JPH03225233A - Burnout circuit for 3-wire resistor under test - Google Patents

Abstract

PURPOSE:To detect the disconnection of a resistor to be measured and to forcedly distribute the output of a microprocessing unit (MPU) to the upper or lower limit by monitoring the changing rate of a voltage corresponding to voltage detecting terminal by the MPU. CONSTITUTION:A constant current source 2 is supplied to a temperature measuring resistor 1 through lead wires A, B2 and a constant current (i) is returned to the source 2 through a COM line 9. Voltage generated on the lead wire A is impressed to the input terminal e1 of a differential amplifier 4 having high impedance through a low pass filter(LPF) circuit consisting of a resistor R1 and capacitor C1. Then, the output e01 of the amplifier 4 is converted into a digital signal by an A/D converter 7 through an input switching circuit 6 for switching plural analog input lines and the ditial signal is fetched to the MPU 8. Voltage generated on a voltage detecting lead wire B1 by regarding the COM line 9 as a reference potential is impressed to the input terminal e2 of a differential amplifier 5 through an LPF circuit consisting of a resistor R2 and capacitor C2 and the output e02 of the amplifier 5 is converted into a digital signal by the A/D converter 7 through the circuit 6 and the digital signal is fetched to the MPU 8.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a burnout circuit for a temperature/electrical signal converter using a three-wire resistor to be measured, and particularly to a burnout circuit when the resistor to be measured is disconnected. , relates to a burnout circuit that operates to swing the output to the positive or negative side.

[Conventional technology]

In general, in automatic process control systems, if a resistance temperature detector breaks due to fatigue or damage, it must be detected and some measure taken to ensure that the control system operates safely. . For this reason, in a resistance temperature detector type temperature/electrical signal converter, in order to detect a disconnection of the resistance temperature detector, an operation is performed to swing the output signal outside the standard signal range on either the positive side or the negative side. It is desirable to have a circuit to do this. This is generally called a burnout circuit.

On the other hand, the main requirements for a resistance temperature detector-type temperature/electrical signal converter are (1) the resistance of the conductor to connect the resistor to be measured installed at the point to be measured and the converter installed in the control room; Being able to automatically compensate for the impact.

(2) Predetermined unified electric signal corresponding to any measurement range (for example, using JISC1604 platinum resistance temperature detector Pt100ΩatO°C as the resistor to be measured, measurement range 0
When measuring a temperature of ~50°C, it is necessary to convert an input resistance change of 100~120Ω into an electric signal of DC4~20mA and output it. Therefore, in the above example, the zero point reference resistance 1 is equivalent to 5 times the input range of 20Ω.
It is necessary to be able to cancel out 00Ω by electrically biasing.

Conventionally, circuit systems for achieving the above two requirements include a bridge system, a voltage drop system using an adder/subtractor, or a differential amplifier. The third embodiment of the burnout circuit in the latter voltage drop type resistance temperature detector type temperature/electrical signal converter is shown in the third example.
As shown in the figure. 1 is a three-wire temperature measuring resistor having a current supply lead wire A at one end and a voltage detection lead wire B1 and a common lead wire B2 at the other end, and resistors r1 to r3 are connected remotely. This is the equivalent conductor resistance of each of the lead wires A, Bl, and B2 from the resistance temperature detector 1 to the converter installed at the measurement point. 2 is a constant current source, and the constant current i is connected to the lead wires A and B.
2. The constant current source 2 is connected as the COM line 4 via the resistor R5.
Return to The voltage generated on the current supply lead A with the COM line 4 as a reference potential is applied to the positive-phase input terminal of the differential amplifier 3 having a high input impedance via a low-pass filter circuit consisting of a resistor R1 and a capacitor C1. The voltages generated in the voltage detection lead wire B1 are applied to the anti-phase input terminal of the differential amplifier 3 having a high input impedance via a low-pass filter circuit consisting of a resistor R2 and a capacitor C2. Here.

The differential amplifier 3 performs the calculation as shown in equation (1), where the COM line 4 is a reference potential, the voltage applied to the positive phase input terminal is el, the voltage applied to the negative phase input terminal is ez, and the output voltage is eO. It is configured to do the following.

eo=G(et-2ez)...11
) In equation (1), G is the conversion gain of the differential amplifier 3. Here, the differential amplifier 3 has a circuit configuration shown in FIG. 5, and in FIG. 5, 11.12 is the first . The second operational amplifier R11 to R14 are resistors and are connected as shown.

