JPH035994Y2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a burnout circuit for an electromotive force transmitter that derives thermoelectromotive force from a thermocouple via an amplifier.

Generally, in electromotive force transmitters using thermocouples,
A burnout circuit is provided to swing the output of the electromotive force transmitter in a certain direction when the thermocouple is disconnected.

FIG. 1 shows a circuit diagram of a conventional electromotive force transmitter with a burnout circuit. In the same figure,
The thermoelectromotive force ei generated by the thermocouple TC is applied to the amplifier M1 via the resistor R, and is amplified to derive the output voltage E0. In Figure 1, Vs is the power source of the reference voltage ES, Ra and Rc are resistors for determining the bridge circuit current, Rb and Rd are zero point adjustment resistors, R1 is a high impedance element, and R2 and R3 are gain adjustment resistors. It is. In such a conventional circuit, when the thermocouple Tc is disconnected, the reference voltage ES becomes
It is applied to a series circuit of resistors R1 and R and capacitor C. Here, if the potential at the (+) input terminal of amplifier M1, that is, point b, is eb, It can be expressed as

However, ei is the voltage that was charged in the capacitor C just before the thermocouple TC was disconnected.

As described above, since the resistor R1 has a high impedance, as is clear from the above equation, the potential at point b gradually rises, and eventually the amplifier M1 outputs an output voltage that has completely swung in the + direction. However, in such a conventional circuit, the resistor R1 has a high impedance and the burnout circuit operates slowly, so there are drawbacks such as a delay in the alarm operation for wire breakage and a large fluctuation in the operation signal.

The purpose of this invention is to provide a burnout circuit that eliminates the drawbacks of the conventional circuits described above and operates at high speed.

In order to achieve the above object, the burnout circuit of this invention divides the input terminal of an amplifier for thermoelectromotive force amplification between first and second high impedance elements, and also has a set voltage source and a set voltage source. A comparator that compares the voltage with the potential divided by the first and second high impedance elements, and switching means that turns on/off according to the output of this comparator are provided, and the switching means turns on/off according to the output of the comparator. Forcibly controls the potential of the input terminal of the amplifier to a predetermined value,
The output voltage is made to swing instantaneously in a predetermined direction.

This invention will be explained in detail below with reference to embodiments shown in the drawings.

FIG. 2 is a circuit connection diagram of a burnout circuit showing an embodiment of this invention. In the same figure, the first
Components with the same reference numerals as those in the circuits shown in the figures indicate the same components. The feature of this embodiment circuit is that the high impedance elements Z1 and Z2 are connected in series to both ends of the reference voltage source VS, and the connection point of both high impedance elements Z1 and Z2, that is, the voltage dividing point a, is connected to the non-voltage terminal of the amplifier M1. Connect to the inverting input terminal (point b), and connect the filter capacitor C between point b and ground to the switching transistor.
A burnout setting voltage source VZ, a comparator M2 that compares the potential at point a with the setting voltage EZ from this power supply VZ are provided, and the output of this comparator M2 is connected to the diode d. , resistance R
5, the switching transistor SW is turned on/off in addition to the switching transistor SW. Note that the burnout setting voltage EZ is set higher than the maximum value of ei.

In this example circuit, assuming that the thermocouple TC is normal, the impedance seen from point a on the input side is Ri + Rb (Ri: internal resistance of the thermocouple TC), and the impedance of impedance elements Z1 and Z2 is set to Ri + Rb. Since it is sufficiently large compared to
The potential at point a is ei, and this potential is applied to the non-inverting input terminal of amplifier M1. On the other hand, as described above, the burnout setting voltage EZ is higher than the potential ei at point a, so the output of the comparator M2 is turned on (+B), the switching transistor SW is turned on, and the capacitor C is connected to ground. Therefore, amplifier M1 generates thermoelectromotive force from thermocouple TC during normal operation.
Amplify ei.

If the thermocouple TC is disconnected, the point a, that is, the input circuit of the amplifier M1 becomes open, so the potential at the point a instantaneously rises to (Z2/Z1+Z2)·ES. Since this potential is higher than the burnout setting voltage EZ, the output of the comparator M2 immediately turns off (-B), so that the filter capacitor C is disconnected from the circuit. As a result, the voltage at the non-inverting input terminal of the amplifier M1 suddenly rises, and its output E0 immediately swings to the maximum direction.

FIG. 3 is a circuit connection diagram showing the main parts of the burnout circuit of another embodiment. In the burnout circuit shown in FIG. 2, the output of the comparator M2 connects or disconnects the filter capacitor C from the circuit, but in this embodiment, the output of the comparator M2 short-circuits the impedance element Z1, Alternatively, it is distinctive in that it releases short circuits. That is, a switching transistor SW is connected in parallel to the impedance element Z1, and the switching transistor SW is turned on/off by the output of the comparator M2 to short-circuit or release the impedance element Z1. Note that the polarity of the input terminal of comparator M2 is opposite to that shown in FIG. Other aspects are the same as shown in Figure 2.

