)

Q10: How should the magnet approach the sensor IC?

The two approaches are "Head-On" or "Slide-By". In the case of "Head-On", the magnet is centered on the sensor face and the air gap is reduced until the sensor switches. In the case of "Slide-By", the magnet moves past the sensor face, at an air gap such that the sensor will turn on as the magnet approaches alignment with the sensor IC.

Q11: Which parameters affect the switch point of the sensor IC?

Parameters affecting turn-on are: sensor sensitivity/operate point (B_OP specification), air gap, and field strength. Parameters affecting turn-off are the sensor's release point (B_RP specification), air gap, and field strength.

Q12: Will a strong magnetic field damage a Hall-effect sensor IC?

No. Most sensor ICs are designed to operate within a narrow, well-defined magnetic range, seldom exceeding 1000 gauss. Having said this, flux densities exceeding 10,000 gauss will not damage the sensor ICs, nor (in the case of Allegro sensor ICs) add to the sensor's designed-in hysteresis.

Q13: What kind of magnet should be used?

There are many materials used for permanent magnets. Alnico, ceramic, neodymium, and samarium cobalt are all popular, and in many applications, flexible materials are available such as nitrile rubber with a barium ferrite filler.

**General Attributes of Various Permanent Magnets**
Material	Energy Product (BH)*	Temp. Stability (Tc in %/ºC)	Cost
Alnico	5 to 10	Excellent thru +150ºC(-0.02)	Moderate
Ceramic	2 to 4	Moderate thru +150ºC(-0.2)	Low
Flexible	0.6 to 1.5	Typ. Limitation of 100ºC(-0.2)	Lowest
Neo	30 to 40	Typ. Limitation of 125ºC(-0.12)	High
Sam	20 to 30	Excellent thru +150ºC(-0.04)	Highest

* BH is the product of flux density and field strength. In general, this value represents the energy density of the magnet and is used to grade permanent magnets. The higher the value, the stronger the magnet.

BH is measured in mega gauss oersteds (MGOe)

Q14: How does the table in question 13 relate to the air gap between the sensor and the magnet?

In general terms, the weakest magnets (flexible) would typically operate in a 0.25 mm to 2 mm range, while the strongest (neodymium or samarium cobolt) could allow an air gap of 4 mm to 6 mm. Note that these are very general values with the magnet material, size, and sensor sensitivity being determining factors.

Q15: Can a Hall-effect sensor IC survive autoclaving?

As long as the autoclave temperature does not exceed 170ºC and the sensor IC is completely dried, there should be no problem caused by autoclaving. It should be noted that it is unlikely that a Hall-effect sensor will operate properly while in an autoclave.