Contact Resistance Anomalies in Reed Contacts - Influence of Temperature and External Magnetic Field

Since their invention in the late 1930's by Dr. Ellwood from Bell Laboratories sealed reed contacts, used in reed relays and switches, have been used in many industries with various environmental requirements. In the space industry the reed relays are primarily used as position indicators actuated via external magnetic fields. The reed relay, as a telemetry indicator in space mechanisms applications, has to survive high levels of environmental loads: mechanical (shocks and vibrations), thermal (high/low temperatures) and electrical (switching loads) while maintaining high stability of its contact resistance and pull-in/drop-out characteristics. The paper compares the experimentally obtained performance of the reed switch contacts with anomalous contact resistance readings versus performance of the clean reed contacts. Comparison is done for various strength of the external magnetic field generated via a pair of Helmholtz coils and for various temperature extremes. Acquired data allows estimating the actual contact forces. The pull-in and drop-out characteristics of the reeds at thermal hot are also determined. Testing of the reed switch component confirmed that the high telemetry resistance values were not related to the coaxial T-switch PCA circuitry and that the root cause was high contact resistance of the reed switch itself. Surface analyses of the anomalous reed contacts via EDX (Energy Dispersive X-Ray Analysis) and SEM (Scanning Electron Microscope) complemented better understanding of the contact condition. Those analyses confirm the presence of contaminating films and particles generated during manufacturing of the reed relay. The paper also describes the test setups used.