Physics
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Electromagnetic hysteresis
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Electromagnetic hysteresis
Ferromagnetic materials are characterized by a magnetic induction not expressible as a function of magnetic field. The relation in an isotropic material being scalar since the fields assume the same direction (but not necessarily the same direction), it is representable on a plane where a hysteresis cycle takes place.
Residual induction can be a problem in several applications, for example in the magnetic field it keeps the anchor of a relay attracted when the control signal ceases. Hysteresis is also one of the causes of energy dissipation in variable regime: magnetic in transformers, and electrical in resistors, which is added to the ohmic one. On the other hand in the magnetic field it is the basis of magnetic storage in tapes and hard disks. In these last devices the direction of the residual magnetization represents a bit: 0 or 1. To change the magnetization state it is necessary to know the previous state, according to which varies the field to be applied.
To avoid this problem, a technique called bias is used, which consists in bringing the system to a known value before writing. The same technique is used in audio tape recorders, where sometimes there is a selector for the type of bias to be used depending on the ferromagnetic material used in different types of tape. In electronic circuits the Schmitt Trigger is used, which generates a hysteresis similar in appearance to that generated in magnetic materials. The circuit is used to eliminate the noise present on the signals. It can be realized by discrete components (operational amplifiers or transistors) but it is present on the inputs of many logic circuits of various families (TTL, CMOS, etc.).
It is also possible to simulate hysteresis on an analog input (ADC) connected to a microprocessor (often integrated into it in microcontrollers or embedded systems). In this case the hysteresis is an algorithm in the software or firmware.
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