Patent GB-716709-A (GB)
Title: Improvements in and relating to electrical timing devices
IPC: H03K17/288
Filed: 1951-01-30
Abstract: 716,709. Valve relay circuits. SIEMENS & GENERAL ELECTRIC RAILWAY SIGNAL CO., Ltd., and RIDDLE, H. J. N. Jan. 30, 1952 [Jan. 30, 1951], No. 2337/51. Class 40 (6). [Also in Group XXXVIII] Apparatus for performing electrical timing comprises in combination a first condenser to be charged to a potential the value of which is dependent upon the duration of a charging period, switching means whereby said first condenser may be disconnected from the supply and connected to a discharge circuit, a second condenser with its own discharge circuit, and means for so associating said condensers that if the potentials thereon are dissimilar any excess of potential on said first condenser is shared with said second condenser, but not vice versa, a timed period being determined by the time elapsing between initial connection of the first condenser to its discharge path and the moment when after partial discharge the second condenser arrives at a predetermined potential. In the embodiment described for control of road-traffic signals (see Group XXXVIII) the timed period is that between operation and release of a relay 1. The relay is operated by momentary closure of contacts 2 and is held operated by a weak current through a resistor 18. Release of the relay 1 is effected by current in a reverse winding after closure of contacts 9 and the leaking away through resistor 10 of any negative charge on the grid 5 of a valve 3. The negative charge is occasioned by change-over contact 11 which in its operated position allows first condenser 12 to charge via resistor 14 and a second condenser 8 to take up a charge by means of grid current in valve 3. When contact 11 reverts the point 17 is connected to cathode potential and condenser 12 rapidly discharges. Condenser 8 cannot' discharge except slowly through timing resistor 10 and hence the grid 5 is driven negative. Subsequent operations of contact 11 can only increase the negative charge on grid 5 (and thereby prolong the timing period) if they produce a positive potential on condenser 12 greater than the negative remaining on condenser 8, whereby further grid current is caused to flow. Thus the conditions shown graphically in Fig. 3 are obtained, where the continuous lines indicate potentials on condenser 12 and the dotted line the voltage of grid 5. Operation of contact 11 for a moderate period charges condenser 12 as indicated at a and, via condenser 8, takes the grid 5 only slightly positive owing to grid current flow. When condenser 12 is suddenly discharged as at b the grid 5 goes negative as at B and slowly rises as condenser 8 discharges through resistor 10. Operation of contact 11 for a short period to charge and discharge condenser 12 as at c, d causes variation C, D in grid voltage, which then continues on its original path E since the variatiom was not sufficient to produce grid. current. Short operation of contact 11 when the grid voltage has nearly reached zero or long operation as at e,. will cause grid current to flow and prolong the timing period, as .indicated at F If only a single make contact is available instead of change-over contact 11 a metal rectifier may be connected across resistor 14 (as in Fig. 4, not shown) to provide the rapid discharge path when charging ceases. A modification (Fig. 5, not shown) provides a resistor between condenser 12 and supply negative such that the potential at point 17 for extremely short reversals of contact 11 rises due to the drop across this resistor and compensates for the otherwise negligible effect of such fast operations. Condenser 12 is preferably of greater capacity than condenser 8. Specifications 591,295, 621,636 and 647,268, [Group XXXVIII], are referred to.