Improvements in or relating to synchro control systems and synchro devices for use therein

1,106,899. Transistor switching circuits. SMITHS INDUSTRIES Ltd. 3 Dec., 1965 [11 Dec., 1964; 20 Oct., 1965], Nos. 50456/64 and 44430/65. Heading H3T. [Also in Divisions G1 and H2] In apparatus for detecting when an A.C. voltage is outside a predetermined range, the test voltage on terminals 21a, 21b (Fig. 2), is opposed by the voltage across a resistor 30, derived from the same A.C. source, and the resultant applied to a transistor 32, whose output is rectified at 35 and fed to a transistor 36. If the test voltage is outside the acceptance range, the rectified difference signal causes transistor 36 to change from conducting to non-conducting, after a delay determined by capacitor 37. This changes the state of subsequent transistors 39, 40, the lack of collector current in 40 causing a warning device 22 to operate an indicator. The difference voltage fed to the first transistor 32 has its out-of-phase component removed by a rejection circuit 33 connected in negative feedback between the collector and base of transistor 32. The apparatus also incorporates means for comparing D.C. potentials at terminals 21e, 21f with that at 21c, and operating the warning device 22 if either or both potentials change from their normal value. A D.C. voltage obtained from bridge rectifier 42 is fed through two similar parallel circuits of a selsyn control system (Fig. 1, not shown) via terminals 21c, 21d and is also used to energize the transistors of the circuit. The potentials at 21e, 21f are fed to the bases of transistors 44, 45 connected as a long-tailed pair. The collectors of 44, 45 are connected to transistor 39 via rectifiers 47, 48 and to transistor 38 via resistors 49, 50. Transistors 44, 45 are normally both conducting, but a change in the potential at one or both terminals 21e, 21f causes one or both transistors to change its state, followed by changes to transistors 36, 39, 40, whereby warning device 22 is operated.