JPH0117877Y2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Industrial Application Field This invention relates to a two-wire transmitter, and particularly to a two-wire transmitter featuring a damping circuit.

(b) Prior Art Figure 1 shows a circuit connection diagram of a two-wire transmitter that has been commonly used in the past. In the same figure, 1 is a physical quantity P such as pressure, temperature, or flow rate as its input.
a conversion circuit that converts the physical quantity into capacitance change, inductance change, resistance change, etc., and outputs voltage changes e ₁ and e ₂ corresponding to this change;
is composed of resistors R1, R2, etc., and voltage changes e ₁ ,
An addition (operation) circuit that adds _e2 , 3 is an operational amplifier. - (minus) input terminal of operational amplifier 3,
A damping circuit 4 is constituted by a variable resistor R _D connected between the cc potentials, an output terminal of the operational amplifier 3, and a capacitor C connected to the movable terminal of the variable resistor R _D. The output terminal of the operational amplifier 3 is connected in series with a transistor Tr and an output resistor R _p to form an output circuit 5 and is connected between a power supply circuit (constant voltage source or constant current source) 6 and cc potential.

Bf is a feedback resistor, and the voltage ef generated across this resistor is passed through the resistor R3 to the operational amplifier 3.
It is designed to be fed back to the + (plus) input terminal of the In addition, 7 is a power supply (for example, DC23V), and 8 is a load. This power source 7 and load 8 are placed on the control room side, and are connected to the conversion circuit side by a two-wire transmission line 9.

In the above circuit, resistor R _D and capacitor C
The time constant circuit or damping circuit 4 is provided to reduce fluctuations in the output signal when the input signal sometimes fluctuates greatly (for example, the flow rate signal immediately after the pump). This damping circuit 4 has a maximum time constant (R2R _D ) when the movable terminal of the variable resistor R _D is moved so that its potential becomes the same potential as the negative input terminal of the operational amplifier ₃ ; When the movable terminal of is moved so that its potential becomes cc potential, the time constant becomes minimum (0).

The disadvantage of a conventional two-wire transmitter including such a damping circuit 4 is that when the movable terminal of the variable resistor R _D is suddenly moved, the output fluctuates not only due to a change in the time constant but also due to the charging voltage of the capacitor C. occurs, and due to the initial condition of the capacitor C, that is, under the initial condition that the output terminal of the operational amplifier 3 is at 0 potential, when the power is turned on, a voltage is applied to the capacitor C from the power supply circuit 6 through the transistor Tr or the IC forming the operational amplifier 3. A charging current flows and its output current rises once as shown in FIG.

(C) Purpose The purpose of this invention is to eliminate the drawbacks of the conventional two-wire transmitter mentioned above, and to create a two-wire transmitter that does not cause output fluctuations even when the time constant is changed, and does not cause an output current increase when the power is turned on. There is a transmitter to provide.

(d) Configuration In order to achieve the above object, the two-wire transmitter of this invention connects a variable resistor forming a time constant circuit between the output side of the adder circuit and one input terminal of the operational amplifier, and A capacitor constituting a time constant circuit is connected to the variable terminal of this variable resistor and the other end (two-wire transmission line connection side) of the feedback resistor.

(e) Examples This invention will be explained in more detail below using examples.

FIG. 2 is a circuit connection diagram of a two-wire transmitter showing an embodiment of this invention.

In the figure, the damping circuit 10 is provided, and the adding circuit 2 is connected to resistors R1, R2, R3, R4.
The other circuit sections are constructed in the same manner as the circuit shown in FIG. 1, except that the circuit shown in FIG. The damping circuit 10 is composed of a variable resistor R _D and a capacitor C.

The variable resistor R _D is the resistor R that constitutes the adder circuit 2.
A capacitor C is connected between the movable terminal of the variable resistor R _D and the connecting end of the two-wire transmission line 9 of the feedback resistor R _f . is connected.

In the example circuit configured as described above, even if the resistance value of the variable resistor R _D is changed, the input impedance of the operational amplifier 3 is normally considered to be infinite, and in a steady state, the current flows through the variable resistor R _D. Since no current is flowing, the input of operational amplifier 3 does not change.
Therefore, no output fluctuation occurs.

Also, when power supply 7, ie 24V DC, is turned on, the voltage across capacitor C is 0 (when power is turned off, resistor R3
Therefore, the output e ₀ of the operational amplifier 3 starts from 0, and the output current becomes as shown in FIG. 4, without any overshoot.

Note that in FIGS. 3 and 4, ti indicates the time when the power is turned on.

(f) Effect As described above, according to the two-wire transmitter of this invention, the variable resistor for configuring the time constant circuit of the damping circuit is connected between the adder circuit and one end of the input of the operational amplifier, and the time constant circuit Since the configuration capacitor is connected to the variable terminal of the variable resistor and the two-wire transmission line connection end of the feedback resistor, even if the time constant is changed by changing the resistance value etc., there will be no fluctuation in the output. Moreover, since the voltage across the capacitor is always 0 when the power is turned on, it is possible to prevent the phenomenon in which the output current temporarily increases when the power is turned on, that is, overshoot. Moreover, in order to achieve the above-mentioned improvement, only the connection between the variable resistor and the capacitor is changed, so there is no increase in cost.

[Brief explanation of drawings]

Fig. 1 is a circuit connection diagram of a conventional two-wire transmitter, Fig. 2 is a circuit connection diagram of a two-wire transmitter showing an embodiment of this invention, and Fig. 3 is a circuit connection diagram of a two-wire transmitter shown in Fig. 1. FIG. 4 is a diagram showing the output current characteristics of the two-wire transmitter shown in FIG. 2 when the power is turned on. 1: conversion circuit, 2: addition circuit, 3: operational amplifier, 5: output circuit, 6: power supply circuit, 8: load,
9: Two-wire transmission line, 10: Damping circuit,
R _f : Feedback resistance, R _p : Output resistance, Tr: Output transistor, R _D : Variable resistance (for time constant circuit configuration),
C: Capacitor (for time constant circuit configuration).

Claims (1)

[Claims for Utility Model Registration] A conversion circuit 1 that receives physical quantities such as pressure, temperature, or flow rate and converts them into electrical signals e ₁ and e ₂ , and a circuit that receives electrical signals e ₁ and e ₂ from this conversion circuit 1 An output in which an adder circuit 2 that adds both signals, an operational amplifier 3 that amplifies the signal added by this adder circuit 2, and a transistor Tr that receives the output of this operational amplifier 3 and an output resistor R _p are connected in series. a circuit 5, a power supply circuit 6 that supplies power supply voltage to each of these circuits, a feedback resistor R _f whose one end is connected to the reference potential terminal CC of each of the circuits, the power supply circuit 6 and the feedback resistor.
In a two-wire transmitter including a load circuit 8 connected to the other end of R _f via a two-wire transmission line 9, between the output side of the adder circuit 2 and one input terminal of the operational amplifier 3. A variable resistor R _D constituting the time constant circuit 10 is connected to the variable resistor R _D .
A two-wire transmitter characterized in that a capacitor C constituting a time constant circuit 10 is connected between the variable terminal of the feedback resistor R f and the other end of the feedback resistor R _f .