Patent GB-1493036-A (GB)
Title: Static relay
IPC: G04C13/04
Filed: 1975-03-11
Abstract: 1493036 Semi-conductor switching circuits COMPTEURS SCHLUMBERGER 11 March 1975 [11 March 1974] 10009/75 Heading H3T A switching circuit for supplying current pulses, Fig. 2c of alternate polarity to an inductive load 12, Fig. 1, in which pairs of switches 20, 34 and 30, 26 operating alternately in response to interleaved control pulses Figs. 2a, 2b, on lines 116, 118, repeatedly connect the load across a supply 22, is characterized in that the switches are protected against excessive (a) load current and (b) reverse voltage, and in that discharge paths for the energy stored in the load inductance at the end of each current pulse are provided. Excess current protection.-All load current passes through an adjustable resistor 36 so set that normally the voltage across it will not make a transistor 92 conduct. Excessive load current however makes transistor 92 conduct thereby reducing the potential of the bases of phase splitters 42, 44, and hence reducing the conductivity of the particular pair of switches 20, 34 or 30, 26 that is on. Furthermore, once transistor 92 begins to conduct, a capacitor 110 begins to charge, so that if the excessive load current permits long enough a transistor 102 becomes conductive, and then by positive feedback action transistor 92 is driven to saturation thereby reducing the load current to zero. Excess voltage protection.-Excess reverse voltage across either switch pair is conducted away by diodes 18, 32, 28, 24 through either the supply 22 and resistor 36, or through a Zener 112 (with a Zener voltage just above the supply voltage) and the resistor 36 if the battery is defective or disconnected. Discharge of stored energy.-Discharge paths for energy stored in the load inductance at the end of a control pulse are established either by maintaining one of the switches, of the pair that have just been conducting, conducting for a further period and by shunting the other switch, or (Fig. 4, not shown) by shunting one of the switches, or (Fig. 5, not shown) by using an SCR for one switch of each pair, or (Fig. 3, not shown) by shunting the load. This is achieved for the transistor switches of Fig. 1, by capacitors 62, 64. When, for example, phase splitter 42 is conducting, to turn on switches 20, 34, capacitor 62 charges, so that when the phase splitter 42 turns off, switch 34 turns off but switch 20 is maintained on and gradually turns off as capacitor 62 discharges through resistors 50, 52. During this time, energy stored in the inductance causes current to flow via switch 20 and diode 28, the energy being dissipated in a resistor in series with the inductor. In the first modification, (Fig. 4, not shown) rather than maintaining switch 20 on at the end of a control pulse, a triac (139) shunting the switch 20 is turned on; in the second modification (Fig. 5, not shown) switch 20 is constituted by an SCR (150) which automatically remains conducting at the end of a control pulse until the current falls below its hold-on value, and in the third modification (Fig. 3, not shown) the load inductance is shunted by a triac (124) which is triggered when an excess voltage appears across the load.