Patent GB-711976-A (GB)
Title: Improvements in or relating to electrical remote indicating or repeating systems
IPC: G08C15/00
Filed: 1951-10-18
Abstract: 711,976. Electric indicating systems. CRACK, K. G. Oct. 16; 1952 [Oct. 18, 1951], No. 24370/51. Class 40 (1). In an electrical remote indicating system of the kind in which a movable member responsive to the input quantity carries a first contact engageable with a second contact of a follow up member operated by a movement energized in series with a remote repeater movement by the current in a thermionic valve controlled by a circuit including the input and follow up contacts such that when the contacts are closed they tend to open and vice versa and are maintained in a floating intermittently making and breaking relationship, the contacts are con- .nected so that their make and break operation varies a current, independent of the grid current of the value, which flows in the circuit determining the valve grid control potential. Transmitting. follow up contacts represented by switch S in series with the primary of transformer TR1 and a secondary winding of transformer TR2 are operated respectively by a movable input member (not shown) and a follow up movement T in series with a receiver movement R and resistance R10 in the cathode return to earth of triode V10 which is supplied with rectified and smoothed anode current from transformer TR2. The control grid of V10 is connected through resistors R12, R13, R15 and high resistance R14 to the junction of resistors R16, R17, which together with resistor R18 form a potentiometer across the valve, and a condenser C12 is connected between the earth line and the junction of resistors R13, R14. The smoothed output of a bridge rectifier M3 across the secondary of transformer TR1 is connected across resistors R15, R19 in series. Circuit constants are such that when the follow up contacts are open the junctions of R16, R17 and of R14, R15 are positive with respect to the cathode of V10, and condenser C12 slowly charges positively over resistor R14 so that the grid potential and thus the anode current of V10 steadily rise at a rate controlled by the retroactive cathode resistor R10, until the follow up movement T closes the contacts. Transformer TR1 is then energised and its rectified output has polarity such as to drive the junction of R15 and R19 negative whereby a negative potential is applied over R14 to discharge C12, so that the valve anode current decreases at a steady rate to a value at which the contacts are again opened. The contacts are thus maintained in a continuously floating make and break relationship and the rectified output potential of transformer TR1 alternately appears across R15 and R19 as contacts are made and broken. During a make period the condenser C14 charges rapidly through rectifier M4 to bias rectifier M5 non-conductively and during a break period it discharges slowly through resistor R20, so that after a break has persisted for an appreciable time (as when the input member moves to a new position in a direction to open the contacts) rectifier M5 conducts and condenser C12 rapidly charges from the junction of over resistors R13, rectifier M5, resistors R20, R19 and R15, so that the valve anode current rapidly increases until the follow up movement responds to re-make the contacts S. The charging current into C12 develops a positive bias across R13 which further increases the valve current, and this bias is suddenly removed when the contacts re-make, to reduce the valve current and prevent overshoot. When the input member moves to close the contacts S, condenser C15 (which is charged through high resistance R21 from the slightly positive potential at the junction of R20, R21 during normal floating of the contacts) is charged negatively over resistor R21 from the rectified output of transformer TR1, and on break discharges through rectifier M6. If make persists for an appreciable time, the condenser continues,to charge until the negative potential is such that rectifier M7 conducts and the condenser discharges through M7 and R13. The additional bias developed across R13 rapidly reduces the valve anode current whereby the deflection of the follow up member is reduced so that the contacts are again broken. On break, the bias developed across R13 is suddenly removed to increase the value current and prevent overshoot. An additional steady rectified and smoothed current is supplied through R23 to the cathode circuit of valve V10 in opposition to the valve anode current so that a finite current flows in the valve for a zero indication at which the follow up and receiver members 'are undeflected. Shunt condenser C17 across the cathode return stabilises the make and break operation of the contacts and a standby valve V11 is provided which takes over automatically on failure of V10, the voltage drop across its bias resistor R18 being compensated by a corresponding change of potential across C12 so that V11 passes the necessary current to maintain the contacts in a floating relationship. In a modification, closure of the follow up contacts is arranged to earth a point of a D.C. fed potentiometer whereby to vary the grid bias (derived from another point of the potentiometer) of a triode whose cathode circuit contains the repeater and follow up movements. A retroactive condenser interconnecting the anode and grid of the triode restrains and slows down the' anode current variations when the contacts are opened or closed, and a standby valve is provided to take over on failure of the primary valve.