ELECTRONIC CIRCUIT ARRANGEMENT

Abstract: Electronic circuit arrangement in a temperature measurement circuit based on a platinum resistor (PT) as a temperature sensing resistor, whereby in addition to the platinum resistor there is provided a reference resistor (R1). The measurement circuit (S) is adapted to apply equally large DC currents (I5, I6) to both resistors having a common return lead (3) from these, and to sense the resulting voltage difference (U) across the resistors. By means of at least one operational amplifier (G) the voltage difference is adapted to cause an output DC current from the measurement circuit (S) being proportional to the voltage difference and thereby constituting a measure of the temperature of the platinum resistor (PT). The measurement circuit (S) comprises means for compensating for non-linearity of the temperature-resistance characteristic of the platinum resistor. Two transistors (T1, T2) are provided, of which the collector output currents (I3, I4) are each adapted to be added to its said respective equally large DC current (I6, I5) being applied to the platinum resistor (PT) and the reference resistor (R1) respectively. The emitter-base voltage of the transistors is adapted to vary depending upon the output DC current from the measurement circuit (S).

Description: ELECTRONIC CIRCUIT ARRANGEMENT This invention relates to an electronic circuit arrangement in a type of temperature measurement circuit which is based on a platinum resistor as temperature-sensing resistor. In this particular type of measurement circuits a reference resistor is also provided in addition to the platinum resistor, and the measurement circuit is adapted to apply equally large DC currents to both resistors, which have a common return lead. The resulting voltage difference across the resistors is sensed, and by means of at least one operational amplifier this voltage difference is adapted to cause an output DC current from the measurement circuit. This DC current shall be proportional to the voltage difference in order to constitute a measure of the tem¬ perature of the platinum resistor. In view of non-linearity occurring in the temperature-resistance characteristic of the platinum resistor, such measurement circuits are known which comprise means for compensating for this non-linear¬ ity. A frequently employed and more or less standardized platinum temperature sensor has the type designation PT-100 and this has a primary interest in this connection. Known temperature measurement circuits for co-operation with such platinum resistors are available in the form of an integ¬ rated circuit or chip, being among other things based on a three conductor principle at the input side for connection to the platinum resistor and the reference resistor respec¬ tively and with a common return lead as mentioned above. As far as the output of the measurement circuit is concerned, the invention is directed to a situation where the output DC current mentioned appears at two output terminals, for simple and practical adaption to the circuits or the system in which the temperature measurement is to be involved. More particularly there exist previously known circuits which carry out the necessary linearizing in connection with, for example, the platinum resistor of type PT-100 mentioned, including solutions based on an increase of the applied current in the sensor when the temperature rises. Thus US Patent 3 783 692 shows a measurement circuit in which a platinum sensing resistor is incorporated in a bridge configuration. Linearizing of the sensor is obtained by increasing the current therein by means of a specific current source having a negative resistance. There is here the question of a rather complicated circuit in which it is difficult to calculate the component values for adaption to different temperature ranges to be covered by the measure¬ ments. US Patent 3 754 442 describes a measurement circuit adapted to co-operate with different sensors having differ¬ ent non-linearity. The sensor is connected into a feed-back loop in an operational amplifier. A second operational amplifier controls an indicator and acts to vary the current n the sensor depending upon the output voltage. Also here a rather complicated circuit is involved, due to among other things that it is to co-operate with different sensors. European Patent 0 101 956 relates to a measurement circuit designed with a common differential amplifier in co¬ operation with a traditional source of current in a four- conductor measurement principle. The output voltage controls the source of current so that the current in the sensor increases with a rising temperature. US Patent 4 114 446, similarly to some of the patent specifications above, is directed to a relatively comp¬ licated measurement circuit comprising a source of current and various amplifiers which by means of several feed-back loops are adapted to liniearize the sensing resistor. The object of this invention is to provide an arrange¬ ment of temperature measurement circuits of the kind described, being based on other and much simpler means than what is previously known, in order to carry out the desired linearizing. In the following description the invention will be explained more closely with reference to the drawings, in which: Figure 1 shows a simplified diagram of a temperature measurement circuit based on an embodiment of the arrange- ment according to the invention, and Figure 2 illustrates a typical characteristic of platinum sensing resistors having a non-linearity to be compensated for. In the diagram of fig. 1 there is shown a platinum resistor PT and a reference resistor Rl being both connected to a main component in the measurement circuit, i.e. the block S which according to known techniques can consist of an integrated circuit especially manufactured for co¬ operation with this particular type of temperature sensing resistor PT. As seen in fig. 1 the platinum resistor PT and the reference resistor Rl at the input side of the circuit S is connected thereto through three terminals or leads 1, 2A and 3, whereby the leads 1 and 2A constitute the inputs to an operational amplifier G which is incorporated in the integrated circuit S. Thus at the input side there is a structure for taking advantage of the so-called three- conductor measurement principle, which is known in connec¬ tion with measurement circuits for platinum sensing resis¬ tors. Circuit S has two outputs 11, 12 for delivering constant DC currents II and 12 respectively, adapted to flow through reference resistor Rl and temperature sensor PT respecively, and having a common return lead 3 to a common point or earth for the complete measurement circuit includ¬ ing the integrated circuit S. These two DC currents being in principle constant, generate a higher or lower input voltage to the operational amplifier G depending upon the existing resistance value of the sensor PT, determined by the temperature sensed thereby. The integrated circuit S, on the basis of the output from the operational amplifier G via internal circuits not shown, is adapted to cause an output circuit Y to deliver a DC current at the output terminals 4 and 5. The amplitude of this DC current 17 is proportional to the input voltage on the operational amplifier G. For example the integrated circuit S can be of the type XTR101 supplied by the firm Burr Brown, USA. Thus what has been described this far with reference to , fig. 1, is related to a known and conventional measurement circuit, being intended for example for use in instrumen¬ tation, control systems or programmable logical controls. The output DC current 17 through terminals 4 and 5 in such conventional circuits can, for example, be adapted to be within the range from 4 to 20 mA, in order to co-operate with other parts or circuits in such systems. The non-linearity problem which leads to difficulties and complications in such measurement circuits is illust¬ rated in fig. 2. This figure shows electrical resistance as a function of temperature for a typical platinum resistor, in the form of the curve or charactistic 20, which over a long distance coincides quite closely to a rectilinear, ideal characteristic 21, but at higher temperatures falls off and deviates relatively much from the straight line. This non-linearity will result in inaccuracies in the temperature measurements, if precautions are not taken for compensating for the non-linearity by providing specific means in, or connected to, the measurement circuit. What is aimed at is, that the DC current which constitutes the output signal from the measurement circuit shall have a high degree of proportionality to the temperature of the temper¬ ature sensing resistor, over the whole temperature range which it is desirable that the measurement concerned shall cover. In the diagram of fig. 1 the dashed line block L indicates a qircuit solution based on the present invention, which serves to bring about the required linearizing in a very simple but at the same time accurate manner. Essential components .in the circuit part L are two transistors Tl and T2 adapted to deliver DC currents 13 and 14 respectively, which are added to said constant currents II and 12 respec¬ tively, with resulting total DC currents 15 and 16 which are applied to the sensing resistor PT and the reference resistor Rl respectively. The additional currents 13 and 14 being supplied by transistors Tl and T2 are determined by a voltage generated by the output DC current 17 at terminal 4, which results in a voltage drop across the series resistor R2 shown, having a diode D in series therewith. An increas¬ ing DC current 17 gives an increase in the currents 13 and 14 and a corresponding increase of the currents 16 and 15. Thereby, a rising temperature at sensor PT will involve a higher voltage increase input to the operational amplifier G than would otherwise have been the case, and the inherent non-linearity of the sensor PT is compensated for. With current directions and polarities as shown, the conducting direction of diode D will be as shown in the diagram, and the transistors Tl and T2 are taken to be PNP- transistors. The additional or compensational currents 13 and 14 are supplied from the collectors of the two transis¬ tors, the emitter-base voltage of each of the transistors being determined by the magnitude of the currents 13 and 14. As will appear from the diagram the voltage drop across the resistor R2 and the diode D will have an equal influence on the emitter-base voltage of both transistors and thereby ensure that the resulting total DC current being applied to the platinum resistor PT and the reference resistor Rl respectively, are equal all the time. The magnitude of the additional or compensational currents 13 and 14 can be adjusted by means of the resistors R3 and R4 in the emitter circuit of both transistors. In this way the compensation can be adapted according to the temperature range in which the sensing resistor PT is to operate. The diode D is inserted in order to compensate for the influence of ambient temperature on the transistors Tl and T2 and thereby on the additional currents 13 and 14. As transistors Tl and T4 it is suitable to employ transistor types having a high current amplification at low collector currents. Based on embodiments in which the constant DC currents II and 12 from the integrated circuit S have a nominal magnitude of 1 mA, the additional currents 13 and 14 in frequently occurring applications of such a measurement circuit advantageously can lie in a range of from 5-150 μh.

