TEMPERATURE CONTROLLING OR DETECTING DEVICE

Abstract: A temperature controlling or detecting device has a circuit which comprises a temperature-sensitive element (1) whose electrical resistance changes with temperature. Changes in the resistance of the said element (1) are detected by an arrangement including a Wheatstone bridge and an operational amplifier (15). The amplifier (15) produces an output signal at a predetermined temperature or at the upper and lower limits of a predetermined temperature range in response to changes in resistance of element (1). At least a sufficient part of the circuit is encapsulated in a resin to prevent ready adjustment of the said predetermined temperature or temperature range.

Description: TITLE:- Temperature controlling or detecting device TECHNICAL FIELD: This invention relates to a temperature controlling or detecting device, and in particular to a temperature controlling device (hereinafter referred to as a thermostat) . BACKGROUND ART: Conventional thermostats generally rely on __ physical movement of adjacent components to cause a switching action at a desired temperature. Such components may include bimetallic strips, or bellows actuated by liquid expansion, which inturn act on a mechanical switch. Such conventional thermostats have a number of disadvantages. For one thing, they are inclined to be inaccurate. Furthermore, 'it is easy for an unauthorized person to alter the temperature at which the switching action mentioned above takes place. This is of particular significance in offices, factories, warehouses and other industrial and commercial premises, where it may be not only advisable to avoid excessive expenditure on heating, but where legally specified maximum temperatures may be exceeded, with a consequent risk of prosecution. DISCLOSURE OF THE INVENTION: According to the present invention there is provided a temperature controlling or detecting device having a circuit which cor.prises a temperature-sensitive element whose electrical resistance changes with temperature and means for detecting changes in the resistance of the said element and producing an output signal at a predetermined temperature or at the upper and lower limits of a predetermined temperature range in res¬ ponse thereto, and means for encapsulating at least a sufficient part of the circuit to prevent ready adjustment of the said predetermined temperature or temperature range. BRIEF DESCRIPTION OF THE DRAWINGS: Figure 1 shows a circuit for use in the thermostat according to the invention; and Figure 2 is a general view of the thermostat. DESCRIPTION OF PREFERRED EMBODIMENTS: The illustrated circuit comprises a sensor 1 which is a temperature-sensitive silicon chip, for example that available from Texas Instruments under the code TSF 102G. This has a resistance of approximately 900 ohms at 20°C and a sensitivity of +7.1 ohms per °C. The sensor is positioned in one side of a Wheatstone bridge. The arm which contains the sensor 1 also contains a resistor 2 which is normally short-circuited by a link 3. The purpose of this will become apparent from the ensuing description. Two of the remaining sides of the bridge have resistors 4 and 5 respectively, the resistances of those resistors being equal to one another. The fourth side of the bridge cont- ains a series combination of a resistor 7, an adjustable resistor 8, a resistor 9, and a further resistor 10 which is normally short-circuited by a link 11. Power is supplied to terminals 12 and 13 of the bridge from a five volt DC stabilized supply 14. An operational amplifier 15 is connected between the terminals 16 and 17 of the bridge and acts as a voltage comparator. For convenience the voltage at terminal 16 is referred to below as V and the voltage at terminal 17 is referred to as V, . At some particular ambient temperature the bridge is in balance, i.e. V = V.. When the temperature is lower than that at which the bridge is balanced V will be lower than V, , and hence the output voltage of the amplifier at terminal 18 (V ) will be high. If the ambient temperature were then to rise, Va would rise towards V, ø and eventually a transition point would be reached at which V was once more equal to V, and at which the output voltage V was substantially zero. The change in amplifier output from high to low can be made very rapid by using an amplifier with high gain, and this can further be enhanced by positive feedback via a resistor 20. From what has been said so far it will be understood that the temperature at which the above mentioned transition occurs in Vc can be adjusted by adjusting the variable resistance 8. The voltage transition at Vc is used to control a relay 21 or an equivalent electronic component, for example a triac or thyristor. Control is effected through a buffer switch 22, which is in the form of a power FET of VM0S type or a transistor, or a second comparator. If the ambient temperature is lower than that at which the bridge is balanced the relay will be energised, but as the temp¬ erature passes through that at which balance is achieved the relay will de-energise. Thus, if the bridge is balanced at a temperature N°C the relay will be energised (i.e. its contacts will be closed) at temperatures below N°C and de-energised (i.e. its contacts will be open) at temperatures above N°C. Thus, the relay 21 can be used to control a heating, cooling, ventilating or air- conditioning system so as to maintain the temperature of the space in which the thermostat is located substantially constant. The relay, or its equivalent, may be remote from the thermostat. OMP The description thus far assumes that the thermostat will operate at a single fixed temperature. However, it is undesirable, for example where the thermostat is used to control a heating system, that the heating system shoul be rapidly turned on and off as the ambient temperature fluctuates about the temperature at which the bridge is balanced. Accordingly, it is desirable to provide a dead band such that if the heating system switches off at N°C it will not switch on again until the temperature has fallen to (N-d) °C, where d is the width of the dead band. This dead band is in fact provided by the resistor 20. When the amplifier output V goes through a transition the output voltage change will cause a change in the voltage V. via resistor 20. This change is such as to move the voltage V, in the opposite direction to the cha¬ nge. in voltage at V ,so that after rising through a transition the voltage at V will then have to fall further to reverse the transition. The exact voltage difference, or hysteresis, can readily be calculated knowing the values of the resistances in the right hand side of the bridge, the value of the resistance of the resistor 20, and the total voltage transition at the output of the amplifier. In practice a value for d of 2 °C or thereabouts is likely to be suitable in most cases. As mentioned above, the resistors 2 and 10 are normally short-circuited by links 3 and 11. These links are made so as to be severable, and provide a means of adjusting the temperature at which the bridge balances. For example, if the resistor 2 has a resistance of 7.1 ohms (the temperature sensitivity of the sensor 1) cutting links 3 will cause the thermostat to operate one degree lower than previously. Similarly, cutting link 11 will cause the thermostat to operate one degree higher . CMPI Various alternative arrangements of links are possible. For example, resistor 10 and link 11 could be positioned in series with resistor 2 and link 3. If, then, resistor 10 has a value of 14.2 ohms, cutting link 3 would cause the thermostat to operate 1°C lower, cutting link 11 will cause it to operate 2 °C lower, and cutting both will cause it to operate 3°C lower. Figure 2 shows, partly in cross-section and partly in side view, an embodiment of the thermostat according to the invention. The illustrated embodiment comprises a housing 50 having an open front which is covered by a heat-conducting metal front plate 51. The plate 51 is spaced from the casing 50 by two insulating gaskets 52. An inner housing 53 is mounted inside the assembly of housing 50 and plate 51 through heat-insulating gaskets 54 by means of which it is secured to the front plate 51 with a gap therebetween. In Figure 2 one side wall of the inner housing 53 has been partly cut away to show a circuit board 55 on which is mounted the circuit of Figure 1. The circuit board 55 carries the necessary circuit components on the side thereof remote from the front plate 51, with the conduct¬ ors being located on the side adjacent the front plate 51. The only exception to this is in the case of the sensor 1 which, although connected to the same side of the circuit board as the other circuit components extend through an aperture formed in the circuit board and through an aperture in the adjacent gasket 54 so that it comes into contact with the front plate 51. The temperature sensed by the sensor 1 is, therefore, the temperature of the front plate 51, which is, for intents and purposes, the same as the ambient temperature. Figure 2 also shows the links 3 and 11 which are also shown in Figure 1. Power is supplied to the thermostat, and an output signal taken therefrom via leads 56 which extend through an aperture in the housing 50. The interior of the inner housing 53 is largely filled with a thermosetting, electrically insulating resin 57. The presence of this resin prevents access being obtained to the circuit board 55 after the circuit board has been installed. The links 3 and 11 protrude out of the surface of the resin 57, so that these links can be severed if it is desired to alter the temperature response of the thermostat. Various modifications may be made to the embodiment of the invention described above. For example, if the link 3 were replaced by a remotely controlled electronic switch, this would permit the operating temperature of the thermostat to be remotely turned down to a lower control temperature, for example to provide temperature control during the night. Another possible modification is to incorporate a second operational amplifier to monitor the voltage difference between V and V. , thereby allowing remote reading of the acutal ambient temperature at the thermostat. It should also be pointed out that by using a single sensor with a plurality of similar bridge circuits it is possible to provide independent control outputs at a plurality of specific temperatures. Although the invention has been described above with reference to a thermostate, the same circuit can be used, if desired, as a fire detector. For example, a plurality of devices of the type described above can be connected in series or in parallel to a suitable monitoring system which would detect any anomalous temperature increases, for example such increases caused by fire.

