GB2148554A - Temperature sensors - Google Patents

Abstract

A temperature sensor for a system for controlling the temperature of e.g. a building or refrigerator comprises two or more thermistor sensing elements 11A, 11B etc respectively connected in series with first reference resistors 12A, 12B, etc. Each series circuit is connected in parallel and the parallel network is connected in series with a second reference resistor 13. A comparator device 16 is coupled to compare the output of each respective series circuits 11A, 12A; 11B, 12B, etc with a reference input 15 and an output device (OR-gate 18) delivers a control signal. The arrangement is such that the output is related to a weighted average of the temperatures of the various thermistors 11, the weighting being in favour of the warmest thermistor and being dependant upon the values of resistors 12, 13 and upon the temperature difference between the thermistors. So that any failed thermistor is effectively ignored. <IMAGE>

Description

SPECIFICATION Temperature sensors This invention relates to temperature sensors.

Temperature controllers are well known and comprise an electronic temperature sensor such as a thermistor connected to a control device capable of delivering or removing heat to or from the environment in which the sensor is located such as to cause the temperature sensed by the sensor to come into balance or equality with a reference temperature. Usually the reference temperature is adjustable by manual means.

In many cases the environment to be controlled behaves in a non-uniform manner so that a sensor with only a single sensing element enables the reference temperature to be attained only at the immediate vicinity of the sensing element. Clearly, this is disadvantageous. By way of example, a non-homogenous environment is to be found within a space-heated building. Another example is large capacity refrigerators or freezers, the latter usually being subjected to uneven loading of products to be stored resulting in blockage of air passageways which produces uneven distribution of icing.

In one proposal for overcoming this problem the sensor has been provided with at least to sensing elements from which an average sensed temperature has been derived but this proposal suffers from the disadvantage that failure of one or more sensing elements results in a markedly erroneous average sensed temperature.

It is an object of the present invention to provide an improved form of temperature sensor for use in a non-homogeneous environment.

According to the present invention there is provided a temperature sensor comprising a plurality of thermistor sensing elements respectively electrically connected in series with a plurality of first reference resistors, each such series circuit being electrically connected in parallel and the parallel network so formed being connected in series with a second reference resistor, comparator means coupled to receive inputs from the respective series circuits and from a reference input, and output means coupled to the comparator means to deliver an output signal.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawing in which Fig. 1 illustrates the electrical circuit of a sensor having only two thermistor sensing elements and Fig. 2 is a temperature phasediagram illustrating operation of the sensor of Fig. 1.

The sensor 10 shown in Fig. 1 comprises two thermistor sensing elements 11 A, 1 1 B respectively electrically connected in series with two first reference resistors 1 2A, 1 2B, the two series circuits thereby formed being connected in parallel and the thus formed parallel network being connected in series with a second reference resistor 1 3. Leads 1 4A, 1 4B, connected to the junction between 11 A, 1 2A and 11 B, 12B respectively provide inputs to a comparator device 1 6 to which a reference input 1 5 is also supplied. The series-parallel circuit provided by components 1 1 A, 1 1 B, 12A, 12B and 13 is electrically energised by a D.C. supply voltage V.

In this embodiment the comparator device 1 6 comprises respective comparators 1 7A, 1 7B connected to leads 14A, 14B, each comparator 1 7A, 1 7B also receiving the reference input 1 5. Each comparator 1 7A, 1 7B is connected to the input of an OR gate 1 8 which functions as an output device to deliver a sensed-temperature control signal to a control device (not shown).

It will be understood that although Fig. 1 illustrates elements 11 A, 1 1 B in close electrical proximity they are intended to be in relatively remote physical proximity as is required for a non-homogeneous environment. Furthermore additional series circuits of components 11 C, 1 2C; 11 D, 1 2D etc. may be connected in parallel with the series circuits shown with a corresponding enlargement of the number of comparators 1 7C, 1 7D etc.

Fig. 2 illustrates the temperature phase diagram for the circuit shown in Fig. 1 and is two-dimensional because there are only two thermistor sensing elements 11 A, 11 B. The diagram illustrates that when resistor 1 3 is zero and resistors 1 2A, 1 2B equal in value but non zero the characteristic 20 is achieved in that the output device 1 8 selects the warmer thermistor 1 1 A, 1 B when the voltage signal provided by the series circuit containing that thermistor exceeds the reference voltage provided by reference input 1 5 without any effect being achieved by the cooler thermistor.

At the other extreme when resistor 1 3 is non zero and resistors 1 2A, 1 2B are zero characteristic 21 is achieved where the output device 1 8 provides an output signal when the mean or average of the thermistor voltages exceeds the reference voltage. However, when both resistor 1 3 and resistors 1 2A, 1 2B are non zero (and fixed in value) intermediate characteristics are achieved, referred to as 'weighted average' characteristics, of which only one, 22, is illustrated in the interests of clarity. It will be appreciated that any one of these weighted average characteristics is selected by means of the relative numerical values of resistors 1 2A, 1 2B and resistor 1 3 in relation to the thermistor resistance values at the normal working temperature of the sensor.

Typically, characteristic 22 provides a weighting in favour of the warmer thermistor 1 1A or 1 1 B by a 4:1 factor when the temper ature difference between the thermistors 11A, 11 B is small but as temperature difference increases the weighting factor increases and a limit is reached when the warmer thermistor 1 1 A or 11 B reaches a preset value, preferably of the order of 5QC, above the reference or normal working temperature so that the other thermistor may be in open-circuit condition as would result from a broken thermistor without seriously affecting the sensor.

By way of example, characteristic 22 is achieved by selecting thermistors 11 A, 1 1 B to be negative temperature coefficient devices having a resistance value of 25 kohms at a normal working temperature of - 1 5 degrees Centigrade; resistors 1 2A, 1 2 B each having a value of 6.8 kohms and resistor 1 3 having a value of 2.7 kohms.

Claims (4)

1. A temperature sensor comprising a plurality of thermistor sensing elements respectively electrically connected in series with a plurality of first reference resistors, each such series circuit being electrically connected in parallel and the parallel network so formed being connected in series with a second reference resistor, comparator means coupled to receive inputs from the respective series circuits and from a reference input, and output means coupled to the comparator means to deliver an output signal

2. A sensor as claimed in claim 1, wherein each thermistor element has a negative temperature resistance coefficient.

3. A sensor as claimed in either preceding claim, wherein said output means comprises an OR gate.

4. A temperature sensor substantially as hereinbefgore described with reference to the accompanying drawing.