Sensors Products Page 3 Sensors Products

2017 Littelfuse, Inc. Specifications are subject to change without notice Littelfuse.com 3 REED SWITCHES A reed switch has two ferromagnetic blades (reeds) contained within a tubular glass envelope that is hermetically sealed at each end. The contacts on each reed blade have a thin layer of precious metal material deposited on them. There is usually nitrogen gas on the inside of the glass envelope to eliminate the presence of oxygen and ensure that the contacts will not oxidize. Reed switches are activated by a permanent magnet or an electromagnet. The reed switch and magnetic field combination is commonly known as the "magnetic circuit." The relative stiffness of the reed blades along with the small gap and overlap between the two contacts controls the sensitivity of the reed switch. The sensitivity of the switch is the amount of magnetic field that is required to actuate the contact into an open or closed mode. It is measured in units of ampere-turns (AT). Most reed switches have a sensitivity range of 10-30 AT, where 10 AT is more sensitive than 30 AT. REED SENSORS Reed sensors are reed switches that are packaged within an external housing for simplified mounting/ connecting and additional protection against environmental influences. These sensors are typically mounted in mechanical systems. A bare reed switch can easily be mounted on circuit boards. However, for an application such as a door security sensor, the reed switch needs a protective shell/housing for handling and mounting. These packages offer resistance to mechanical stress by protecting the bare glass of the reed switch. REED RELAYS A reed relay is made by combining a reed switch with a copper coil. Like other relays, this provides galvanic isolation between the coil input and the controlled contact(s). However, because of the small size and magnetic efficiency of the reed switch, the power required to drive the coil is lower than most other types of relays. Other advantages include high insulation resistance, low contact resistance and long contact life. Reed relays are used in many applications, including automotive, test equipment, security, medical and process control equipment. HALL EFFECT SENSORS A Hall effect device is a semiconductor-based integrated circuit with Hall plates that respond to magnetic fields. Additional circuitry is added for power supply and signal conditioning, temperature compensation and EMC/ESD protection. Hall effect devices provide digital or analog output signals that are used for proximity and continuous rotary or linear positioning. Unlike a reed switch, a Hall effect device contains active circuitry, so it draws a small amount of current at all times. Hall effect devices come in two- or three-wire versions. Some devices are programmable. MAGNETIC ACTUATORS Littelfuse offers a wide range of magnetic actuators that are packaged in shapes similar to the relative mating sensors. We also offer a limited family of bare magnets with various grades of materials, including ferrite (ceramic), AlNiCo and neodymium iron boron (NdFeB) materials. Comparing Reed vs. Hall Effect Although there are differences between reed switch and Hall effect technologies, both platforms offer practical advantages for various applications. Here is a comparison of the benefits of each technology. Figure 1-Magnetic field lines must be aligned parallel to a reed switch's contacts in order to open/close the contacts S N S N S N OPEN Hold Close MAGNET -X mm X mm X -X O Y mm S N Y Figure 2-Two-pole linear activation, parallel orientation OPEN Hold Close MAGNET -X mm X mm X -X O Y mm S N Y Close OPEN Hold Figure 3-Single-pole linear activation, perpendicular orientation INTRODUCTION TO MAGNETIC SENSING Reed Switch Hall Effect Sensor Noncontact sensing element Zero power consumption High-Frequency operation Digital switching Linear sensing Integrated circuitry Ultra-small package size Electrical load capability EMC/ESD immunity

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