JPS6356953B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a burnout circuit for a resistance/electrical signal converter using a 4-wire resistor to be measured.
In particular, the present invention relates to a burnout circuit that outputs a burnout upscale when a resistor to be measured is disconnected. In a typical automatic process control system, if a resistor to be measured breaks due to fatigue or damage, it must be detected and some measure taken to ensure safe operation of the control system. Therefore, it is desirable that the converter include a circuit that operates to swing the output signal outside the standard signal range, either the upper limit or the lower limit, in order to detect a disconnection in the resistor to be measured. This is generally called a burnout circuit. FIG. 1 shows an embodiment of a temperature converter composed of a four-wire resistance temperature detector and a differential amplifier as the resistor to be measured. In FIG. 1, 1 has a branched first current supply lead wire A1 and a first voltage detection lead wire B1 at one end, and a second current supply lead wire A2 and a first voltage detection lead wire at the other end. It is a 4-wire type resistance temperature detector having two voltage detection lead wires B2 branched from each other.
1 to r4 are the respective lead wires A1 and B from the resistance temperature detector 1 installed at a remote measurement point to the converter.
1, B2, and A2. A series circuit of constant current source 2 and bias resistor R _b is connected to resistance temperature detector 1 via current supply lead wires A1 and A2.
A constant current i is supplied to as indicated by the arrow in the figure. Then, with the intersection of the constant current source 2 and the bias resistor R _b as the reference potential COM, the voltage detection lead wire B
A generated voltage e1 that changes in response to a change in resistance of the temperature sensing resistor 1 obtained through the temperature sensing resistor 1 is connected to the positive phase input terminal of the differential amplifier 3. In addition, the current supply lead A obtained via the voltage detection lead wire B2
The generated voltage corresponding to the value of the equivalent conductor resistance r4 of 2 is divided by a voltage divider circuit consisting of a resistor R1 and a resistor _R2 , and the added voltage e2 of this voltage and the voltage generated across the bias resistor Rb is a differential voltage. It is connected to the negative phase input terminal of the amplifier 3. Here, as shown in the calculation formula (1), the output voltage e ₀ of the differential amplifier 3 is the input/output conversion gain for the input voltage e2 at the negative phase input terminal, and the input/output conversion gain for the input voltage e1 at the positive phase input terminal. It is configured to have a conversion gain twice the input/output conversion gain. e ₀ =K|e ₁ −2e ₂ | (1) In equation (1), K is the conversion gain of the differential amplifier 3. In addition, ZD is a Zener diode connected to both ends of the constant current source 2, and when the resistance temperature detector 1 is disconnected, the load of the constant current source 2 is opened, and the input terminal of the differential amplifier 3 is connected to the zener diode. This is to prevent excessive voltage from being applied and to protect the constant current source 2 itself. Note that the zener voltage V _z of the zener diode ZD is selected to be V _z >>(r1+R _t +r4+R _b )i so as not to affect the measurement circuit during steady state. Here, R _t is the resistance value of the resistance temperature detector 1. Note that the resistance value R _t of the resistance temperature detector 1 is expressed as in equation (2). R _t = R _tb + ΔR _t ...(2) Here, R _tb is a constant bias resistance value determined by the type of resistance temperature detector and the lower limit of the measurement temperature range,
ΔR _t is the resistance change corresponding to the temperature change from the lower limit of the measurement temperature range determined by the type of resistance temperature detector and the measurement temperature range. Furthermore, when the input impedance of the positive phase input terminal of the differential amplifier 3 is Z1 and the input impedance of the negative phase input terminal is Z2, (r2+R _t +r4+R _b )<<Z1 (r3+r4)<<(R1+R2) [R1・R2/ The input impedances Z1 and Z2 of the differential amplifier 3 are selected to be sufficiently large so as to satisfy the condition (R1+R2)+R _{b ]} <<Z2. In the temperature converter shown in FIG. 1 constructed in this way, first, in a steady state,
Since ZD is in the cut-off state, the constant current i from the constant current source 2 is as shown by the arrow in the figure, r1 → R _t → r4 → R _b
The voltages e1 and e2 applied to the input terminal of the differential amplifier 3 are given by the following equations. e1=( _Rt +r4+ _Rb )i=( _Rtb + _ΔRt +r4+ _Rb )i...(3) e2=r4・i・R2/(R1+R2)+ _Rb・i...(4) Here, the voltage divider circuit If the values of the resistors R1 and R2 are both set equal to R1=R2, the generated voltage r4・
Since i is divided into 1/2, equation (4) is expressed as equation (5). e2=(1/2・r4+R _b )i...(5) The generated voltages e1 and e2 shown by equations (3) and (5) are calculated by the differential amplifier 3 as shown in equation (1). Therefore, e ₀ =K(R _tb +ΔR _t −R _b )i...(6) In equation (6), the value of the bias resistance R _b is calculated as the constant bias resistance R _tb included in the resistance temperature detector 1. If the value of the bias resistor R _b is adjusted to R _tb = R _b (7), then e ₀ =K・ΔR _t・i (8). That is, in steady state, as the output of the differential amplifier 3, as expressed by equation (8), the resistance change ΔR _t in an arbitrary range of the resistance temperature detector, which is not affected by the lead wire resistance, is converted into a desired range. It is possible to obtain the output e ₀ converted into an electrical signal. Next, in the embodiment shown in FIG.
