TEMPERATURE SENSING CIRCUIT

Abstract: A temperature sensing circuit including a temperature sensing element 32 located at a medium to sense the temperature of the medium. The sensing element is a resistive element with a range of resistive values corresponding to a range of sensed temperatures. The sensing element is connected to circuitry 30, 31, 34 and 40 for detecting variations of the resistance of the element 32. The circuitry provides an output signal of one state when the element resistance increases beyond a first value and provides an output signal of a second state when the element resistance decreases beyond a second value.

Description: The present invention relates to a temperature sensing apparatus and a respective temperature sensing method that provide an output relative to two temperature transition points. Temperature sensing mechanisms are required in many applications to determine temperature levels to endure proper operation of equipment. Traditionally, temperature sensing has been accomplished by mechanical devices that rely upon mechanical changes of the device in response to temperature changes of the mechanism. These mechanical devices often develop reliability problems or vary from their designed operational characteristics. From EP-A-0 208 318 which is prior art in accordance with Article 54(3) EPC an electrical temperature sensing switch for an electric load and the application of the switch in automotive vehicles is disclosed. The switch comprises a bridge circuit and a temperature sensitive element within the bridge. The circuit is realised in the form of an integrated circuit comprising three resistors, two comparators, a flip-flop controlled by the outputs of the comparators and a transistor the collector of which is connected to a terminal of the integrated circuit. In this circuit arrangement the trigger and threshold inputs of the integrated circuit are tied together. From the periodical Elector, Vol. 9, No. 7/8, July/August 1983, pages 7-66 and 7-67 a circuit arrangement for the control of the interior temperature for cars is known which enables the desired temperature within the passenger compartment of the car to be set between two limits. The ambient temperature is sensed by an NTC thermistor, the resistance value of which is used to set the level at one of the inputs of an operational amplifier within an integrated L121 circuit. This operational amplifier sets the triggering level of the internal logic which drives an output stage. By means of a potentiometer, the temperature at which the circuit operates may be adjusted. From the periodical Electronic Design, Vol. 30, No. 16, August 1982, page 217, a circuit is known which monitors the temperature and the level of a liquid bath using a 556 integrated circuit which consists of two 555 timers. One of the timers is to monitor the level whereas the other is to monitor the temperature of the bath. It is the object of the present invention to provide a reliable mechanism for sensing temperature. This object is attained by the features of claim 1. A preferred embodiment of the invention is the subject matter of the dependent claim. A method of sensing temperatures is the subject matter of claim 3 whereas a further embodiment of the method is the subject matter of claim 4. In the present invention, a temperature sensing circuit is provided that includes a temperature sensing element with a resistive value that varies according to the temperature measured. Circuitry connected to the sensing element detects variations of resistance within the sensing element. The circuit provides an output of a first state when the detected resistive value increases beyond a first point and provides an output of a second state when the detected resistive value decreases beyond a second point. In an embodiment of this invention, a temperature sensing circuit is provided that includes a temperature sensing element or thermistor with a resistance that varies according to the temperature measured. An external resistor network is connected to the thermistor. A circuit is connected to this resistor network and the thermistor. The circuit includes two comparators, a latch and an internal resistance network. The external resistor network is connected to the comparators. An internal resistor network is also connected to the comparators. The circuit receives a voltage which is placed across both resistive networks and sensing elements. The voltage across the thermistor varies as a result of thermistor resistance changes from temperature variances. The comparators compare the resulting voltages of the two resistive networks and set or clear the latch accordingly. The output of the latch is an output signal with two states. The first state is provided when the element resistance increases beyond a first transition point. The second state is provided when the element resistance decreases beyond a second transition point. The values of resistors in the first resistor network are selected in combination with the resistive range characteristics of the thermistor to provide these transition points at preselected temperatures. In a further embodiment of this invention, the output is connected to a relay which may be used to activate a switch. An application for this embodiment would be an air conditioner wherein the thermistor is placed on the evaporator coil and the relay is connected to power the compressor. The temperature sensing circuit would insure proper operation of the compressor relative to the temperature condition of the evaporator coil. In a further embodiment, the temperature sensing circuit may be self contained within a housing providing a single unit temperature probe. BRIEF DESCRIPTION OF THE DRAWINGS The above and other features and advantages of this invention will become more apparent as the invention becomes better understood by the detailed description that follows, when considered in connection with the accompanying drawings. FIGURE 1 is a schematic diagram of a timer circuit. FIGURE 2 is a schematic diagram of an embodiment of the temperature sensing circuit invention. FIGURE 3 is a schematic diagram of a temperature sensing circuit embodiment including a variable resistor for adjusting a transition point. FIGURE 4 is a pictorial view of a temperature probe. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT This invention includes a temperature sensing circuit with two temperature transition points. One transition point provides an off-to-on output transition state and the second transition point provides an on-to-off output transition state. FIGURE 2 is a schematic diagram of an embodiment of this invention. The device 40 is a 555 timer circuit and is illustrated in FIGURE 1. Device 40, the 555 timer, is detailed in the specification information for the National Semiconductor LM555 contained in the National Semiconductor Linear Data Book-1983 and the specification information for the Intersil ICM 7555 contained in the Intersil Data Book , both herein incorporated by reference. Line 41 is connected to Vcc and line 44 connected to ground. Line 42 is the threshold input and line 43 is the trigger . Line 45 is the control voltage line connected to ground via capicitor 33 and line 46 provides the output signal to coil 34 which activates switch 35. The resistive values of resistors 30 and 31 (R30 and R31) are selected to set high and low temperature trigger points according to the resistive/temperature characteristics of thermistor 32 (R32). For the low temperature trip point, the values of R30 and R31 are selected to satisfy the equation: For the high temperature trip point, the values of R30 and R31 are selected to satisfy the equation: As an example of the embodiment in FIGURE 2, for a thermistor with a resistive value of 30 kΩ at 0°C and 26 kΩ at 3°C, the values of R30 and R31 are 27,333 kΩ and 24,667 kΩ respectively. The resulting circuit provides a temperature hysteresis between the transition of 0°C and 3°C. It should be apparent that a variable resistor or potentiometer may be used to adjust the resistive values to adjust the temperature trip points. FIGURE 3 is an illustration of the invention with a variable resistor. Device 80 is the 555 timer with lines 81 and 84 being Vcc and ground respectively. Lines 82 and 83 are the threshold and trigger lines with lines 85 and 86 being the output and control voltage lines. In this embodiment, thermistor 53 is connected in parallel with variable resistor 55 which provides an adjustment of the thermistor resistive characteristics for a temperature range. Additionally, R54 is provided in addition to R51 and R52 to provide trimming of the temperature difference between the high and low temperature trip points. This configuration results in a versatile circuit that may be adjusted to yield the desired operating characteristics for triggering switch 59 with coil 58. The 555 timer is an integrated circuit that is approximately equivalent to two comparators connected to a flip-flop or latch. The two comparators are connected to an internal resistive network. It is the combination of the values of the internal resistive network together with the external resistive networks that provide the operating characteristics of the present invention. Thus, the proper operation of the invention is independent of minor variations in Vcc. Using a CMOS version of the 555 timer will reduce the power requirements of the invention and enable the sensing circuit to be used almost any place where voltage is available for Vcc. FIGURE 4 illustrates a still further embodiment, wherein the invention is implemented as a temperature probe having a thermistor 91 connected to a 555 integrated circuit 94 with two resistors 92 and 93 all mounted upon a board 97 and environmentally isolated by a case 96. The power and output lines to and from the probe are provided on pins 95. One application for this probe would be with an evaporator coil of an air conditioner. The probe would be located having the thermistor measure the evaporator coil temperature. The output of the probe would activate a switch which powers the compressor. When the temperature of the evaporator coil falls below a low temperature trip point the probe would deactivate the compressor. The compressor would remain deactivated until the evaporator coil temperature rises above a high temperature trip point wherein the compressor would be reactivated. The output of the invention would therefore be used to control the compressor to ensure proper operation of the unit in relation to the temperature of the evaporator coil.

