All main powered devices require a supply which is maintained within certain limits. Thus a low supply voltage can cause malfunction, whilst a high supply voltage will cause serious damage. Micro-processor based and other sensitive equipment requires not only a stable voltage but a clean supply free of transient spikes and other electrical noise.

At the heart of an EM stabilizer, a transformer has its secondary winding connected between the mains supply and the load, with the primary winding voltage controlled by a motor-driven variable transformer. A solid state servo amplifiercontinuously monitors the output voltage of the stabilizer, and any error voltage is amplified to operate the servo motor of the variable transformer, driving its brushgear to a point where it is injecting a voltage that will correct the supply voltage by adding or subtracting from it.