Regarding the operation in various abnormal states of
This will be explained using ~e. Note that the values of the equivalent conductor resistances r1 to _r4 of each lead wire shown in FIG.
The resistances r1 to r4 of the lead wires are omitted in Fig. 2 because they are sufficiently small values compared to the values of the input impedances Z1 and Z2, and there is no problem in explaining the operation in abnormal conditions. . First, when the first current supply lead wire A1 of the resistance temperature detector 1 is disconnected, the circuit shown in FIG. 1 is represented as shown in FIG. 2a. At this time, the constant current i from the constant current source 2 is the zener diode ZD
The voltage e1, expressed by equations (3) and (5), which was applied to the input terminal of the differential amplifier 3 during steady state,
Both e2 becomes zero. Therefore, the output e ₀ of the differential amplifier 3 also becomes zero. Next, when the first voltage detection lead wire B1 of the temperature sensing resistor 1 is disconnected, the circuit shown in FIG. 1 is represented as shown in FIG. 2b. At this time, the constant current i from the constant current source 2 flows in the same path as in the steady state as shown by the arrow in the figure, and the negative phase input terminal of the differential amplifier 3 is A generated voltage e2 similar to that is applied. However, the positive phase input terminal of the differential amplifier 3 having a high input impedance is in an open state, and the output e ₀ of the differential amplifier 3 becomes unstable due to line noise and the like. Further, when the second voltage detection lead wire B2 of the temperature sensing resistor 1 is disconnected, the circuit shown in FIG. 1 is represented as shown in FIG. 2c. In this case, as in the case where the lead wire B1 is disconnected, the constant current i from the constant current source 2 goes around the same path as in the steady state as shown by the arrow in the figure, and the positive phase input terminal of the differential amplifier 3 The generated voltage e1 is the same as in the steady state as shown in equation (3).
is applied. On the other hand, at the negative phase input terminal of the differential amplifier 3, e2=R _b・i・Z2/(R2+Z2)
voltage is applied. As a result, the differential amplifier 3
The output e ₀ of is obtained by substituting the values of e1 and e2 into equation (1) as follows: e ₀ =K[R _tb +ΔR _t +r4+R _b −2R _b ·Z2/(R2+Z2)]i (9). Here, the input impedance Z2 at the negative phase input terminal is a sufficiently large value compared to the value of the resistor R2, and the value of R2 can be ignored. Furthermore, since R _tb =R _b is selected, equation (9) can be expressed as equation (10). e ₀ = K(ΔR _t +r4)i ...(10) In equation (10), the voltage r4・i generated by the lead wire resistance r4 is the voltage ΔR _t generated in response to the resistance change ΔR _t of the resistance temperature sensor. - Since it is a smaller value than i, the output e ₀ of the differential amplifier 3 has a value almost equal to the output voltage in the steady state shown by equation (8). Further, when the second current detection lead wire A2 of the temperature sensing resistor 1 is disconnected, the circuit shown in FIG. 1 is represented as shown in FIG. 2d. The input impedances Z1 and Z2 of the differential amplifier 3 are sufficiently large values, and the zener voltage V _z of the zener diode ZD is selected so that (R _t +R1+R2+R _b )i≫V _z . The constant current i from the constant current source 2 is connected to a zener diode as shown by the arrow in the figure.
