Patent GB-1462190-A (GB)
Title: Reversing switch
IPC: H03K17/66
Filed: 1973-12-13
Abstract: 1462190 Control of D.C. motors J L FEAVER 12 Nov 1974 [13 Dec 1973] 57797/73 Addition to 1424776 Heading H2J [Also in Division H3] The reversing switch of Specification 1,424,776 is provided with the following additional features: (1) Clamping transistors sa, sb whose bases are supplied with a common logic command or respective separate commands from external means (not shown), whereby transistor sa and/or sb is/are switched on to connect to earth any input command signal(s) applied to the transistors pa, pb, thereby to prevent normal operation of the bridge circuit abcd or change it from the condition determined by the input command present. As shown, the transistors sa, sb are controlled by circuit members er, et,fr, ft, gr, hr connected to a common point t, whereby resistor er or fr produces a voltage to turn on its associated transistor et, ft when excess current flows through the motor m in one or other direction, thus switching on transistors sa, sb to de-energize the motor until the current has fallen below a preset value. Hence the current is limited by intermittent switching of transistors sa, sb producing intermittent application of the command signal to transistor pa, pb and intermittent energization of the motor. Members er, et, fr, ft may be replaced by one resistor shunted by two reversed base-emitter junctions with an inverting junction in one of the collector paths. (2) The circuit shown in Fig. 2 which automatically produces reverse-current braking of the motor if its speed is high enough when it is de-energized, the latter speed condition being sensed by members 1, 2, 3 to turn on transistors 1, 6 if the speed exceeds a predetermined value. With an input command signal to transistor pb to energize the motor in one direction, application of the same command signal to a terminal 9 causes transistors 4, 5, 7 to turn off. Removal of the signal from pb and 9 deenergizes the motor and turns on transistors 4, 5, 7 for a time determined by the time constant of members 13, 14. Thus if the speed is high enough, transistors 6, 7 conduct simultaneously to turn on a transistor 8 and produce at terminal 23 an output signal, which is applied to transistor pa to energize the motor in the opposite direction. When the speed has fallen below the predetermined value, transistors 1, 6 and 8 turn off to remove the signal from terminal 23, the absence of command signals from transistors pa, pb causing the bridge abed to change to the condition where the motor is connected to member w to produce further braking as in Specification 1,424,776. If desired the member w may be replaced by two anti-parallel connected combinations each comprising a low voltage cell in series with a diode, Fig. 3a (not shown), whereby one or other cell is charged according to the direction of rotation of the motor and the energy thus stored is subsequently returned to a main battery (for example in a D.C. traction system) through a D.C.-D.C. converter. The circuit of Fig. 2 is also used to produce braking of the motor when it is used in a positional servo system, a transducer of which produces an error signal which is applied to terminal 9. Transistor 4 is off when the error signal is increasing and also if it is decreasing slowly at a rate below that determined by the time constant of members 13, 14. If the rate of decrease is above the latter however, transistors 4, 5, 7 turn on and, if transistors 1, 6 are on, cause production of a signal at terminal 23 for application to transistor pa. If there is no input command at transistor pb the motor is braked by reverse current. If such a command is present, the bridge is put into the condition where transistors bd are turned on to produce a braking circuit for the motor through appropriate ones of the transistors b, d and the diodes in parallel with them.