GB614472A - Improvements in or relating to time delay circuits - Google Patents

Abstract

614,472. Valve relay circuits. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., and PARKER, J. E. T. Feb. 19, 1946, No. 5100. [Class 40 (vi)] In a time delay circuit, a switching operation connects a charged condenser in the grid cathode circuit of a cold cathode discharge tube, and provides means for discharging and recharging the condenser in the opposite sense through a parallel connected resistance, such that the striking voltage of the tube is reached after a time delay. In the Figure, a potentiometer YJ, YK, YL, YM and YN is connected across the H.T. supply. Condenser C7 is charged through contacts DS1 and DS2, the upper plate being negative. Contacts DS1 and DS2 are controlled by relay DS (not shown) which is operated by the external switching operation. Contact DS1 connects the upper plate of the condenser to the control grid of cold cathode discharge tube NA, and contact DS2 disconnects the lower plate from the positive supply, which is then applied to the anode of tube NA. Condenser C7 now commences to discharge and then charge in the opposite direction, through the high resistance YP, YQ, YR; YS which is connected to a point on the potentiometer. After a time delay, the voltage on the upper plate of condenser C7 is sufficient to cause tube NA to conduct, and relay AL, which may control an alarm circuit, then operates. The effective value of resistance YP-YS may be adjusted by means of tapping 18, to provide a choice of delay periods, and fine adjustment, to cater, for example, for different cold cathode tubes, may be effected by means of the tapping 24 on the potentiometer. Element ARA which is a non-linear resistance material such as that known as " Atmite" '' (Registered Trade Mark), is provided to maintain the delay interval substantially constant in spite of changes in supply voltage. The resistance of ARA falls as the current through it increases, so that the proportional change in voltage across resistance YJ for any change in supply voltage is increased. The proportional change in the initial charge given to condenser C7 is therefore also increased. This results in a stabilization of the delay period. Specification 609,120, [Group XXXIX], is referred to.