GB634774A - Improvements in and relating to electronic time delay circuits - Google Patents

Abstract

634,774. Valve relay circuits. BRITISH THOMSON-HOUSTON CO., Ltd. July 30, 1945, No. 19402. Convention date, Aug. 5, 1944. [Class 40 (vi)] An electronic timing circuit comprises a voltage divider connected across an A.C. supply circuit, a tetrode valve, a capacitor which may be charged through the grid cathode circuit so that it develops a voltage which controls the anode current, a switch in the grid cathode .circuit and a voltage regulating device across part of the divider circuit. In Fig. 1, the resistors 14, 15, 16 and 17 form a potential divider circuit across the A.C. supply applied to terminals 4. The potential across resistors 15, 16 and 17 is stabilised by discharge device 24. When switch 3 is in the open position, the voltage applied across the anode-cathode circuit of valve 6 is equal to that across resistor 14. The voltage waveform applied to the control grid from the tapping on resistance 16 results in condenser 19 being charged through the grid-cathode circuit, thereby applying a negative bias potential to the grid equal to the volts drop across resistor 15 and the upper part of resistor 16. The small anode-cathode voltage, and the fact that the screen is at cathode potential, results in anode current being insufficient to operate the relay 1. When the switch 3 is closed, the full A.C. supply is applied across the anode-cathode circuit, and a flat topped waveform due to the voltage. drop across resistances 17 and the lower part of 16, which is in phase with the anode voltage. is applied to the grid in series with the negative potential due to the charge on condenser 19. Condenser 19 then commences to discharge through resistance 20, and after a predetermined time interval the grid potential has risen sufficiently to allow anode current to flow during each half-cycle, thereby operating relay 1. The condenser resistor elements 12, 13 keep the relay operated during the quiescent half-cycles. Owing to the stabilising action of the discharge device 24, the voltage to which condenser 19 charges, and therefore the delay time interval is independent of voltage supply variations. The condenser resistor elements 22, 21 cause the screen voltage to be slightly lagging in phase and assists consistent operation. A modification of the circuit is shown in Fig. 3, in which the switch is placed in the voltage dividing circuit, instead of in the cathode lead, and a gaseous discharge valve 29 is used to operate the relay. The full A.C. supply is thus always across the anode-cathode circuit of valve 6. With switch 3<1> open, a voltage waveform equal to the voltage across device 24 and in phase with the anode voltage is applied to the grid, superimposed on the potential across condenser 19 which charges up to this potential. Under these conditions, anode current in valve 6 is sufficient to produce a negative bias on the control grid of valve 29 through the integrating network 31 and 32, so that valve 29 does not conduct and relay 1 is not operated. When switch 31 is closed, the bias waveform superimposed on the voltage across condenser 19 is reduced to that across resistors 17 and the lower part of 16, which results in anode current in valve 6 being cut off for a predetermined interval, when condenser 19 has sufficiently discharged through resistance 20 to allow valve 6 to again conduct. While valve 6 is cut off. the increase in the potential of the control grid of valve 29 allows this valve to conduct and operate relay 1 for the predetermined interval. A further modification (Fig. 2, not shown) describes a circuit similar to that in Fig. 3 except that the relay is operated by the anode current of valve 6, as in Fig. 1, and the gaseous discharge valve 29 and associated elements are excluded. In this case, closure of the switch 3<1> results in the relay 1, previously operative, being non-operative during the predetermined time interval. Specification 496,641 is referred to.