Wiegand effect

Wiegand effect is a nonlinear magnetic effect, produced in specially annealled and hardened wire called Wiegand wire. Initially, Wiegand wire is low-carbon Vicalloy (an alloy of cobalt, iron, and vanadium) wire, fully annealled. In this state the alloy is "soft" in the magnetic sense- that is, it is attracted to magnets (it is ferromagnetic and so magnetic field lines will divert preferentially into the metal) but the metal retains only a very small residual field when the external field is removed. Then, the wire is subjected to a series of twisting and untwisting operations, to cold-work the outside shell of the wire, while retaining a soft core within the wire, and then the wire is aged.

The result is that the magnetic coercivity of the outside shell is much larger than the inner core. This high coercivity outer shell will retain an external magnetic field even when the field's original source is removed. The wire now exhibits a very large magnetic hysteresis loop - if a magnet is brought near the wire, the high coercivity outer shell excludes the magnetic field from the inner soft core until the magnetic threshold is reached, at which point the entire wire (both the outer shell and inner core) rapidly switch magnetisation polarity. This switchover occurs in a few microseconds, and is termed the Wiegand effect.The Wiegand Effect is exhibited when the internal magnetic fields in specially prepared wires swiftly reverse polarity when exposed to external magnetic fields. This reversing magnetic field can be captured as a negative-going electrical pulse.

The Wiegand data protocol incorporates two separate logic lines, a "one" line and a "zero" line. As the Wiegand effect can only produce negative-going electrical pulses, both lines will individually go low to indicate the presence of a bit. Wiegand wire (wire specially developed to exhibit the Wiegand Effect) is very hard to duplicate. This is why the technology is heavily used in security access cards and "pin" cards. This simple decoder circuit can be used to convert dual channel, negative-going electrical pulses from a Wiegand magnetic reader head to standard hexadecimal format. The circuit consists of a 74HC00 quad NAND gate and two cascaded 74LS164 8-bit shift registers.

Where necessary, the number of shift registers can be increased to suit the bit depth of the Wiegand data stream. For example, we used 26-bit Wiegand for our experiments (two parity bits + three bytes of data) and this required four 74HC164s.The value of the Wiegand effect is that the switchover speed is sufficiently fast that a significant voltage can be output from a solenoid using a Wiegand-wire core. Because the voltage induced by a changing magnetic field is proportional to the rate of change of the field, a Wiegand-wire core can increase the output voltage of a magnetic field sensor by several orders of magnitude as compared to a similar coil with a non-Wiegand core. This higher voltage can easily be detected electronically, and when combined with the high repeatability threshold of the magnetic field switching, making the Wiegand effect useful for positional sensors.

Once the Wiegand wire has flipped magnetization, it will retain that magnetization until flipped in the other direction. Sensors and mechanisms that use the Wiegand effect must take this retention into account. Besides direct sensors, the Wiegand effect can be used for security key-card door locks; the card reader need only provide a fixed magnetic field, while the user pulls the card through the field. As the magnetic field flips at each wire to indicate a 1 (or fails to flip, indicating a zero) in a non-moving coil, a second set of wires provide a clock track. The resulting digital code is then sent to a host controller to determine whether to electrically unlock the door.