Evaluation of wrist strap monitors from an MR head perspective

Magnetoresistive (MR) heads are very sensitive devices that can be damaged by applying a current that exceeds the normal bias current of the head, which is typically around 10 mA for anisotropic magnetoresistive (AMR) devices. Contact with the head from a charged object can cause damaging current transients. Depending on the magnitude of the charge, these currents can reach peak values of hundreds of milliamps during this very fast discharge. All monitors tested resulted in a voltage on the operator under worst case conditions. This worst case voltage was AC for capacitive monitors and DC or pulsed DC for resistive and charge detecting monitors. Under best case conditions, there was at least one monitor in each category that exhibited near zero voltage on the operator. If the operator is holding a pair of metal tweezers and makes contact with an MR head, the voltage on the operator will cause current transients that are potentially harmful to the head. By considering the advances in MR technology in recent years, the data in this paper aims to assist the end user in selecting the most suitable wrist strap monitoring technology.