It flows only through ZD and generates Zener voltage _Vz . This zener voltage V _z is applied directly to the positive phase input terminal of the differential amplifier 3 via lead wires A1 and B1, and is also applied via a voltage dividing circuit consisting of a resistance temperature detector 1 and resistors R1 and R2. It is applied to the negative phase input terminal of the differential amplifier 3. That is, the respective voltages applied to the input terminal of the differential amplifier 3 are expressed by equations (11) and (12). e1= _Vz ...(11) e2= _Vz・(R2+ _Rb )/ _Rt +R1+R2+ _Rb )...(12) In equation (12), the value of the resistance _Rt of the resistance temperature detector and the bias resistance _Rb The value is sufficiently smaller than the values of the voltage dividing resistors R1 and R2, and R _t and R _b are ignored. Furthermore, since the values of the resistors R1 and R2 are selected to be R1=R2, equation (12) can be expressed as equation (13). e2≒1/2・V _z ...(13) As is clear by substituting the relationship between equations (11) and (13) into equation (1), the differential amplifier 3 in this case
The output voltage e ₀ of is almost zero. Finally, when the resistance wire R _t of the resistance temperature detector 1 is broken, the circuit shown in FIG. 1 is represented as shown in FIG. 2e. In this case, as in the case where the lead wire A2 is disconnected, the constant current i from the constant current source 2 flows only through the zener diode ZD as shown by the arrow in the figure, generating the zener voltage _Vz . This zener voltage V _z is applied to the positive phase input terminal of the differential amplifier 3 via the lead wires A1 and B1, so that e1=V _z . on the other hand,
The voltage e2 at the negative phase input terminal of the differential amplifier 3 becomes zero. Here, since the Zener voltage V _z is selected to be sufficiently large compared to the steady state generated voltage e1 shown in equation (3), by substituting the values of e1 and e2 into equation (1), it becomes clear that As shown, the output e ₀ of the differential amplifier 3 swings to the plus side (upper limit side). Hereafter, this will be referred to as the burnout scale. In this way, in a temperature converter using a conventional 4-wire RTD, when various abnormal conditions occur due to disconnection of the RTD, the direction of the output cannot be determined, and the control system cannot be operated safely in the event of an abnormality. There was a drawback that the direction of movement could not be selected. In particular, as mentioned above, when the voltage detection lead wire B2 is disconnected, it outputs a value that is almost the same as the steady state output value.
There was a possibility that the user would not notice that there was an abnormal condition. In addition, if the current supply lead wire A1 or A2 is disconnected, the output will be zero, and the steady state input resistance ΔR _t = 0 (lower limit value of the temperature measurement range)
This was a serious inconvenience, as it was not possible to distinguish between the output and the output when . The purpose of the present invention is to eliminate the drawbacks of the conventional circuit as described above, and to ensure that the output of the converter is adjusted to the upper limit (plus The object of the present invention is to realize a burnout circuit with a simple configuration, which performs a so-called burnout upscale operation in which the output voltage is swung out to the side (side). The main point of the present invention is to connect a plurality of resistors and a plurality of unidirectional semiconductor elements in series and parallel between a measurement circuit connecting a four-wire resistor to be measured and a converter, and to prevent various types of damage caused by disconnection of the resistor to be measured. When an abnormal condition occurs,
A voltage generated by passing a current from a constant current source to the resistor through the unidirectional semiconductor element for supplying a constant current to the resistor to be measured is applied to the differential converter constituting the converter. In addition to the input terminal of the amplifier, the output is forced to burnout upscale. Hereinafter, the burnout circuit according to the present invention will be explained in detail using the embodiment shown in FIG. Note that in FIG. 3, the same parts as in FIG. 1 are indicated using the same symbols. The differences between FIG. 3 and FIG. 1 are as follows. A resistor R3 is provided between the first voltage detection lead wire B1 and the positive phase input terminal of the differential amplifier 3, and a resistor R4 is provided between the second voltage detection lead wire B2 and the voltage dividing resistor R1. are newly connected. Further, a series circuit of a high impedance resistor R5 and a voltage source 4 having a voltage value V _a is connected between the intersection of the first voltage detection lead wire B1 and the resistor R3 and the common potential COM with the polarity shown, and a second A series circuit of a high impedance resistor R6 and a voltage source 5 having a voltage value (-V _b ) is connected between the intersection of the voltage detection lead wire B2 and the resistor R4 and the common potential COM. Also,
For example, a first unidirectional semiconductor element is connected between the intersection of the first current supply lead A1 and the constant current source 2 and the intersection of the resistor R3 and the positive phase input terminal of the differential amplifier 3. Diode D1 is connected with the polarity shown,
For example, a diode D2 is connected as a second unidirectional semiconductor element between the intersection of the resistor R1 and the resistor R4 and the intersection of the second current supply lead wire A2 and the bias resistor _Rb with the polarity shown. It is. In the embodiment of FIG. 1, the resistors R1 and R2
A voltage divider circuit was constructed, and the values of the resistors R1 and R2 were made equal to divide the generated voltage r4·i corresponding to the equivalent conductor resistance r4 of the lead wire A2 to 1/2. In the figure, resistor R2 is replaced by resistor R
2', resistor R4, resistor R1 and resistor R
2' constitutes a voltage dividing circuit, and the relationship between the respective resistance values is selected to be R2'=R1+R4, so that the generated voltage r4·i is divided into 1/2.
