SE300835B - - Google Patents

Abstract

1,011,749. Switching inductive loads. TELETYPE CORPORATION. June 5, 1962 [July 5, 1961], No. 21578/62. Heading H3T. A circuit for energizing an inductor in response to a signal provides a steady potential across the inductor while the current therein is increasing and then holds the current at a steady value when a predetermined magnitude is reached. The invention is described in connection with the selector magnet of a teleprinter which is to be controlled by signals having mark and space periods. In the space period transistor 11 is cut off by the absence of signal on its base, and transistor 15 is thus conducting. Transistor 19 is cut off by the voltage drop across voltage-sensitive resistors 30, 31. Transistor 48 is conducting, while transistor 60 is cut off by the voltage drop across 64. The input circuit of transistor 11 has ohmic resistors 25, 26 and voltage-sensitive resistors 27, 28 which are switchable to cater for different magnitudes and datum levels of the mark signal applied at 23. When a mark signal appears it turns on transistor 11 and cuts off transistor 15 through lead 16, thus reducing the voltage drop across 30. Transistor 19 now conducts and current is able to flow through the inductor 45 via transistors 48 and 19. As soon as this current starts, the base and emitter of transistor 60 go more positive to extents that enable this transistor to conduct; transistor 49 is then cut off since its base potential is maintained first by capacitor 73 and then by clamping diode 74 in conjunction with resistors 71 and 72. Thus the rising current through the inductor is carried by transistor 60 instead of 48. When, however, the emitter of 50 becomes sufficiently positive transistor 48 will conduct again and cut off 60, the inductor current now being through transistor 48 and high resistance 54. The circuit is stabilized during the period of rising inductor current by positive feedback through diode 57 which holds transistor 15 cut off. At the end of the "mark" period, the input signal cuts off transistor 11 and thus turns on transistor 15. The collector voltage of 15 is transferred via diode 65 to the base of transistor 19 and cuts it off (together with diode 55), thus opening the path of current through the inductor. The back E.M.F. across the inductor causes diodes 55 and 66 to conduct and drives the collector of 19 and the base of 15 more negative via diode 57, ensuring that the latter is held conducting. During this changeover period transistor 48 remains conducting and transistor 60 remains cut off. NPN transistors can be used instead of PNP. The voltage-sensitive resistors 27, 28, 30, 31 act as current-limiters for surges and provide a fixed voltage drop; they comprise PN and NP junctions in parallel. Specification 556,700 is referred to.