GB1279183A - Temperature sensitive devices - Google Patents

Abstract

1279183 Automatic temperature control HONEYWELL Ltd 11 June 1970 [18 June 1969] 28245/70 Heading G3R A temperature sensing unit 10 Fig. 1 comprising a thermistor 20 includes an anticipatory heater in heat exchange relationship with the thermistor and having a linear energizing current/power dissipation characteristic. In a space heating arrangement shown the thermistor 20 responds to Room temp. and the heater is a transistor 26. Alternatively it may be a Zener diode Fig.2 (not shown). The thermistor is connected in a bridge unit which produces an output at terminal 12 when the sensed temp. deviates from a desired value. The unbalance output is detected by a level detecting and sequencing unit 35 which sequentially turns ON or turns OFF thyristors 40 to control four electric heater units (not shown) via heater relays 17, depending upon whether the room temp. is to be raised or lowered. In addition, the unit 35 operates sequentially the switches 50, simultaneously with corresponding thyristors. Closure of a switch 50 causes conduction of a transistor 60 which in turn passes collector/emitter current through Tr26. Each switch closed increases this current and hence the heat output of the anticipatory heater. This heat rise is sensed by thermistor 20 and thus, if the space temp. is increasing, the temp. signal at 12 drops more rapidly than if it were representative of the rising space temp alone. This causes the detector 35 to turn OFF an SCR (in a known manner not described) and open a corresponding switch 50 at an earlier moment than otherwise. The heat output of the transistor 26 is a linear function of the amount of heat supplied by the heating system by virtue of its bias arrangement which maintains a substantially constant collector/emitter voltage regardless of its collector emitter current, so that its power dissipation varies linearly with said current. The level detector 35 may alternatively turn OFF all the thyristors 40 and open all of the switches 50 simultaneously.