The rest is the same as the embodiment shown in FIG. Here,
The forward breakdown voltage V _D1 of diode D1 is V _D1 > r
1.i, and the forward breakdown voltage V _D2 of the diode D2 is selected to be V _D2 >r4.i. Also,
The value of resistor R5 is R5≫(r2+R _t +r4+R _b )
At the same time, the values of resistor R5 and voltage V a are set so that V _a /R5≪i so that the current of V _a /R5 does not flow from r2 → R _t → r4 → R _b during steady state and cause _a measurement error. are selected. Also, the value of resistor R6 is (r3+r4+R _b )≪(R4+R1+R2')
≪Selected as R6, −V _b /R6 at steady state
The values of the resistor R6 and the voltage (-V _b ) are selected so that |-V _b /R6|<<i, so that the current does not flow from r3→r4→R _b and cause a measurement error. In the embodiment of FIG. 3 configured in this way, in a steady state, both diodes D1 and D2 are in a cut-off state, and the values of resistors R5 and R6 are also high impedance, V _a /R5, -V _b /R6. The current can also be ignored. Therefore, in steady state, the diodes D1 and D2, the resistors R5 and R6, and the voltage source 4,
5 is equivalent to being removed from the circuit and does not affect the measurement circuit. That is, as in the embodiment shown in FIG. 1, the constant current i from the constant current source 2 flows in the direction of r1 → R _t → r4 → R _b as shown by the arrows in the figure, and its operation and effect are also the same as in the embodiment shown in FIG. 1. This is the same as detailed in the example. That is, in a steady state, the generated voltages e1 and e2 applied to the positive phase input terminal and the negative phase input terminal of the differential amplifier 3, respectively, are as described above.
The values shown in equations (3) and (5) are the same, and the output e ₀
is also given by equation (8), and accurate measurement conversion is performed without being affected by the resistance of the lead wire. Next, we will explain the operation of the resistance temperature detector in various abnormal states in the embodiment shown in FIG.
This will be explained using e. Here, the values of the equivalent conductor resistances r1 to r4 of each lead wire shown in FIG. 3 are as follows:
This is a small value compared to the values of circuit resistances R1, R2', R3 to R6, and _Rb , and there is no problem in explaining the operation in an abnormal state, so the lead wire resistances r1 to r4 are are omitted. Note that, as is clear from the relationship between equations (1) and (8), the conditions for ensuring that the output e ₀ of the differential amplifier 3 reaches burnout upscale are as shown in equation (14) when an abnormality occurs. All you have to do is satisfy. e1-2・e ₂ >ΔR _ts・i (14) Here, ΔR _ts is the resistance value change span of the resistance temperature detector 1 corresponding to the temperature change span from the lower limit value to the upper limit value of the measurement temperature range. be. First, when the first current supply lead wire A1 of the resistance temperature detector 1 is disconnected, the circuit shown in FIG. 3 is represented as shown in FIG. 4a. Here, compared to the resistance value R _t of the resistance temperature detector 1, the input impedance Z1 of the positive phase input terminal of the differential amplifier 3 and the resistance R
5 is very large, and the value of the resistor R6 is also very large compared to the value of the bias resistor _Rb , so the constant current i from the constant current source 2 is D1 as shown by the arrow in the figure.
