JP2512934B2 - Temperature measurement circuit - Google Patents

Description

The present invention relates to a temperature measuring circuit using a resistance temperature detector.

[Prior Art] FIG. 4 shows a conventional example of a temperature measuring circuit using a resistance temperature detector. In the figure, RTD is a resistance temperature detector, and A, B, and b are connection terminals. One end of the resistance temperature detector RTD is connected to the terminal A through the lead wire l1, and the other end is connected to the terminals B and b through the lead wires l2 and l3, respectively. r1 to r3 indicate resistance values of the lead wires l1 to l3, and r1 = r2 = r3 = r. IS is a constant current source, RS is a reference resistor, the constant current source IS is connected to terminal A,
The reference resistor RS is connected between the terminal b and the reference potential point COM via a connection line l4. BU is a buffer amplifier and SU is a differential amplifier. The connection terminal A is connected to one input terminal of the differential amplifier SU via the buffer amplifier BU, and the terminal B is directly connected to the other input terminal of the differential amplifier SU.

In such a configuration, the constant current I supplied by the constant current source IS is from the terminal A to the lead wire l1 → low temperature measuring antibody RTD →
It flows to the reference potential point COM through the lead wire l3 → reference resistance RS. This current I causes the voltage drop at terminals A and B to be V1 and V2.
If RT is the resistance temperature detector RTD and RS is the resistance value of the reference resistance RS, then V1 = (RT + RS + 2r) * IV2 = (RS + r) * I. Therefore, when the output voltage of the differential amplifier SU is Vo, Vo is expressed by the following equation.

Vo = 2V2-V1 = (RS-RT) (1) If the value of the reference resistance RS is set to the same value as the resistance value of the RTD RTD at 0 ° C in the formula (1), the output voltage Vo is Corresponds to the measured temperature.

Here, in the circuit of FIG. 4, the reference resistance RS is the connection line l4.
Although it is connected to the terminal b through, the connecting line l4 also has a resistance. This resistance is shown as rs in the figure, but in the circuit of FIG. 4, this resistance rs is the measured value Vo.
Will cause an error.

[Problems to be solved by the invention]

The present invention has been made to solve such a problem, and is intended to prevent the resistance value of the connection line l4 connecting the reference resistance RS from affecting the measurement result.

[Means for solving problems]

The present invention achieves the above object by applying to the terminal b a voltage corresponding to the product of the resistance value of the resistance temperature detector at 0 ° C. and the current flowing through the resistance temperature detector. Examples will be described in detail below.

〔Example〕

FIG. 1 is a connection diagram of an embodiment of a temperature measuring circuit according to the present invention. In FIG. 1, the same parts as those in FIG. 4 are designated by the same parts as those in FIG. In FIG. 1, OP is an operational amplifier, the output terminal of which is connected via a connecting line 15 and the inverting input terminal thereof is connected via a connecting line 16 to a terminal b.
It is connected to the. When the resistance value of the resistance temperature detector RTD at 0 ° C. is RT (0), the voltage represented by E = I · RT (0) is applied to the non-inverting input terminal of the operational amplifier OP. With this configuration, the applied voltage E is made constant by the operational amplifier O and applied to the terminal b. Therefore, in such a configuration, V1 and V2 are represented by V1 = E + (RT + 2r) .I V2 = E + r.I, and Vo is Vo = 2V2-V1 = E + RT.I = {(RT (0) -RT } · I (2) As is clear from the equation (2), the output voltage Vo corresponds to the temperature to be measured Note that the output terminal of the operational amplifier OP and the inverting input terminal are connected by the connecting line l5. , b via l6
The resistors r5 and r6 are also present in those connecting lines. However, these resistors do not cause an error because the potential of the terminal b is constant.

In the conventional example shown in FIG. 4, when the input is switched using a scanner or the like, an error occurs in the output voltage Vo due to variations in the contact resistance of the relay of the scanner. 2 and 3 are designed so as not to be affected by the contact resistance of such a scanner relay. SC is the scanner in FIGS. 2 and 3, and terminals A, B, and b in FIG.
Are all switched by the scanner contacts s1 to s5. In this case, since the scanner contacts s4, s5 are connected to the connection lines l5, l6 connecting the terminal b and the operational amplifier OP, not only the scanner contacts s1 to s3 but also the scanner contacts s5, s5.
The contact resistance of 6 also does not cause an error as described in FIG. In FIG. 3, the terminal b is shared by all channels. In this case, the error factors due to the connection lines l5 and l6 are eliminated as described in FIG. In some cases, the terminal b may be shared between channels, and at that time, it is necessary to use another scanner contact at the points X and Y on the connecting lines l5 and l6. There is no effect.

[Effect of the present invention]

As described above, in the present invention, since the potential of the connection terminal b of the temperature-measuring low antibody connected by the three-wire system is configured to be a constant voltage, the resistance value of the connection wire connected to this connection terminal b Alternatively, a temperature measuring circuit that is not affected by the contact resistance of the scanner contact can be obtained with an extremely simple structure.

[Brief description of drawings]

1 to 3 are connection diagrams of an embodiment of a temperature measuring circuit according to the present invention, and FIG. 4 is a connection diagram of an example of a conventional circuit of this type. RTD: RTD, A, B, b ... Connection terminals, IS ... Constant current source, SU ... Differential amplifier, OP ... Operational amplifier, E ... Voltage.

Claims (1)

(57) [Claims] 1. A resistance temperature detector is connected using three terminals A, B, b, and a differential amplifier is used to detect the difference in voltage drop between terminals A, B caused by a constant current supplied from terminal A. In the temperature measuring circuit to be taken out, a voltage corresponding to the product of the resistance value of the resistance temperature detector at 0 ° C. and the current flowing through the resistance temperature detector due to the constant current is applied to one input end of the other end. A temperature measuring circuit comprising an operational amplifier having an input terminal and an output terminal connected to the terminal b via resistors.