Claims: A temperature sensing apparatus comprising: a first and second set point temperature sensing circuit including a thermistor (32;53;91) for placement in operative association with a medium and connected to a voltage divider having first and second resistors (31;30;52,51;93,92) of preselected values coupled in series with the thermistor (32;53;91) for generating a first voltage indicative of the medium's first set point temperature at a first node (A) between the first and second resistors (31,30;52,51;93,92) and for generating a second voltage indicative of the medium's second set point temperature at a second node (B) between one (31;52;93) of said first and second resistors and said thermistor (32;53;91); and a 555 timer (40;80;94) having its threshold and trigger inputs (42,43;82,83) coupled to the voltage divider for monitoring first and second voltages from the set point temperature sensing circuit wherein the threshold input (42;82) is electrically connected to one (A) of said first and second nodes and the trigger input (43;83) is electrically connected to the other (B) of said first and second nodes, said 555 timer (40;80;94) outputting first and second logic state output signals indicative, respectivley, of temperatures below the first set point temperature and above the second set point temperature wherein a hysteresis effect is generated for switching between the first and second logic state output signals. The sensing apparatus of claim 1 comprising a third adjustable resistor (55) coupled to the voltage divider (51,52,53) to adjust the first and second voltages and thereby adjust the first and second set point temperatures. A method of monitoring the temperature of a medium and generating a control output when the medium temperature varies from a predetermined range defined by first and second set point temperatures comprising the steps of: a) sensing the medium temperature using a temperature sensitive resistive network having a thermistor (32;53;91) in heat transfer relationship with the medium for outputting medium temperature responsive signals from said network series coupled with two resistors (31,30;52,51;93,92) to form a voltage divider network having first (A) and second (B) nodes; b) energizing the voltage divider network to provide a first network output signal at one node and a second network output signal at a second node; c) coupling network output signals from a first node to the trigger input of a 555 timer circuit so that when the sensed medium temperature is altered in one sense direction to one set point temperature the output state of the 555 timer changes; d) coupling network output signals from a second node to the threshold input of said 555 timer circuit so that when the sensed medium temperature is altered in the other sense direction to the other set point temperature the output state of the 555 timer changes again; and e) coupling an output device to the 555 timer circuit output so that the timer output states provide a control output for governing the device in response to sensed medium temperature. The method of claim 3 wherein the sensing step additionally comprises the step of coupling a variable resistor to one node of said voltage divider network to adjust the medium temperature responsive signals to vary the set point temperatures.

Inventors: GILLETT, JIMMIE D.
Applicants: RANCO INCORPORATED OF DELAWARE
Application year: 1986
Main classification: G05D23
IPC codes: F24F11, F25B49, B60H1, G05D23