GB1187402A - Overload Protection Circuits - Google Patents

Abstract

1,187,402. Protecting transistor circuits. INTERNATIONAL COMPUTERS Ltd. 24 May, 1968 [14 April, 1967], No. 17352/67. Heading H3T. [Also in Division H2] A circuit has a load 26 in series with a switching device, e.g. a transistor 2, a bi-stable circuit 12 means to apply an input signal to the switching device to render it conducting and to the circuit 12, to initiate a change to a state in which it renders the switching device 2 non- conducting and means to prevent the circuit 12 changing its state providing the device 2 conducts normally within a predetermined time. In the arrangement shown, a signal at a terminal 4 causes a transistor 1 to conduct to apply an input signal to the transistor 2 to render it conductive and to the circuit 12 where current flows through a resistor 18 and a Zener diode 16 to charge a capacitor 17. Providing no overload condition exists, the collector potential of the transistor 2 becomes such as to unblock a diode 22 to prevent the input signal charging the capacitor. However, should there be an overload condition, then the collector potential of the transistor 2 remains or becomes sufficiently positive to block the diode 22, the capacitor 17 charges so that after the predetermined time the potential across the capacitor causes the transistors 13, 14 to conduct and connect the base of the transistor 2 to a negative potential sufficient to stop it conducting. The transistors 13, 14 remain conducting until the fault condition is removed and a test/reset switch 24 is operated. A fault-indicating lamp 25 is provided.