In Figure 5, R12=R13 ・R13+R14 given that Therefore, the input/output relationship is as shown in equation (1).

In FIG. 3, 5 is a burnout operation direction switching circuit that performs an operation to swing the converter output to the positive side (hereinafter referred to as burnout upscaling) when the resistance temperature detector 1 becomes abnormal. This selects whether to perform a burnout downscaling operation or to perform an operation to shake it out to the negative side (hereinafter referred to as burnout downscaling). Reference numeral 6 denotes a constant voltage source for burnout/upscaling, which is connected between the COM line 4 and the U12 terminal of the burnout operation direction switching circuit 5 with the polarity shown. Reference numeral 7 denotes a constant voltage source for burnout/downscaling, which is connected to the COM line 4 and the D1212 terminal of the burnout operation direction switching circuit 5 with the illustrated polarity. U of burnout operation direction switching circuit 5
The ll and Dll terminals are connected to the intersection of the lead wire B1 and the resistor R2 via the high resistance R3, the D21 terminal is connected to the intersection of the node line B1 and the resistor R2, and the U21 terminal is connected to the opposite side of the differential amplifier 3. At the phase input end, U22 and B22 are each connected to the intersection of the lead wire B2 and the resistor R5 via a diode D3.

Furthermore, the intersection of the lead wire A and the resistor R1 is connected to the negative phase input terminal of the differential amplifier 3 via a series circuit of diodes Di, D2, and resistor R4.

In the temperature/electrical signal converter of FIG. 3 constructed in this manner, the resistance value Rt of the temperature measuring resistor 1 is expressed by the following equation.

Rt=Rtb+ΔRt...(2
) Here, Rtb is a constant bias resistance value (zero point reference resistance) determined by the type of resistance thermometer and the lower limit of the measurement temperature range, and ΔRt is the temperature from the type of resistance thermometer and the lower limit of the measurement temperature range. This is the resistance change corresponding to the change.

First, in a steady state, the voltage el applied to the positive phase input terminal of the differential amplifier 3 is e+=(r 1 +Rt b+ΔRt + r 3+R
5) i...(3) Also, the voltage e2 applied to the negative phase input terminal is ez=cr3+R5)i - (4)
Here, using the same wire type, same wire diameter, and same wire length as the lead wires A, Bl, and B2, r=rl=r2
= r33), and substituting equation (4) into equation (1), eo=G(Rt b+ΔRt −R5)i = 4
5) In equation (5), the constant bias resistance Rub included in the resistance temperature detector 1 and the value of the bias resistance R5 are set equal to R.
If tb=R5 is selected, equation (6) is obtained.

eo=G・ΔRt-1 (6) As described above, in the steady state, the output of the differential amplifier 3 is the resistance rl of the lead wire as shown in equation (6).
, r2 and r3, it is possible to convert the resistance change ΔRt of the resistance temperature sensor in any range into an electrical signal in a desired range.

Here, (Rt + r l ) i <(VD1+
The forward breakdown voltage V of diode D1 is
Since the forward breakdown voltage VD2 of DI and the diode D2 is selected, no current flows through the series circuit consisting of the diodes Di, D2, and the resistor R4, and the circuit is immediately in an open state. Or, since the forward breakdown voltage Voa of the diode D3 is selected so that r3・i<Voa, the diode D3 is in an open state and a high resistance R3 is selected, so that the voltage is determined by the constant voltage source E1 or E2. There is no impact.

Next, the operation of the temperature measuring resistor 1 in various abnormal states in the temperature/electrical signal converter shown in FIG. 3 will be explained using FIG. 4. Note that the values of the equivalent resistances rl, r2, and r3 of each lead wire shown in FIG. 3 are the circuit resistance R.
1, is a sufficiently small value compared to the values of R2 and R5,
The resistances r1 to r3 of the lead wires are omitted in FIG. 4 so as not to hinder the explanation of the operation in an abnormal state.

First, FIGS. 4(a) to 4(d) describe the operation of the resistance temperature detector 1 in various abnormal states when the burnout operation direction switching circuit 5 shown in FIG. 3 is selected for burnout upscaling. Explain using. At this time, the burnout operation direction switching circuit 5 connects the Ull terminal and the U12
12 terminal, U21 terminal and U2222 terminal are shorted, and D
Open the ll terminal and the D1212 terminal and the D21 terminal and the D22 terminal. Note that the output eo of the differential amplifier 3
The conditions for ensuring burnout and upscaling are:
As is clear from equations (1) and (6), it is expressed by equation (7).

