GB1065052A - Improvements in electrical power supply arrangements - Google Patents

Abstract

1,065,052. Transistor switching circuits. COAL INDUSTRY (PATENTS) Ltd. Jan. 9, 1964 [Oct. 9, 1962], No. 38214/62. Heading H3T. [Also in Division H2] A protective arrangement for a supply system for control circuits, e.g. of a mining machine (see Division H2), comprises a Wheatstone bridge circuit (Fig. 4) with a stabilized voltage supply connected across one pair of diagonally opposite points of the circuit and a transistor balance detector connected across the other pair of diagonally opposite points. The neutrals of two three-phase intrinsically safe supplies are connected to points Y and Z in the 4th arm of the bridge so that a short circuit between any conductor of one supply and any conductor of the other supply will short-circuit a resistor R25 arid unbalance the bridge. When this occurs, a relay BLA is operated to effect the cutting off of all the supplies. If the supply system includes an intrinsically safe D.C. supply, the positive conductor of this supply is connected to point X so that a short-circuit between this supply and one of the three-phase supplies will result in the short-circuiting of the resistor R26, or of resistors R26 and R25. The various supply conductors may have earthed screens and the resistor R27 may be mounted remotely so that any earth fault or open circuit will unbalance the bridge. The intrinsically safe D.C. supply may be provided with the over-voltage protective circuit shown in Fig. 3. In this circuit a stable reference voltage is applied to base of a transistor VT21 by Zener diodes MR20, MR21 supplied via a resistor R6. The emitter of a transistor VT19 is held at this reference voltage level so that transistors VT19, VT20 and VT22 are normally non-conducting. Should the voltage of the protected - 20 v. line rise, VT19 conducts so causing VT20 and VT22 to conduct. A relay RLN is now energized and opens its contacts N1, N2. Transition from the non- conducting to the conducting state is assisted by voltage feedback through a resistor R86.