→ R3 → R _t → R _b and completes a cycle. The zener voltage V _z of the zener diode ZD is V _z > V _D1 + (R3
+R _t +R _b ) i is selected, Zener diode ZD
is in a blocked state. Therefore, the voltage e1 applied to the positive phase input terminal of the differential amplifier 3 becomes e1=(R3+R _t +R _b )i (15) if the lead wire resistance is ignored. In addition, the voltage R _b・i generated across the bias resistor R _b is the voltage R2′ of the voltage divider circuit and (R1+
It is applied to the negative phase input terminal of the differential amplifier 3 through the parallel combined resistance of R4), but the input impedance Z2 of the negative phase input terminal is very large compared to the value of the parallel combined resistance, so the lead wire resistance is ignored. Then, e2=R _b・i (16). By substituting the relationships between equations (15) and (16), equations (2), and equations (7) into equation (14), which is the burnout/upscale conditional equation, equation (17) is obtained. (R3+ΔR _t )i>ΔR _ts・i (17) As is clear from equation (17), even when the input, which is the worst condition for burnout operation, is the lower limit of the measurement range, that is, ΔR _t = 0, By selecting the value of the resistor R3 to be larger than the value of the measurement input span resistance ΔR _ts , it is possible to satisfy equation (17) and swing the output _e0 of the differential amplifier 3 to the positive side. , can instantly cause burnout and upscaling. For example, a platinum resistance temperature detector is used as the resistance temperature detector 1.
If JIS P _t 100Ω is used and the measurement temperature range is 0 to 500°C, the resistance change is 100Ω to 284.02Ω, so the resistance change span is ΔR _ts =184.02Ω. Therefore, the value of the resistor R3 may be set to 184.02Ω or more, for example, 220Ω. Next, when the first voltage detection lead wire B1 of the temperature sensing resistor 1 is disconnected, the circuit shown in FIG. 3 is represented as shown in FIG. 4b. At this time, the constant current i from the constant current source 2 flows as shown by the arrow in the figure, as in the steady state, and the voltage e2 applied to the negative phase input terminal of the differential amplifier 3, ignoring the lead wire resistance. For example, as in the case of the breakage of the lead wire A1, the voltage is expressed by equation (16). On the other hand, the voltage V _a of the voltage source 4 is applied to the positive-phase input terminal of the differential amplifier 3 via the resistor R5 and the resistor R3, but the value of the resistor R5 and the positive-phase input terminal Since the input impedance Z1 of is very large, if the resistance R3 is ignored, e1=V _a ·Z1/(R5+Z1).
The relationship between the relational expressions of these e1 and e2 and equation (7) is expressed as (14)
Substituting into equation (18) is obtained. V _a・Z1/(R5+Z1) > (ΔR _ts +2R _tb )i...(18) In other words, the resistor R5,
By selecting the value of the voltage V _a , the output of the differential amplifier 3 can be rapidly brought to burnout and upscale. For example, if the resistance temperature detector 1 is a platinum resistance temperature detector JIS P _t 100Ω and the temperature measurement range is 0 to 500°C, ΔR _ts =184.02Ω, =100Ω, as in the above-described specific example. Also, constant current i of constant current source 2 = 2mA,
If the input impedance of the positive phase input terminal of differential amplifier 3 is Z1 = 10MΩ, the right side of equation (18) is (ΔR _ts + 2R _tb )i = (184.02Ω + 2 × 100Ω) × 2mA≒0.77V. Become. Therefore, for example, if the value of resistor R5 is
1MΩ, and if the voltage V _a of voltage source 4 is set to 1V (18)
The left side of the equation is V _a・Z1/(R5+Z1)=1V×
10MΩ/(1MΩ+10MΩ)≒0.91V,
Equation (18) is satisfied, and the output _e0 of the differential amplifier 3 swings to the positive side, resulting in burnout upscale. Further, when the second voltage detection lead wire B2 of the temperature sensing resistor 1 is disconnected, the circuit shown in FIG. 3 is represented as shown in FIG. 4c. In this case as well, the constant current i from the constant current source 2 flows as shown by the arrow in the figure as in the steady state, and the voltage e1 applied to the positive input terminal of the differential amplifier 3 is the same as in the steady state. For example, e1(R _t +R _b )i. On the other hand, the voltage e2 applied to the negative phase input terminal is applied to the resistors R1, R2', R6
Since the value _of resistance R _b is small compared to the value of
+R1+R2')+R _b・i. These e1,
By substituting the relationship of e2 and the relationships of equations (2) and (7) into equation (14) and rearranging, equation (19) is obtained. -2(-V _b )R2' / (R6+R4+R1+R2') > (ΔR _ts -ΔR _t ) i ...(19) In equation (19), if we consider the worst condition for burnout operation, the input is at the lower limit of the measurement range. When the value, that is, ΔR _t = 0, equation (19) becomes (20)
