Patent GB-624686-A (GB)
Title: Improvements in and relating to electric control circuits
IPC: B23K11/24
Filed: 1944-11-01
Abstract: 624,686. Control systems for resistance welding and other toads. BRITISH THOMSONHOUSTON CO., Ltd. Nov. 1, 1944, No. 21365. Convention date, Nov. 1, 1943. [Class 38 (iv)] A load such as a resistance welding transformer 10 is supplied from an A.C. source 11. through a pair of reversely connected grid-controlled valves 12, 13 of the gas-filled type, wherein a hold-off bias applied by a transformer 22 in the grid circuit is reduced by a bias applied through a transformer 24 with a switch-controlled timer 25 in the primary circuit, and is overcome by a peaked voltage applied from a transformer 20 energized from a phase-shifting circuit 29 controlled by a regulating circuit 30 which includes means, e.g. the filamentary cathode of a diode 64, responsive to the load current, means also being provided for energizing the regulator circuit during the off intervals in response to variations in the voltage of the supply circuit as compared with an electrical reference quantity. Preferably the reference quantity is provided by a condenser 100 .which is charged during the " on " period and is connected in a circuit for maintaining the filament of the diode energized during " off " periods. A D.C. voltage is applied across the conductors 50, 51 of the regulating circuit 30 by a full-wave rectifier 52 and discharge devices 57-60, preferably of the cold cathode type connected in series and providing different levels of voltage. The filament of diode 64 is energized by a transformer 65 the primary, winding of which is energized by the split secondary 80, 81 of a transformer 82 with its primary winding connected to a current transformer 84 in the primary circuit of welding transformer 10. The diode 64 has its anode connected to a resistance 68, and with the valves 59, 60 forms a bridge circuit from which the point 69 and conductor 63 apply a controlling bias to an amplifying valve 70. The D.C. voltage across conductors 42, 43 is thus varied in accordance with the energization of the diode filament. Conductor 42 is connected to the centretapped secondary of a regulating transformer 45, and conductor 43 provides a controlling bias to the cathodes of a pair of discharge tubes 31, 32, whereby peaking pulses are applied over a circuit, including the primary winding of peaking transformer 20. A minimum voltage control across conductors 42, 43 is provided by connecting across discharge tube 57 a voltage divider 76 with a tapping connected to the cathode of a valve 77 which has its anode connected to the anode of amplifying valve 70, so that as the conductivity of the latter valve decreases the anode of 77 becomes more positive and at a point determined by the tapping on resistance 76 the valve 77 connects the conductor 43 directly to the resistance tapping. The condenser 100 is connected in the anode-cathode circuit of a pentode 101 with control grid connected to the anode of amplifying valve 70 through resistor 104, so that during " on " time, the condenser is charged to a voltage dependent on the output of the regulating circuit 30. A controlled discharge path for the condenser is provided by a double triode 109 which has one anode 110 connected to the positive side of the condenser, this anode having a control grid 115 connected to the anode 120 of a "switching" valve 117, also a double triode, with both anodes connected to a feed conductor 62 through resistances. The grid potentials of valve 117 are controlled in accordance with the welding periods by connecting them to the output circuit of a full wave rectifier 132 energized from the secondary of a transformer 91 coupled to the welding transformer 10. As soon as the welding transformer is de-energized the left-hand half of valve 117 becomes non- conductive and the right-hand half conducts, thus causing the right-hand half of valve 109 to be non-conductive and interrupt the discharge path of condenser 100. However, the right-hand half of valve 109 is conductive and renders the control grid of pentode 101 negative, thereby interrupting the charging of the condenser, the voltage of which remains at the value existing at the last period of energization. The " off " time filament supply comprises a circuit 140 with the anodes of two valves 166, 167 connected through the split secondary windings 168, 169 of transformer 170 to the split primary of transformer 66, which energizes the filament of diode 64. An " off " time phase-shifting circuit 141 controlled by the comparative voltage of condenser 100 provides voltages of peaked wave form which are applied across a transformer to the control grids of valves 166, 167 in a switching circuit, including a discharge tube 139, the firing of which is controlled by the potential applied to its cathode from the anode 118 of firing valve 117. The control system eliminates voltage ripple across the points 63, 69 of the bridge circuit associated with diode 64 by producing an opposing ripple across a resistor 73 in the anode circuit of a full-wave rectifier 94 energized by a secondary winding 95 on transformer 66 which energizes the diode filament.