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The widespread use of semiconductor rectifiers, with their extremely high conversion efficiency, ability to operate at high reverse voltages and consequent small size, has brought in its train problems peculiar to these devices. It is most important, therefore, that the electrical ratings should be established on both a systematic and rigorous basis with the specific problems in mind. Certain electrical parameters such as forward and reverse voltage and current, thermal resistance and junction temperature, have obvious effects on the electrical rating of the rectifier. Others, such as thermal capacitance and carrier storage time, affect the rating indirectly, but are no less important. In the paper, some of the factors which influence the rating of germanium and silicon junction rectifiers are considered, and the procedure for obtaining the curves of rectified current rating as function of ambient temperature is given. Determination of overload characteristics and the associated use of thermal-electrical analogue techniques are described. Methods for deriving the derating factor for parallel operation, operation at higher mains frequency and at high altitude are also given.