Claims: CLAIMS : 1. A temperature controlling or detecting device having a circuit which comprises a temperature-sensitive element whose • electrical resistance changes with temperature and means for detecting changes in the resistance of the said element and producing an output signal at a predetermined temperature or at the upper and lower limits of a predetermined temperature range in response thereto, and means for encapsulating at least a sufficient part of the circuit to prevent ready adjustment of the said predetermined temperature or temperature range. 2. A device according to claim 1, wherein the said circuit comprises a Wheatstone bridge having a plurality of arms one of which contains the said temperature- sensitive element. 3. A device according to claim 2, wherein at least one arm of the bridge contains a resistance normally short-circuited by a severable link, thereby to permit the balance point of the bridge to be altered by severing the link. 4. A device according to claim 3, wherein the encapsulating means is a resin which leaves unencapsulated the said link or links. 5. A device according to any preceding claim which comprises an outer housing closed by a heat-conducting plate, and an inner housing mounted within the outer housing and separated therefrcm by heat-insulating gaskets, the sand circuit being mounted substantially within the inner housing but wifh the temperature - sensitive element extending out of the inner housing and into thermal contact with the said heat-conducting plate. 6. A device according to any preceding claim, wherein 5 the detecting means comprises an operational amplifier. 7. A device according to claim 6, wherein the operational amplifier is arranged to provide a dead band such that the output signal produced thereby occurs ] _0 at a lower temperature when the temperature is falling and at a higher temperature when the temperature is rising.

Inventors: WRIGHT ARCHIBALD ROBERT, WRIGHT, ARCHIBALD, ROBERT
Applicants: SERVICE FIRST INTERNATIONAL LI, SERVICE FIRST INTERNATIONAL LIMITED
Application year: 1982
Main classification: G05D23
IPC codes: G05D23