(el-2e2)>ΔRts-1...(7) Here,
ΔRts is the resistance 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.

First, when the wire of the resistance temperature detector 1 is disconnected, the circuit shown in FIG. 3 is expressed as shown in FIG. 4 (,). At this time,
Since the positive phase input terminal of the differential amplifier 3 has high impedance, the constant current i from the constant current source 2 is as shown by the arrow in the figure.
It flows from D1→D2→R4→D3→R5. Therefore, the voltage e1 applied to the positive phase input terminal of the differential amplifier 3 is expressed as follows.

e1=Vot+Voz+Vo3+(R4+R5)i-(
8) Here, Vol+ Vo2. Voa are connected to diodes Di, D2 . This is the forward breakdown voltage of D3.

On the other hand, the voltage e2 applied to the negative phase input terminal of the differential amplifier 3 is expressed as follows.

e z: Vo3+ R5・i −(
9) Here, the diodes Di, D2 . Select D3, R4=R5
Select resistors R4 and R5 so that (8).

By substituting equation (9) into equation (7), we get Vo>ΔRt s−i −(to
) where the diodes DI, D2 . Forward breakdown voltage of D3 Vost Vo2. If Voa is selected to be a sufficiently large value compared to ΔRtS-1, the output voltage e of the differential amplifier 3. can swing to the positive side.

Next, when the current supply lead wire A of the temperature sensing resistor l is disconnected, the circuit shown in FIG. 3 is expressed as shown in FIG. 4(b). This is the same operation as in the case where the strands of the temperature sensing resistor 1 are disconnected, so the explanation will be omitted.

Next, when the voltage detection lead wire B1 of the resistance temperature detector 1 is disconnected, the circuit shown in FIG. 3 is expressed as shown in FIG. 4(c). At this time, the constant current i from the constant current source 2 is
As in the steady state, the resistance R of the resistance temperature detector 1 is increased as shown by the arrow in the figure.
t, the voltage e1 flowing through the bias resistor R5 and applied to the positive phase input terminal of the differential amplifier 3 is the same as the normal voltage in the steady state shown by equation (3). On the other hand, the voltage e expressed by equation (4) that was applied to the negative phase input terminal of the differential amplifier 3 during steady state
2, when the lead wire B1 is disconnected, the constant voltage source 6 (El) connected in series with the resistor R3 causes C2→R2→R3.
Since the current iB flows and (R2+R3)<Z2, . 2=E 2 (1, (R2+R31・),
= (11, where Z2 is the input impedance of the negative phase input terminal of the differential amplifier 3. The conditions for burnout and upscaling of the output eo of the differential amplifier 3 are (
As shown in equation 7), substitute equation (11) into equation (7)...(12) Here, considering that the burnout time is generally 60 seconds or less, the constant voltage Source E2. By selecting resistors R3 and R2, burnout and upscaling can be performed after an arbitrary time.

Next, when the common lead wire B2 of the resistance temperature detector 1 is disconnected, the circuit shown in FIG. 3 is expressed as shown in FIG. 4(d). This is the same operation as in the case where the current supply lead wire A of the temperature sensing resistor 1 is disconnected, so a description thereof will be omitted.

Next, the operation of the resistance temperature detector 1 in various different states when the burnout operation direction switching circuit 5 of FIG. 3 is set to burnout/downscale will be explained in FIGS. This will be explained using h). At this time, the burnout operation direction switching circuit 5 connects the Dll terminal to the D12 terminal.
Short-circuit the 12 terminal, the D21 terminal and the D2222 terminal,
Ull terminal and U12 terminal, and U21 terminal and U222
Open 2 terminals. Note that the output e of the differential amplifier 3.

In order to ensure burnout and downscaling,
When the value of eo in equation (1) is ΔRt=0 in equation (6), e
. A value smaller than the value of is immediately expressed by the condition of equation (12).