Claims: C l a i m s 1. Electronic circuit arangement in a temperature measure¬ ment .circuit based on a platinum resistor (PT) as a tempera¬ ture sensing resistor, whereby in addition to the platinum resistor there is provided a reference resistor (Rl), and the measurement circuit (S) is adapted to apply equally large DC currents (15, 16) to both resistors having a common return lead (3) from these, and to sense the resulting voltage difference (U) across the resistors, said voltage difference being by means of at least one operational amplifier (G) adapted to cause an output DC current from the measurement circuit (S) being proportional to the voltage difference and thereby constituting a measure of the temperature of the platinum resistor (PT), and the measure¬ ment circuit (S) comprises means (L) for compensating for non-linearity of the temperature-resistance characteristic of the platinum resistor, c h a r a c t e r i z e d in that there are provided two transistors (Tl, T2), of which the collector output currents (13, 14) are each adapted to be added to its respective said equally large DC current (16, 15), being applied to the platinum resistor (PT) and the reference resistor (Rl) respectively, and that the emitter-base voltage of the transistors is adapted to vary depending upon the output DC current' f om the measurement circuit (S) . 2. Arrangement according to claim 1, c h a r a c t e r i z e d in that a series resistor (R2) is inserted in an output lead for the output DC current, said series resistor also being incorporated in the emitter-base circuit of the transistors in order to give the varying emitter-base voltage. 3. Arrangement according to claim 2, c h a r a c t e r i z e d in that a diode (D) is inserted in series with the series resistor (R2), said diode being also incorporated in the emitter-base circuit of the transistors.

Inventors: JEZIOROWSKI MAREK, JEZIOROWSKI, MAREK
Applicants: NORAPP JOH H ANDRESEN, NORAPP - JOH. H. ANDRESEN
Application year: 1990
Main classification: G01K7
IPC codes: G01K7