GB253948A - Improvements in and relating to temperature regulators - Google Patents

Abstract

253,948. British Thomson-Houston Co., Ltd., (Assignees of Hull, A. W.). June 22, 1925, [Convention date]. Vapour and vacuum or low-pressure apparatus, arrangements of.-The temperature of an electrically heated device, e.g. a resistance furnace, is kept constant by means of a resistance responsive to temperature which varies the polarity of an electromotive force applied to the input circuit of an electron discharge device, the varying current in the output circuit being employed to control the temperature. A furnace 10 is heated by a resistance 11 supplied from mains 12, and controlled by a switch 13 which may short-circuit a section 11<a> of the resistance. The electromagnet 14 which operates the switch 13 is controlled by a device operated by variations in the resistance 20, having a positive temperature coefficient, due to temperature variations in the furnace. A Wheatstone bridge 15 is connected to an alternating current supply 21, one of its resistances being the resistance 20. An electron discharge device 16 has its filament 24 connected to an alternating current supply through a transformer 24<a>, while the grid 25 is connected to a step-up transformer 29 so as to be responsive to the voltage across the terminals 27, 28 of the bridge. The filament is connected to a point 32 in the resistance 18, so that the grid has a negative bias. The plate 26 is connected through the magnet coil 14 to the point 23 and thence to the supply source 21. When the furnace is slightly above the predetermined temperature, the potential of 27 will be negative relatively to 28 when the point 23 and plate 26 are positive. The transformer 29 then applies a negative potential to the grid, which reduces the plate current to such a value that the magnet 14 releases the switch 13 and allows it to open. The consequent insertion of the resistance 11<a> reduces the heating in the furnace. If the furnace falls below the required temperature, the potential of 27 becomes positive relatively to 28, producing a positive potential on the grid, and an increase in plate current which closes the switch 13 and short-circuits the resistance 11<a>. The temperature of the furnace may be controlled within Œ0.1‹ C. In a modification, two stages of amplification are used, each stage consisting of a push-pull circuit, and in another modification a mercury vapour rectifier is used for controlling the supply of current to the furnace, in combination with a Wheatstone bridge, with or without a thermionic valve amplifier.