(e 1-2 e z)<O→el<2ez
(12) First, when the wire of the resistance temperature detector 1 is disconnected, the circuit shown in FIG. 3 is expressed as shown in FIG. 4(e). At this time, the input impedance of the positive phase and negative phase input terminals of the differential amplifier 3 is very high, and the bias resistor R5
Since the value of resistor R3 is selected to have high impedance compared to the value of , the constant current from constant current source 2? ! ! Flow 1 is
The flow flows in the order of D1→D2→R4→R2→R5 as indicated by the arrows in the figure. The voltage e1 applied to the positive phase input terminal of the differential amplifier 3 and the voltage e2 applied to the negative phase input terminal are respectively expressed by the following equations.

(13) Also, since <, R5=Rtb as described above, this relationship and equation (13) are substituted into the condition of equation (12) to obtain equation (14).

VL11+VD2+(R4-Rt b)i <R2・i
...(14) Burnout/downscaling can be achieved by selecting the value of the resistor R2 so as to satisfy the condition of equation (14).

Next, when the current supply lead wire A of the resistance temperature detector 1 is disconnected, the circuit shown in FIG. 3 is expressed as shown in FIG. 4(f). This is the same operation as in the case where the strands of the resistance temperature detector are disconnected, so the explanation will be omitted.

Next, when the voltage detection lead wire B1 of the resistance temperature detector 1 is disconnected, the circuit shown in FIG. 3 is expressed as shown in FIG. 4(g). The constant current i from the constant current source 2 at this time is
As in the steady state, the resistance Rt of the resistance temperature detector 1 is as shown by the arrow in the figure.
, the voltage e1 flowing through the bias resistor R5 and applied to the positive-phase input terminal of the differential amplifier 3 is the same as the normal voltage in the steady state shown by equation (3). On the other hand, the voltage e2 expressed by equation (4) that was applied to the negative phase input terminal of the differential amplifier 3 during steady state
When the lead wire B1 is disconnected, the constant voltage source 7 (R2) connected in series with the resistor R3 causes R3→R2→C2.
and current iB flows, and since (R2+R3)<22, e 2= E 1 (1-E (R2+ps+ts)
−(15). The conditions for burning out and downscaling the output eo of the differential amplifier 3 are as shown in equation (12), so by substituting equation (15) into equation (12), the burnout time is generally 60 If you select the constant voltage source El and the resistor R3°R2 taking into account that the time is less than 2 seconds,
Can be burnout/downscaled after an arbitrary amount of time.

Next, when the common lead wire B2 of the resistance temperature sensor 1 is disconnected, the circuit shown in FIG. 3 is expressed as shown in FIG. 4(h). Compared to the value of bias resistor R5, the value of resistor R3 is very large, and moreover, (Vo3+Rt b・1)(E
1, the constant current 1 from the constant current g2 at this time flows from Rt-+D3 to R5 as shown by the arrow in the figure. Then, the voltages e1 and e applied to the differential amplifier 3
2 is expressed by equation (17).

Here, equation (2) and the relationship R5=Rtb and (17)
By substituting the equation into the condition of equation (12), equation (18) is obtained.

ΔRt-i < VO2- (18) Therefore, burnout downscaling can be achieved by satisfying the condition of equation (18).

[Problem to be solved by the invention] As mentioned above, temperature and temperature measurement using the conventional three-wire resistor to be measured
In electrical signal converters, burnout circuits used when various abnormal conditions occur due to disconnection of a resistor to be measured have a large number of components, are complex, cannot be miniaturized, and have high costs.

Furthermore, it is necessary to select the value of the bias voltage or the like depending on the input span, and it is necessary to change the voltage of the constant voltage source for each input span. Furthermore, it was necessary to use a switch to switch between up mode and down mode, resulting in poor work efficiency.

The object of the present invention is to measure temperature and
In order to solve the above-mentioned drawbacks of conventional circuits in electrical signal converters, if the wires of the resistance temperature detector or any of the lead wires are disconnected, the output of the converter is changed to the standard direction in the specified direction. A burnout function that allows the signal to go out of the range,
The goal is to realize this with a simple circuit configuration.

[Means to solve the problem]

The gist of the present invention is to use an MPU for compensating the wiring resistance of the resistor to be measured, and to install two resistors and a unidirectional semiconductor element between the measuring circuit connecting the resistor to be measured and the amplifier. A series-parallel circuit is provided, and the current direction from a constant current source for supplying a constant current to the resistor to be measured is determined when the measuring circuit is opened due to disconnection of the strands or lead wires of the resistor to be measured. A disconnection can be detected by the MPU by switching the unidirectional semiconductor element and the resistor to flow the voltage to the circuit, changing the applied voltage corresponding to the voltage detection terminal of the resistor to be measured in a fixed direction. be.

