Patent GB-1105622-A (GB)
Title: Circuit arrangement for feeding electrical apparatus by way of a transistor circuit
IPC: H03K3/26
Filed: 1965-03-03
Abstract: 1,105,622. Protecting transistor switching circuit. HOLLANDSE SIGNAALAPPARATEN N.V. 3 March, 1965 [6 March, 1964], No. 9126/65. Heading H3T. The base voltage of at least one transistor in a switching circuit is monitored so that if this voltage exceeds a predetermined value it triggers a bi-stable circuit to apply a control voltage to cut off or reduce the current supply which is being switched, thus preventing damage to the switching transistors. In Fig. 3 this principle is applied to a constant-current source 105 comprising a transistor 305 whose collector current is stabilized by a Zener diode 317 and applied to a load 308 via switching transistors 346, 347, 352, 353 under the control of pulses applied at 342 and 343. The protection transistor 306 is initially conducting, thus short-circuiting the load 308. To start operations, a voltage is applied at 300, differentiated by capacitor 301, to operate bi-stable circuit 302 and turn off transistor 306 allowing current to pass to the load. The pulses at 342 and 343 are of equal mark/space ratio and of opposite polarity, and pass through emitter followers 344, 345 and phase-reversing amplifiers 354, 355 so that in one half-cycle transistors 347, 352 are on while transistors 346, 353 are off, the opposite occurring in the other half-cycle. The current in the load is thus periodically reversed in direction. If the load 308 should become opencircuited or develop a high resistance, the increase in base voltage of the switching transistors will cause the collector voltage of 344 or 345 to rise. Diode 334 or 335 will then conduct, producing an increase in collector voltage of transistor 330 or 331; this is applied via an OR gate 311 to the bi-stable circuit 302 which turns on the transistor 306 and diverts the current away from the switching transistors. The capacitors across resistors 322, 340, 341, 358 and 359 ensure that this action occurs rapidly enough to prevent damage to the switching transistors, while the capacitors across resistors 338, 339 prevent false tripping by switching transients. The circuit can be switched off by applying a voltage to the OR gate at 312. In Fig. 4 (not shown), the circuits 352-355 are expanded into a matrix of four circuits to supply four separate loads. By providing separate switching voltages on the bases of appropriate transistors in the matrix, the loads can be switched on selectively. The above arrangement also provides protection against (a) an open-circuit or high-resistance in the switching circuits themselves, and (b) the pulses at 342 and 343 not being exactly antiphase but overlapping in ther mark and/or space periods. The arrangement is applicable to magnetic drum stores where the switching circuit selects the winding to be fed and/or the direction of current in the winding. Modifications.-The shunt transistor 306 can be replaced by a series-connected valve. The control can be effected on the constant-current source itself, the output current being cut off or reduced in value by a voltage from the bistable circuit. In another version (Fig. 2, not shown) the switch 306 is controlled by a coincidence circuit fed from (a) the output of the bistable circuit, and (b) the input operating voltage. The coincidence circuit may contain diodes, transistors or valves.