GB1317482A - Protective devices - Google Patents

Abstract

1317482 Protective arrangements ROBERT BOSCH GmbH 18 June 1970 [18 June 1969 31 July 1969] 29523/70 Heading H2K In a protective arrangement for a D.C. load, e.g. an electrolytic machine 11-13, of the kind having a thyristor 15 to short circuit the load when the load resistance drops below a predetermined value, at least one silicon diode 17 is connected in series with the load to ensure that in the event of the short circuiting of the load there is sufficient voltage across the anode/ cathode path of the thyristor 15 for it to become conductive. In the arrangement shown a plurality of series connected diodes 17 is normally short-circuited by a plurality of parallel transistors T 1 -Tn. Should the gap between the work-piece 11 and electrode 12 become too small, then the potential between a point 22, 35 on the positive supply line and the junction 34 of voltage dividing resistors 30, 31 is sufficient to cause transistors Tb, Ta to conduct to apply a triggering potential to the gate S of the thyristor 15, which conducts to short-circuit the load conductors. At the same time the potential between points 22, 24 is not sufficient to maintain the transistors T 1 -Tn conductive, so that the diodes 17 cease to be short-circuited. Also, this potential is not sufficient to maintain a transistor Tc conductive so that an electromagnet 42 ceases to be energized and contacts 38 open to disconnect the whole arrangement. Over-voltage protection is provided by a resistor 37 and a Zener diode 36 which conducts to trigger the thyristor 15 when the supply voltage is too high. In alternative arrangements, Figs. 2 and 3 (not shown), the silicon diodes also serve to rectify an A.C. supply.