[Effect]

That is, since the wiring resistance from the resistor to be measured is constant and the voltage at the voltage detection terminal of the resistor to be measured under normal conditions is also constant, the rate of change of the voltage corresponding to the voltage detection terminal is By monitoring with the MPU, disconnection of the resistor to be measured can be easily detected, and the output of the MPU can be forced to the upper or lower limit.

〔Example〕

Hereinafter, the burnout circuit according to the present invention will be explained in detail using the embodiment shown in FIG.

In FIG. 1, 1 has a current supply lead wire A at one end, a voltage detection lead wire B1 and a common lead wire B2 at the other end.
It is a three-wire type resistance temperature detector having a branch and a resistance r1 to
r3 is the equivalent conductor resistance of each of the lead wires A, Bl, and B2 from the resistance temperature detector 1 installed at a remote measurement point to the converter.

Reference numeral 2 denotes a constant current source, which is supplied to the resistance temperature sensor 1 through the lead wires A and B2 as shown by the arrow in the figure, and the constant current l is C
It returns to the constant current source 2 as the 0M line 9.

The voltage generated in the current supply lead A is applied to the input terminal el of the differential amplifier 4 having a high impedance via a low-pass filter circuit consisting of a resistor R1 and a capacitor C1, and the output of the differential amplifier 4 is eot is an A/D converter 7 that converts an analog signal into a digital signal via an input switching circuit 6 that switches multiple analog input lines.
is converted into digital data and taken into the MPU 8.

Also, with the C0M wire 9 as a reference potential, the voltage detection lead wire B
1 is applied to the input terminal e2 of the differential amplifier 5 having high impedance via a low-pass filter circuit consisting of a resistor R2 and a capacitor C2, and the output e02 of the differential amplifier 5 is applied to the input terminal e2 of the differential amplifier 5. Via circuit 6,
The data is converted into digital data by the A/D converter 7 and taken into the MPU 8. Further, a diode D1 is connected between the intersection of the resistor R1 and the lead wire A and the input terminal of the differential amplifier 5, and a diode D2 is connected between the intersection of the resistor R2 and the lead wire B1 and the C0M line 9, respectively. Connected with polarity. Further, a high resistance R3 is connected between the lead wire A and the lead wire B1. Further, RO is a bias resistor, which is connected between the circuits extending from the common lead wire B2 to the constant current source 2.

In the temperature/electrical signal converter of FIG. 1 constructed in this manner, the resistance Rt of the temperature measuring resistor 1 is expressed by the following equation.

Rt=Rtb+ΔRt...(19
) Here, Rtb is a constant bias resistance value determined by the type of resistance temperature detector 1 and the lower limit of the measurement temperature range. ΔRt is the resistance corresponding to the type of resistance temperature detector 1 and the temperature change from the lower limit of the measurement temperature range. This is the change.

First, in a steady state, the voltage e1 applied to the input terminal of the differential amplifier 4 is: ex=(r 1 +Rt b+ΔRt+r3+Ro)i
...(20), and the output voltage 8o1 of the differential amplifier 4 is eo□=G
(r 1 + Rtb+ΔRt+r3+Ro)i...
(21) Also, the voltage e2 applied to the input terminal of the differential amplifier 5 is e2=(
r3+Ro)i - (22), and the output voltage e02 of the differential amplifier 5 is eoz=G(r3
+Ro)i (23). The MPU 8 is programmed to perform the calculation shown in equation (24) and output the converter output eout.

eoui= eol −2'eojZ
,・B4) Here, using the same wire type, same wire diameter, and same wire length as the leads 1%A, Bl, and B2, set r=rl=r2=r3, (21), (2
3) Substitute equation (24) into equation (25) to obtain equation (25).

e out = G (Rt b + ΔRt - Ro)i
- (25) In equation (25), if the constant bias resistance value Rtb included in the resistance temperature detector 1 and the value of the bias resistance Ro are selected to be equal, Rt b = Ro, then (26
) to obtain the formula.

e 011 L = G ・ΔRt-i
-(26) As described above, in a steady state, the converter output e for any resistance change ΔRt of the resistance temperature detector 1 without being affected by the resistances rl, r2, and r3 of the lead wires. , 1. is obtained. By performing this operation periodically, the temperature/electrical signal conversion operation is performed.

