Patent GB-707441-A (GB)
Title: Improvements in or relating to an electrical bridge circuit, for a temperature control circuit
IPC: G05D23/24
Filed: 1949-02-17
Abstract: 707,441. Automatic temperature control systems. DUNHAM CO., Ltd., C. A. Feb. 17, 1949, No. 4363/49. Class 38 (4). [Also in Group XXXVIII] General. The invention is described with reference to a system for heating rooms B, C, &c., Fig. 1, from a common control room A, and maintaining each room at a desired individual temperature. Steam from a source 10, 11 is supplied over pipes 12, 12b, entering room B through a valve 13b controlled by a motor 14b; risers 20b conduct the steam to heating radiators 21b and to a " balancer " 25b for measuring the heat output of the system. The " balancer comprises a smal( convection type heater with temperature sensitive resistances located in the air input and output streams. A temperature sensitive resistance is incorporated in a pad 27b which is mounted on the window so as to respond to outside as well as inside temperatures. The temperature sensitive resistances are connected to a terminal box 17b situated in or near the room B ; the terminal box 17b is connected to a control panel 19b in the control room A. Switches on the control panel enable the various resistances to be connected into bridge networks, so that out-of-balance currents produced by variation of temperature in room B cause motor 14b to vary the supply of heat and at the same time to vary a potentiometer to rebalance the bridge. Circuits can also be selected to measure the rate at which heat is transferred to the room and the extent to which valve 13b is open. A terminal box 17 is shown in Fig. 2B with its connections to the resistances 78, 79 in a " balancer" 25 to the resistance 83 and its balancing resistance 84 in the window pad 27, and to the motor 14 ; a control panel 19 is shown in Fig. 2A. The control panel 19, Fig. 2A, includes ganged four-position switches I, II, III ... VIII; in the first position, the system is disconnected ; in the second position the heating of the associated room is automatically controlled; in the third position the rate of heat supply to the room is measured; while, in the fourth position, the extent to which the valve 13 is open is measured. The control panel includes dials 112, 117, 135, 113, and 102, Fig. 7, and 104, Fig. 6, which adjusts the corresponding potentiometers shown in Figs. 2, 3, 4 and 5, so that the measurements are made in, and the controls effected from, the control room. Automatic control.-With the ganged switches in their second position, the circuit is as shown in Fig. 3, and comprises a complex bridge ; arms E, E' are formed by the temperature sensitive resistance 83 on the window and its balancing constant resistance 84 connected by a potentiometer 102; arms F, F<SP>1</SP>, are formed by the resistances 78, 79 of the balancer 25 ; arms G, G', H, H' and I, I' are formed by constant resistances connected by potentiometers 104, 107, 70. Normally the bridge is balanced, but a variation of resistance 83, 79 or 78 will unbalance the bridge to actuate galvanometer 45 which operates motor 14, Fig. 2B, as described below, to adjust the steam valve 13 and vary the tapping on potentiometer 70 to re-balance the bridge. The negative pole of the voltage applied to the bridge is taken to potentiometer 112 which thus controls the ratio of current flow between arms E, E' and F, F' and this potentiometer is calibrated to enable a desired change in the temperature difference between resistances 78, 79 to correspond to a change in temperature of resistance 83 of one degree. Potentiometer 102 is set to determine the point at which the change in resistance 83 causes the steam valve to be closed. Rheostat 113 determines the amount of current flowing in arms G, G', and together with potentiometer 104 allows the steam valve to be adjusted to satisfy unusual requirements. The positive pole of the voltage supplied to the bridge is taken to rheostat 117 supplying potentiometers 107, 70; the setting of rheostat 117 determines the extent to which the steam valve is adjusted in consequence of changes in resistances 83, 78, 79. Measuring rate of heat supplied to room.- With the ganged switches in the third position, the circuit is as shown in Fig. 4. Potentiometer 104 is manually adjusted to balance the bridge, and is calibrated to show the rate of heat supplied to the room, this being dependent on the difference in temperature of the resistances 78, 79. Indicating the extent of opening of steam valve.-With the ganged switches in the fourth position, the circuit is as shown in Fig. 5. The condition of the steam valve is dependent on the setting of potentiometer 70, and thus is shown by the ratio of arms I, I<SP>1</SP>, by manually adjusting potentiometer 104 to balance the bridge ; the setting of potentiometer 104 indicates the condition of the valve. Control of motor 14.-The galvanometer needle 47, Fig. 2A, co-operates with switches 48, 49 which are periodically moved towards the galvanometer so as to be actuated in accordance with the position of the needle 47. These switches 48, 49 selectively actuate relays 38, 39 which operate relays 60, 61 in the terminal box 17, Fig. 2B, to energize the motor 14 in the appropriate direction. The motor is thus operated for short periods of time when the bridge is unbalanced. Time switch.-The system may have as an additional feature a time switch which reduces the heat supply during the night and accelerates the heat supply during the morning " heat-up " period. A clock is arranged to close switch 172, Fig. 9, at night thus connecting a negative source to potentiometer 175 ; this potentiometer is included in a pair of bridge arms connected across the diagonal XY of Fig. 3, so that the bridge is unbalanced and the steam valve closed to an extent depending on the setting of potentiometer 175. In the morning the clock opens switch 172 and closes switch 171 to unbalance the bridge in the reverse direction and thus opens the steam valve to an extent depending on the setting of potentiometer 174. When the room temperature reaches the desired value, thermostat 177 operates to open switch 178 and disconnect the arms from the source of supply, leaving the bridge as shown in Fig. 3. Specification 524,926 is referred to.