It looks like the formula. -2 (-V _b ) R2' / (R6 + R4 + R1 + R2') > ΔR _ts・i ... (20) In other words, the resistor R6,
By selecting the value of the voltage (-V _b ) of the voltage source 5, burnout and upscaling can be achieved quickly. For example, as in the previous example, ΔR _ts = 184.02
If Ω, i = 2mA, the right side of equation (20) is approximately 0.37V
becomes. Here, the value of the resistance of the voltage dividing circuit is (R4+
R1) = R2' = 50kΩ, and the value of resistor R6 is
2.2MΩ, if the voltage (−V _b ) value is 10V (20)
The left side of the equation is -2(-V _b )R2'/(R6+R4+
R1+R2')=-2(-10V)50kΩ/(2.2MΩ
+50kΩ+50kΩ)≒0.43V, satisfying the condition of equation (20), and the output of differential amplifier 3 swings to the positive side, resulting in burnout upscale. Furthermore, when the second current supply lead wire A2 of the temperature sensing resistor 1 is disconnected, the circuit shown in FIG. 3 is represented as shown in FIG. 4d. The value of the resistor _R5 and the input impedance Z1 at the positive input terminal of the differential amplifier 3 are very large compared to the resistance value R t of the resistance temperature detector, and the value of the resistor R4 is very large compared to the value of the resistor R6. Since it is very small, the constant current i from the constant current source 2 at this time is as shown by the arrow in the figure, R _t →R4→
It flows from D2 to R _b and completes a cycle. Therefore, the voltage e1 applied to the positive phase input terminal of the differential amplifier 3 is e1=(R _t +R4+R _b )i, if the lead wire resistance is ignored.
+V _D2 . Here, the zener voltage V _z of the zener diode ZD is selected to satisfy V _z >(R _t +R4+R _b )i+V _D2 , and the zener diode ZD is in a cutoff state. On the other hand, the resistor R4 of the voltage dividing circuit is R1
This value is smaller than the value of R1≈R2'.
Therefore, the breakdown voltage V _D2 of diode D2 is R
The pressure is divided into 2'/(R1+R2')≒1/2. Therefore, the voltage e applied to the negative phase input terminal of the differential amplifier 3
2 becomes e2≈V _D2 /2+R _b ·i. By substituting the relationships between these voltages e1 and e2 and equations (2) and (7) into equation (14) and rearranging, equation (21) is obtained. (R4+ΔR _b )i>ΔR _ts・i ...(21) In equation (21), the worst condition for burnout operation is when ΔR _t = 0 as mentioned above, so (21)
The equation is shown by equation (22). R4・i>ΔR _ts・i (22) In other words, as is clear from equation (22), by selecting the value of resistor R4 larger than the value of measurement input span resistance ΔR _ts , burnout can be prevented. Can be scaled up. For example, if ΔR _ts = 184.02Ω as in the previous example, by setting the value of the resistor R4 to a higher value, for example, R4 = 220Ω, the output of the differential amplifier 3 can be instantly swung to the positive side. can be cut,
Burnout can be scaled up. Finally, when the resistance wire R _t of the resistance temperature detector 1 is disconnected, the circuit shown in FIG. 3 is represented as shown in FIG. 4e. Value of resistor R5 and differential amplifier 3
Since the input impedance Z1 at the positive phase input terminal of is very large, the constant current i from the constant current source 2 at this time flows only through the zener diode ZD as shown by the arrow in the figure, generating a zener voltage _Vz across it. . And this zener voltage _Vz is the lead wire A1, B1
and is applied to the positive phase input terminal of the differential amplifier 3 via the resistor R3, but since the input impedance Z1 is very large compared to the value of the resistor R3, e1= _Vz . On the other hand, the voltage source 5 causes a current of -V _b /R6 to flow through the bias resistor through lead wires B2 and A2.
R _b , and the voltage at both ends is applied to the negative phase input terminal of the differential amplifier 3, but this voltage is extremely small and can be ignored, e2≈0. Therefore, the differential amplifier 3 has an excessive zener voltage V _z applied to the positive phase input terminal.
The output immediately swings to the positive side, resulting in burnout and upscaling. In the embodiment shown in FIG. 3, the zener diode
As mentioned above, when there is a ZD, when the resistance temperature detector 1 is disconnected, the load of the constant current source 2 is opened, and an excessive voltage is applied to the input terminal of the differential amplifier 3. This is provided to prevent the constant current source from being damaged and to protect the constant current source itself, and is not necessarily necessary depending on the circuit configuration of the constant current source 2 and the differential amplifier 3. The presence or absence of the zener diode ZD is related to Figure 4a.