Here, if the forward breakdown voltage Voz of the diode D1 is selected so that (r1+Rt)i<Vol, no current flows through the diode D1. It is immediately open. Also, the resistance R is adjusted so that (r3+Ro)<R3.
If you select 3, the constant current 1 due to the connection of resistor R3
There is no need to think about dividing the flow.

Also, the diode D is set so that (r3+Ro)i<Vol.
Since the forward breakdown voltage VD2 of 2 is selected, the diode D2 is in an open state. Furthermore, the MPU 8 is programmed to process as shown in the flowchart shown in FIG. In the steady state, the output e of the differential amplifier 5
. 2 After the capture process 21, the output e01 of the differential amplifier 4 is captured in the process 23, and based on these data, the wiring resistance compensation calculation process 24 is performed, and the output is output from the converter.
The program runs every calculation cycle.

Note that the constants of each element shown in FIG. 1 are, for example.

It is set as follows.

...(27) The maximum output value eat(ma
x), the minimum output value eot(min) is as is clear from equation (21). Also, the output e from the differential amplifier 5. 2 is (23
) As is clear from the formula, eoz=0.22G −(29
). Therefore, the maximum output value e within the standard signal range from the MPU 8. uv (IlaX) and minimum output value eou
t(+++in) becomes clear from equation (26).

In FIG. 1, the output e02 from the differential amplifier 5 has a value proportional to the wiring resistance r3 of the lead wire B2 and the voltage across the bias resistor RO, as is clear from equation (23). Generally, the temperature effect on wiring resistance is 4%/10
℃, and even if there is a maximum temperature change of 10℃ per hour, if one calculation station period t of the MPU is 0.1 seconds,
The temperature effect on wiring resistance during one calculation cycle of MPtJ is:
It is about I×10-4%, and from equation (23), the change Δeox in the output e02 of the differential amplifier 5 due to temperature change is Δ
eoz= 2.2 x 10-7G -(
31).

Next, FIG. 2 (
This will be explained using a) to (d). In addition, Figure 2 (,) ~
In (d), the same parts as in FIG. 1 are indicated using the same symbols. In FIGS. 2(a) to 2(d), the input switching circuit 6 and A/D converter 7 shown in FIG. 1 are omitted because they are not needed in the explanation of the burnout operation.

First, when the wire of the resistance temperature detector 1 is disconnected, the circuit shown in FIG. 1 is expressed as shown in FIG. 2(a). At this time,
The constant current i from the constant current source 2 is D1 as shown by the arrow in the figure.
→R2→r2→r3→RO. Therefore, the output eoz of the differential amplifier 5 is e02=G(R2+r2+r3+
RO)i .=(32), and by substituting the constant of equation (27) into equation (32), equation (33) is obtained.

e02=2,64G
-(33) Here, since the output e02 of the differential amplifier 5 in steady state is 0.44G, the change Δe02 in eo'2 is Δeo2=2.2G ・-(
34). Here, e in steady state. 2 change Δe
o2 is 2,2 X 10- as shown in formula (31) L-
7G, so the burnout detection criterion Δes by MPU is set to, for example, a safety factor of 5, Δes=l, I X 10-BG
-(35), as is clear from equations (34) and (35), Δes<Δe02, and burnout can be detected.

Next, when the current supply lead wire A of the resistance temperature detector 1 is disconnected, the circuit shown in FIG. 1 is represented as shown in FIG. 2(b). This is the same operation as in the case where the strands of the temperature sensing resistor 1 are disconnected, so the explanation will be omitted.

Next, when the voltage detection lead wire B1 of the resistance temperature detector 1 is disconnected, the circuit shown in FIG. 1 is represented as shown in FIG. 2(c). e at steady state. 2 is the formula (23), so when the voltage detection lead wire B1 is disconnected, eo2 is as follows because the leakage current iB flows in the order of R3→R2→C2. Output e of the differential amplifier 5 for one calculation period. The change Δeo2 in Equation (36), r 1 in Equation (27), and R
t (min).

If calculated by substituting R1, R3, C2, t, Δeo2
=0.22 (1- to -尭°°”) 6 valves 5 x 10-
'G...(37) From equations (35) and (37), Δes<Δeo
2, burnout can be detected.

Next, when the common lead wire B2 of the resistance temperature detector 1 is disconnected, the circuit of FIG. 1 is expressed as shown in FIG. 2(d).

