A Magnetic-Field Regulation and Stabilization System

An inexpensive system for controlling and stabilizing the magnetic-flux density of an electromagnet on the basis of a monolithic Hall sensor is described. The main functional unit of the system is a precision controllable stable-current source with digital control and a regulating unit based on a power MOSFET transistor, which operates in the saturation region. A signal from a magnetic-field sensor is used to control the magnetic-field flux density, while a negative electromagnet-current feedback stabilizes the flux density. In this method, the 1/f noise of a Hall-effect sensor does not affect stability of the electromagnet flux density, while the drift and noise characteristics of the system are determined by the parameters of the used reference-voltage source. The position of the working point of the transistor in the saturation region is stabilized in the entire range of operating currents using a drain-source voltage control circuit. The developed system is used to control the magnetic-field flux density of the mass analyzer of the MI1201IG mass spectrometer and provides the following characteristics of the magnetic-field flux density control: the operating frequency band is 0-10 Hz, the flux-density control range is 0-0.4 T with a step of 1.5 mu T, and the overall drift and noise instability do not exceed +/- 2 x 10(-6) T within 20 min in the operating temperature range of 20-50 degrees C.