Patent GB-899090-A (GB)
Title: A transistor arrangement for the rapid switching on and off of an inductive load
IPC: H03K17/64
Filed: 1959-04-15
Abstract: 899,090. Controlling clutches. LICENTIA PATENT-VERWALTUNGS-G.m.b.H. April 15, 1959 [April 15, 1958 ; Aug. 13, 1958 ; Sept. 18, 1958 ; March 16, 1959], No. 12784/59. Class 80 (2). [Also in Groups XXXV and XL (c)]  In a switching circuit comprising an inductive winding of an electromagnetic clutch and a transistor connected in series therewith for switching on and off the current therethrough, the rate of current build-up is increased by connecting in series with the winding a further transistor which is arranged to conduct during the build-up period and thus apply an increased voltage to the inductive winding. In Fig. 4, the winding 1 is connected to the power supply through a transistor switch 4. When the transistor is first switched on a second transistor 6 is also conducting so that the voltage applied to the winding has a value U greater than the normal steady state value UN. When the current has increased to the desired value the voltage developed across transistor 6 is such as to cause a bistable switching circuit 6, 7 to operate and so render transistor 6 non-conducting. The current accordingly now flows to terminal 11 through a previously blocked rectifier 10 from the normal operating voltage source UN. If the current is now interrupted by rendering transistor 4 non-conducting, the energy in the winding discharges through circuit comprising a rectifier 8 and a zener diode 9a or a set of diodes 9b may be connected in the discharge path so as to cause the discharge current to decay in linear rather than an exponential manner so that a more rapid discharge is obtained without exceeding the maximum permissible collector voltage of transistor 4. A further reduction in collector voltage during discharge may be obtained by connecting a zener diode or a set of diodes in series between the collector electrode and the winding (see Fig. 1, not shown). A transistor 28 connected across the winding becomes conductive when the voltage across the resistor 29 produced by the discharge current is a sufficiently low value so that current flows from a source 27 into the winding to remove remanent magnetism. In an alternative circuit (Fig. 2, not shown), a single voltage source U is used, the increase in voltage applied to the winding during current build-up being produced by providing a resistor in series with the winding which is short circuited by a transistor during the build-up period, the transistor being controlled at the base by a switching circuit operated by the voltage developed across a resistance in series with the winding. In Fig. 3, two transistors 18, 19 are added in series with the winding and are arranged to be conducting when the current is flowing from the source to the winding. When the main transistor 4 is cut off, the discharge current flowing through rectifier 8 renders the transistors 18, 19 non-conducting so that the current flows through resistors 23, 24, 25 which are chosen to provide a discharge time constant such that the maximum collector voltage of transistor 4 is not exceeded. These resistors may be of non-linear type so as to make the discharge occur at a steady rate. A rectifier 22 becomes non-conductive during the switching off period to prevent the negative voltage UN from reaching the base of transistor 19 and thus applying excessive voltage to the transistor.