At this time, the constant current i from the constant current source 2 flows from r1 to Rt −+ r 2 to D2 as shown by the arrow in the figure. Therefore,
The output eoz of the differential amplifier 5 is e o2: G-VD2
- (38) Here, from equation (27), the forward breakdown voltage Voz of diode D2 is 0.7
2V, the output e from the differential amplifier 5. 2 becomes 0.72G and Δeoz=0.5G
−(39)(35), From formula (39), Δes<
Δeo2, and burnout can be detected.

As described above, burnout is detected and the converter output eout is swung to the upper or lower limit based on the burnout operation direction (burnout upscaling or burnout downscaling) preset in the MPU. I can do things.

Furthermore, the diode D2 used in the embodiment of FIG. 2 has a constant current g.
2 is not necessarily required as long as it does not cause any problem if left open. At this time, if the open circuit voltage of the constant current source 2 is Ei2, then eo2=Ei2(1-E-1rl+
R1+'r2+R21c2t)Q...(40) It becomes. Here, if Ei2 is set to IOV and the constant of equation (27) is substituted into equation (40), eoz=9.9
7G, Δeo2=9, 75G=
・From equations (41), (35), and (41), Δes<Δeo
2, and the MPU can easily detect burnout.

In addition, burnout was detected using only the amount of change in the output eoz of the differential amplifier 5 in one MPU calculation cycle; however,
This is not the case. Based on steady state Δeo2, MP
Any method using the U program may be used. For example, if the output e02 of the differential amplifier 5 exceeds a certain standard range, burnout may of course be detected.

〔Effect of the invention〕

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 be operated in a stable direction of burnout upscaling or downscaling, and the control system can be protected. Moreover, only two resistors and one diode are added, which do not require high precision, so the circuit configuration is simple and can be realized at low cost, which is useful for industrial measurement.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an embodiment of a resistance thermometer type temperature converter equipped with a burnout circuit according to the present invention, and FIG.
d) is an equivalent circuit diagram at the time of abnormality of the embodiment shown in FIG. 3 is an equivalent circuit diagram of the embodiment shown in abnormality, FIG. 5 is an equivalent circuit diagram of the differential amplifier 3 shown in FIGS. 3 and 4, and FIG. 6 is a burnout diagram of the embodiment shown in FIG. 1. This is an operational flow of processing. RO, R1, R2, R3...Resistance, C1, C2...
Capacitor, Di, D2...Diode, 4,5...
・Differential amplifier, 6... Input switching circuit, 7... Analog/digital converter, 8... Microprocessing unit Figure 2 Figure 2 Figure 3 eJ=er-2ez Figure 4 Figure M4 Figure Figure 4 Figure 4 Funeral map Figure 4

Claims (1)

[Claims] 1. A three-wire resistor to be measured having a current supply lead wire at one end and a voltage detection lead wire and a common lead wire branched at the other end, and the current supply lead wire and the common lead. a constant current source for supplying current to the resistor through the wire, and a voltage generated through the current supply lead wire with the potential of the negative terminal of the constant current source as a reference potential as an input. a first amplifier; a second amplifier inputting the voltage generated via the voltage detection lead wire; an A/D converter that converts the output signal of the amplifier from analog to digital; Equipped with a microprocessing unit (hereinafter referred to as MPU) that receives the output of the converter as input,
The MPU transmits a first digital input signal corresponding to the voltage from the current supply lead wire and a second digital input signal corresponding to the voltage from the voltage detection lead wire at a certain period (hereinafter referred to as a calculation period). At the same time, based on the first digital input signal and the second digital input signal, the wiring resistance from the resistor to be measured is calculated to compensate for the resistance change of the resistor to be measured. In the temperature/electrical signal converter configured to output a signal corresponding only to the current, a first resistor is connected between the circuit from the voltage detection lead wire to the second amplifier, and between the supply lead wire and the voltage detection lead wire,
A second resistor is connected, and a unidirectional semiconductor element is connected between the current supply lead wire and the input terminal of the second amplifier, and the MPU receives the second digital input signal for each calculation period. Calculating the rate of change and forcing the output of the MPU by making the rate of change greater than a predetermined value when any lead wire of the resistor to be measured having the three lead wires is disconnected. A burnout circuit for a three-wire resistor to be measured, characterized in that the burnout circuit is configured to cause the resistor to be measured to swing out to an upper limit or a lower limit.