As is clear from reference to .about.e, this only applies to the case of FIG. 4e, that is, when the resistance wire R _t is broken. Therefore, the zener diode ZD
Regarding the case where the resistor wire R _t is disconnected when is removed, in FIG. Constant current source 2
The load is substantially equivalent to an open circuit, and no constant current i flows. Therefore, a power supply voltage for driving the constant current source 2 (not shown) appears across the constant current source 2, and this excessive voltage is applied to the lead wires A1, B1 and the resistor R3.
It is added to the positive phase input terminal of the differential amplifier 3 via. Therefore, the output is still burnout upscale. In addition, in the embodiment shown in FIG. 3, the resistance temperature detector 1 has a constant bias resistance R _tb , and in order to subtract this bias resistance, a bias resistance R _b is added to the measurement circuit.
Although the case has been described in which the bias resistor R _b is provided, the presence of this bias resistor R b is not necessarily required for the operation of the burnout circuit of the present invention. For convenience, the present invention has been described with respect to the case of inputting a resistance temperature detector, but it can be applied to any case where resistance changes such as slip resistance input are measured. For example,
When the measurement input range is 0 to 100Ω with a slip resistance input, the resistor under test does not include a constant bias resistance, R _tb = 0, so in this case, the bias resistance R _b is naturally unnecessary (R _b = 0 ). In this case as well, the burnout operation of the present invention is not restricted in any way. In the embodiment shown in FIG. 3, the generated voltage r4·corresponding to the value of the resistance r4 of the lead wire A2 is
After i is divided into 1/2 by a voltage dividing circuit, it is added to the voltage R _b・i generated across the bias resistor R _b , and this added voltage e2=r4・i/2+R _b・i
is added to the negative phase input terminal of the differential amplifier 3 and multiplied by K・2e _{by 2} as shown in equation (1), resulting in K(r4・i+2R _b・
i). That is, if we focus on only the (r4·i) term, the result is only that the gain is multiplied by K. Therefore, like the slip resistance input mentioned above,
When R _tb = R _b = 0, remove the 1/2 voltage divider circuit (resistors R1 and R2' in the embodiment shown in Fig. 3) and change the calculation formula for the differential amplifier 3 to e ₀ = K | e1 An equivalent result can be obtained by setting −e2|. Of course, the burnout circuit of the present invention can also be applied to this case. In the embodiment shown in FIG. 3, the polarity of the input terminal of the differential amplifier 3 is determined by the first voltage detection lead wire B.
The side to which the generated voltage e1 obtained through 1 is applied is connected to the positive phase input terminal (+), and the second voltage detection lead wire B2
Although the side to which the generated voltage e2 obtained through is applied is the negative phase input terminal (-), it is also applicable to a case where the polarity is completely reversed as shown in parentheses in FIG. 3. The calculation formula for the differential amplifier 3 in this case is e ₀
=K(2e ₂ −e ₁ ). Therefore, if the resistance input ΔR _t of the RTD increases during steady state, the output e ₀
increases on the negative side. Similarly, in all burnout operations during various abnormal states due to disconnection, the output swings to the negative side, but since the direction of the output matches the direction of the output when the input ΔR _t increases, it is still a burnout upscale. In addition, in the embodiment shown in FIG. 3, the negative side of the constant current source 2 is connected to the reference potential COM.
This can also be applied when the polarity is reversed to that shown in the figure. However, in this case, the diodes D1, D
2. The polarities of the voltage sources 4 and 5 and the Zener diode ZD may all be connected in opposite polarities to those shown in the figure. Furthermore, in the embodiment shown in FIG. 3, one ends of the voltage sources 4 and 5 are connected to the reference potential COM, but both are connected to the intersection of the bias resistor R _b and the second current supply lead wire A2. is more suitable.
However, if it is connected to the reference potential COM as in the embodiment of FIG. 3, it is convenient because the power supply voltage for driving the differential amplifier 3 (not shown) can be shared as the voltage sources 4 and 5. In the embodiment shown in FIG. 3, it should be noted that since the burnout circuit according to the present invention is added, in order to prevent measurement errors in steady state and to ensure burnout operation, , the input impedance of the differential amplifier 3 is several M
It is necessary to have a high impedance of Ω or more.
For this purpose, for example, a specific circuit example of the differential amplifier 3 as shown in FIG. 5 is suitable. According to the circuit configuration shown in FIG. 5, the non-inverting input terminals of the two operational amplifiers 6 and 7 are both input terminals for the input voltages e1 and e2, so that a very large input impedance can be obtained. In addition, in FIG. 5, the resistors R7 to R1
The value of 0 is R8=R9, R7=R9・R, respectively.
