The paper is an applications report of an electronic model of an s.c.r.for use in predicting the conducting state of a thyristor. The model is entirely logical in operation, relies purely on voltage signals from the thyristor and does not depend on any current measurements in the main power system. This property allows the model to be used in thyristor arrangements without modification to the existing power circuits.Extensive use of the model in cycloconvertor systems to extract firing information and to prevent short circuits has proved its excellent reliability, and its use in other thyristor applications is easily envisaged.