Patent GB-693132-A (GB)
Abstract: 693,132. Bridge circuits. CAMBRIDGE INSTRUMENT CO., Inc. Feb. 26, 1951 [Feb. 25, 1950], No. 4625/51. Class 37. In an electrocardiograph or the like including an electronic amplification circuit operating a recording device, an overload protection circuit is provided for preventing damage to the recording device by excessive signals and includes at least one electron discharge tube and means under the control of the discharge tube for rendering the amplification circuit inoperative whenever the signals exceed a predetermined maximum value. Voltages from contact electrodes 1, 2 on the patient are applied to a two-stage push-pull amplifier 6. The right leg of the patient is connected through an electrode 7 to earth through a fuse 8 shunted by a condenser 9. The amplified signal voltages control the balance of a vacuum-tube bridge circuit 10 which drives a recording mechanism 12 having a stylus 14 recording on a moving chart 16. In the amplifier 6 two valves 20, 22 are arranged in push-pull, the cathode heaters being supplied through a ballast tube 38. The voltages at the anodes 54, 56 of the valves 20, 22 are applied through condensers 66, 68 to the control grids 70, 72 of two push-pull voltageamplifying valves 74, 76. Switches 82, 84 enable the grids 70, 72 to be earthed during adjustment of the equipment. The anodes 102, 104 are connected through sensitivity control potentiometers 118, 120 to the control grids 126, 128 of amplifier valves 130, 132 forming two arms of a bridge, the other two arms comprising valves 146, 148. Potentiometers 110, 138 control the lateral position of the stylus 14 when no signal is applied to the amplifier 6. The cathodes 150, 152 of the valves 130, 132 are earthed through a ballast resistance 154 in series with a fixed resistor 156. The coil 198 of the recording galvanometer 12 is connected between the cathodes 174, 176 of the valves 146, 148. To prevent damage of the coil 198 due to an excessive signal, two gaseousdischarge overload control tubes 206, 208 are arranged with their anode circuits in parallel and their control grids 210, 212 connected through resistors 214, 216 to the control electrodes 126, 128 of the valves 130, 132. The cathodes 220, 222 and the secondary control grids 224, 226 of the tubes 206, 208 are connected to the cathodes 150, 152 of the valves 130, 132. The anodes 228, 232 of the tubes 206. 208 are connected through the actuating coil 234 of a relay 235 to the screen grids 158, 160 of the valves 130, 132, so that the screen current for the valves 130, 132 and the plate current of the tubes 206, 208 are derived from the power supply 64 through a common series resistor 162. The tubes 206, 208 are normally non-conductive due to bias voltage from the power supply terminal 50 through the potentiometers 118, 120. When the voltage applied to the control grid of either of the tubes 206, 208 reaches a predetermined value, the tube ignites and passes current which causes a large voltage drop across the resistor 162 so that the potential on the screen grids 158, 160 is reduced and cut off the anode current, preventing flow of appreciable current through the coil 198. At the same time the relay 235 closes its contacts 238, 242 which connect the anodes of the tubes 206, 208 to earth, thereby de-energizing these tubes. One or more of the electron tubes may be replaced by transistors or other semi-conductive elements.