This paper is aimed at presenting a design of a varistor-based protection circuit for load-connected electronic equipment. The varistor I-V characteristics are described-for the first time-in five regions. Not only the protection-circuit parameters but also the varistor parameters are determined according the load being protected. Because of the nonlinearity of the protection circuit, the describing equations for load and varistor currents and voltages are solved numerically. Different waveforms of the input voltage with and without surge superimposed on the normal operating voltage are investigated. In all cases, the protection circuit has limited the load voltage to a safe value irrespective of how high the input voltage with the superimposed surge.