In the circuit shown in Figure 3, if the thermocouple is normal, the potential ei at point a is the burnout setting voltage EZ
Since the output of the comparator M2 is off, the switching transistor SW is not turned on, and the impedance Z1 is in a short-circuited state.
Therefore, the amplifier M1 is similar to that shown in FIG.
Amplify the thermoelectromotive force ei from a normally operating thermocouple TC.

If the thermocouple TC is disconnected, the potential at point a will change instantaneously (Z
2/Z1+Z2)・Raises to ES. Since this potential is higher than the burnout setting voltage EZ, the comparator M2 is immediately turned on, and thereby the switching transistor SW is also turned on. That is, impedance element Z1 is short-circuited. Therefore, the time constant becomes smaller, and the point b, that is, the non-inverting input terminal of the amplifier M1, rapidly rises to near the reference voltage ES, and the output E0 immediately swings out to the maximum direction.

FIG. 4 is a circuit connection diagram showing the main parts of a burnout circuit according to still another embodiment. The embodiment circuits shown in FIGS. 2 and 3 above are shown for upper limit swing-off circuits, but the embodiment shown in FIG. 4 shows a lower limit swing-off circuit. In comparison with the embodiment circuit shown in FIG. 2, the switching transistor SW, which is turned on and off by the output of the comparator M2, is connected in parallel to the filter capacitor C, that is, between the non-inverting input terminal of the amplifier M1 and the ground. point, and comparator M
The difference is that the polarity of the input terminals of the second embodiment is reversed, the burnout setting power supply VZ is connected to the inverting input terminal, and the point a is connected to the non-inverting input terminal via the resistor R4. In the example circuit shown in Fig. 4, when the thermocouple TC is operating normally, the potential ei at point a is lower than the burnout setting voltage EZ, as in the example circuit shown in Fig. 3. Since the output of device M2 was off, the switching transistor
SW is also off.

However, when the thermocouple TC is disconnected, the potential at point a increases and becomes larger than the burnout setting voltage EZ, so the potential at the non-inverting input terminal of comparator M2 becomes high, its output turns on, and switching transistor SW turns on. The non-inverting input terminal of the amplifier M1 is short-circuited to ground, and its output swings to the lower limit.

FIG. 5 is a block diagram showing an example of process control to which the burnout circuit of this invention is applied. In the figure, the temperature of the controlled object detected by the thermocouple TC is converted into an electrical signal by converter 1 including a burnout circuit, and in addition to controller 2, controller 2 performs predetermined calculations and outputs the manipulated variable. The electric/pneumatic converter 3 converts the signal into a pneumatic signal, which controls the opening and closing of a process control valve 5 via a solenoid valve 4. Note that 6 is a temperature monitor, and 7 is an abnormality indicator lamp. If thermocouple
When the TC is disconnected and an abnormality occurs, the abnormality is detected,
By activating the solenoid valve 4 and holding the immediate control valve 5 in its current state, it is possible to prevent disturbances in the process due to disconnection of the thermocouple TC. It is also possible to hold the current state by switching the controller 2 from automatic to manual mode using a signal from the monitor 6.

As described above, according to the burnout circuit of this invention, when a disconnection abnormality occurs in the thermocouple, the input voltage of the amplifier is immediately forced to a predetermined value and the output voltage is swung in a predetermined direction, thereby preventing burnout. It is faster and abnormalities can be detected faster. Therefore, necessary measures can be taken quickly when an abnormality occurs, and confusion in the control system can be prevented even if an abnormality occurs in the thermocouple.

[Brief explanation of the drawing]

Fig. 1 is a circuit connection diagram of a conventional burnout circuit, Fig. 2 is a circuit connection diagram of a burnout circuit showing an embodiment of this invention, and Fig. 3 is a burnout circuit of another embodiment of this invention. 4 is a circuit connection diagram showing the main parts of a burnout circuit according to another embodiment of this invention. FIG. 5 is a process control diagram to which the burnout circuit of this invention is applied. FIG. 2 is a block diagram showing an example. TC...Thermocouple, M1...Amplifier, VS...Reference power supply, M2...Comparator, VZ...Burnout setting power supply, Z1, Z2...High impedance element,
C...Filter capacitor, SW...Switching transistor.

Claims (1)

[Claims for Utility Model Registration] In an electromotive force transmitter that derives thermoelectromotive force from a thermocouple via an amplifier, the input terminal of the amplifier is divided and connected by first and second high impedance elements, and the setting A voltage source, a comparator that compares the voltage of the set voltage source and the potential divided by the first and second high impedance elements, and switching means that turns on/off according to the output of the comparator, A burnout circuit characterized in that the potential of the input terminal of the amplifier is forced to a predetermined value by turning on/off the switching means.