Curing Interference in Relay Systems

A review is made of methods for EMI suppression for relay coil transients and switch contact arcing. The mechanism of generation of coil transients and contact arcing is described briefly. A relay solenoid is controlled in actuation by a toggle switch S. In closing, the switch exhibits bounce characteristics which cause momentary deenergizing of the relay solenoid, with the subsequent momentary oscillatory transients and switch-bounce current breaking, reflected as switch contact arcing during bounce. When S is opened, the stored energy (L12/2) is returned to the circuit and is dissipated as power losses in the relay core and switch arcing. In addition, the collapsing magnetic field generates a transient, or counter EMF, voltage expressed as E = ¿L(di/dt) volts. As S opens, an arc appears due to breaking current which quenches as the switch opens further. The arc is restruck as the coil voltage transient increases due to the sudden current changes, and the process continues until the coil current reaches zero. It may be noted, when viewing the transient on an oscilloscope, that a series of sawtooth changes reflected from the switch arcing is observed, rather than the expected exponential rise. The general description of this generated EMI is that it is broadband in frequency and impulsive in nature. It can be coupled into the ac power circuits via the power supply, or radiated from the line, the solenoid, and switch.