Application of MR Element to Clamp-on Type Current Meter

A new clamp-type ampmeter employing a magnetoresistive (MR) element has been constructed with a sub-milliampere order sensitivity and a wide frequency bandwidth. The sensor element is a 1000 to 1500 Å thick, 0.2 mm wide and 15 mm long evaporated thin film of 80/20 NiFe exhibiting a 2.5% magnetoresistive change. The MR element is mounted in the 0.2 mm gap of a ferrite core, which is wound with a 188-turn coil to provide a dc bias field (4 Oe) for linear sensitivity and to enable feedback detection. In the non-feedback mode, the system noise level (sensor and circuit) is 0.8 mV, equivalent to a 0.2 mA current. In feedback mode, this ammeter maintains linear sensitivity for currents of up to 0.4 A in the frequency range from dc to 10 kHz. The temperature stability was improved by compensating using element pairs.