A modified type of the resonant turn-off d-c chopprr using an auxiliary SCR, compared with its conventional one, has no limitation about its starting and also operates more steadily. Presented in this paper, making a comparison between the modified type and the conventional one, are the circuit analysis and its efficiency measurement from the point of view of overall efficiency, that is, the ratio of the output to the input. Furthermore, some effects of the source impedance, counter emf load, etc. are discussed. As a result, it has been analytically clarified that this modified type is different from the conventional circuit only by one terminal connection of turn-off capacitor, but has the features of uncompounded circuit configuration, no limitation about its starting and more steady operation.