10/(2R9/R10), the output e ₀ of the operational amplifier 6 is e ₀ =(1+R10/R9)(e ₁
-2e ₂ ), which agrees with equation (1), and the specific circuit example shown in FIG. 5 can be applied as is to the differential amplifier 3. As described above, according to the burnout circuit according to the present invention, when various abnormal conditions occur due to disconnection of the resistor to be measured, the output can always be set to burnout/upscale, and the control system can be operated safely. You can decide the direction. Moreover, since the constant voltage sources 4 and 5 can be shared with the power supply voltage for driving the differential amplifier 3, only a few general resistors and two diodes that do not require high precision are added, making the circuit configuration simple and inexpensive. This is useful for industrial measurement.

[Brief explanation of the drawing]

Figure 1 is an example of a conventional resistance thermometer type temperature converter that does not have a burnout circuit, and Figure 2 a~
e is an equivalent circuit diagram at the time of abnormality of the embodiment shown in FIG. 1, FIG. 3 is a diagram of an embodiment of a resistance temperature detector type temperature converter equipped with a burnout circuit according to the present invention, and FIGS. 4 a to e is an equivalent circuit diagram of the embodiment shown in FIG. 3 at the time of abnormality, and FIG. 5 is a specific circuit diagram of the differential amplifier. DESCRIPTION OF SYMBOLS 1...Resistance temperature sensor, 2... Constant current source, 3... Differential amplifier, 4, 5... Voltage source, 6, 7... Arithmetic amplifier.

Claims (1)

[Claims] 1. A first current supply lead wire and a first voltage detection lead wire are branched at one end, and a second lead wire is provided at the other end.
A four-wire resistor to be measured has a branched current supply lead wire and a second voltage detection lead wire,
A current is caused to flow from a constant current source through the first and second current supply lead wires, and the side through the second current supply lead wire is set as a reference potential, and the first voltage detection lead wire a generated voltage that changes in accordance with the resistance change of the resistor to be measured obtained through the
from a differential amplifier having a high input impedance that receives as input a generated voltage corresponding to the resistance value of the second current supply lead obtained through the second voltage detection lead; In the resistance/electrical signal converter, when the first current supply lead wire of the resistor to be measured is disconnected, the current from the constant current source is passed through the first unidirectional semiconductor element to the first current supply lead wire. When the second current supply lead wire of the resistor to be measured is disconnected, the current from the constant current source is passed through the second unidirectional semiconductor. The voltages generated by passing the voltages through the second resistors through the elements are applied to the inputs of the differential amplifiers, and the voltages to be measured are applied so as to swing the output signals of the differential amplifiers to the upper limit. 4, characterized in that the first and second resistors and the first and second unidirectional semiconductor elements are connected in series and in parallel between the measurement circuit connecting the resistor and the differential amplifier. Burnout circuit for wire-type resistor under test. 2. In claim 1, the first
the first resistor between the voltage detection lead wire and the first input terminal of the differential amplifier; and the second voltage detection lead wire and the second input terminal of the differential amplifier. The second resistor is inserted between the ends, and the first current supply lead wire and the constant current source intersect, and the first resistor and the first input end of the differential amplifier. and between the intersection of the second resistor and the second input terminal of the differential amplifier and the common potential. the second direction in the same direction as the unidirectional semiconductor element;
a third resistor that connects the unidirectional semiconductor elements, and has a high impedance between the intersection of the first voltage detection lead wire and the first resistor and the reference potential, and a first voltage source; and a fourth resistor 6 having a high impedance between the intersection of the second voltage detection lead wire and the second resistor and the reference potential and the first voltage source have opposite polarity. are connected in series with a second voltage source, so that when various abnormal conditions occur due to disconnection of the resistor to be measured, the output signal of the differential amplifier always oscillates to the upper limit. A burnout circuit for a 4-wire resistor to be measured, which is characterized in that it can be disconnected. 3 In claim 2, the second claim
A voltage dividing circuit is inserted between the intersection of the resistor and the second unidirectional semiconductor element and the second input terminal of the differential amplifier, and The input/output conversion gain on the input end side of is configured to have a conversion gain twice that of the input/output conversion gain on the first input end side, and the voltage dividing circuit
A burnout circuit for a four-wire resistor to be measured, characterized in that it is configured to divide the voltage by 1/2 including the resistance of the resistor.