JP2001052282A - Two-wire transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a two-wire transmitter which is so improved as to secure a minimum voltage value for placing itself in operation. SOLUTION: The two-wire transmitter has a power supply means 11 which is provided on a load side and supplies a transmit current to the load and a measurement control means 12B which supplies the transmit current through a transmission line and controls and supplies the transmit current to the load according to a signal corresponding to a measured physical quantity. In this case, the measurement control means 12B is equipped with a feedback resistance which generates a feedback current proportional to the transmit current, a current control means 15B which can generates a primary voltage by controlling the transmit current so that the feedback voltage matches the signal voltage corresponding to the measured physical quantity, an insulating means 16 which converts the primary voltage into a fixed secondary voltage linearly while holding the inverse function relation with a flowing-in primary current, a signal processing means 17 which supplies this secondary voltage to measure the physical quantity and generates a signal voltage based upon the measured physical quantity, and a limiter means which limits the primary voltage to a voltage value where the current control means 15B is able to operate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-wire transmitter, and more particularly, to limiting a circuit to a predetermined constant voltage value in order to ensure the operation of the circuit, and using one circuit to output analog or digital signals. The present invention relates to a two-wire transmitter that can be switched to a two-wire transmitter.

[0002]

2. Description of the Related Art As shown in FIG. 8, a two-wire transmitter according to a first embodiment of the prior art is provided on a load side and supplies power to a transmission current Io to the load. A circuit 11 and measurement control means for supplying the transmission current Io via the transmission lines L1 and L2 and controlling the transmission current Io to a load by controlling the transmission current Io based on a signal corresponding to the measured physical quantity, that is, a measurement control circuit 12 It is composed of

The power supply circuit 11 forms a series circuit with a DC power supply 13 provided on the load side and a load resistor 14 connected in series to the DC power supply 13, and the supply voltage V02 is applied to the load resistor 14. It is configured to be supplied. Terminals T1 and T2 at both ends of the series circuit are connected to transmission lines L1 and L2.
Terminals T1 'and T1 constituting the measurement control circuit 12 through
2 '.

The transmission current I flowing through the transmission lines l1 and l2
o is configured to flow in the range of 4 mA to 20 mA.
Among them, the minimum transmission current Io of 4 mA is the minimum power supply required for the measurement control circuit 12.

The measurement control circuit 12 includes a feedback resistor R1 for generating a feedback voltage Vf1 proportional to the transmission current Io, and a feedback voltage Vf1 for a signal voltage VS1 corresponding to the measured physical quantity.
Are controlled so that the primary voltage V2
, Ie, the current control circuit 15 and a linear constant secondary voltage Vd1 while maintaining an inverse functional relationship with the primary current I2 flowing the primary voltage V2.
(Constant voltage) converting means, that is, the insulating circuit 16
And a signal processing means for supplying the secondary voltage Vd1 to measure a physical quantity and generating a signal voltage VS1 based on the measured physical quantity, that is, a signal processing circuit 17.

[0006] The connection state of the measurement control circuit having such a configuration is first determined by the terminals T connected to the transmission lines L1 and L2.
1 ′ and T2 ′ are the emitter-base of the transistor Q1, the collector-base-emitter of the transistor Q2,
A current control circuit 15 is connected between a circuit in which a resistor R2 and a feedback resistor R1 are connected in series, respectively, and a connection point (point b) between the collector of the transistor Q1 and the resistor R2 and the feedback resistor R1 in the circuit connected in series. Are connected in parallel, and the primary voltage V
2 is generated. The current control circuit 15 connects the insulation circuits 16 in parallel. The insulating circuit 16 connects the signal processing circuit 17 in parallel. Here, the feedback resistor R1 generates a feedback voltage Vf1 proportional to the transmission current Io, and the current control circuit 15 controls the transmission current Io to control the primary voltage V2.
Can occur.

Such a current control circuit 15 includes a transistor Q1, a capacitor C1 connected in parallel between a connection point of the resistor R2 and the feedback resistor R1, a diode DZ1 connected in parallel, and also in parallel. It comprises a series circuit of a collector / emitter of a connected transistor Q4 and a diode DZ2.

[0008] The base of the transistor Q2 is connected to the output terminal of the operational amplifier OP3 via a resistor R3.
The output terminal of the operational amplifier OP3 and the resistor R3 are connected to the base of the transistor Q4 via the resistor R4.

The non-inverting input terminal (+
Side), the feedback voltage Vf1 generated in the feedback resistor R1 is applied via the resistor R5, and at the same time, the signal voltage VS1 generated based on the physical quantity measured by the signal processing circuit 17 is applied via the resistor R7. Are connected to each other. On the other hand, the inverting input terminal (-side) is connected to the connection point between the resistor R2 and the feedback resistor R1.

A primary voltage V2 is applied to input terminals T7 and T8 of the insulation circuit 16, and a signal is isolated by a transformer (not shown). Then, a constant voltage Vd1 which is a secondary voltage is output to the output terminals T7 'and T8', and the terminals T9 and T8 '
The power supply voltage of the signal processing circuit 17 is supplied via T10.

A signal processing circuit 17 converts the number of Karman vortices generated at a predetermined place, that is, a physical quantity measured by a sensor 18 for extracting a measurable physical quantity into a predetermined signal, and converts this signal into a predetermined signal. Signal voltage VS1 converted to voltage
Generate This signal voltage VS1 is applied to the non-inverting input terminal (+ side) of the operational amplifier OP3 via the resistor R7. Further, the signal processing circuit 17, the transistor Q1, and the terminal T
1 ′, the receiving communication circuit 19
Are connected. Further, a transmission communication circuit 2 is provided between the signal processing circuit 17 and the resistor R5 via the feedback voltage Vf1.
0 is connected.

The two-wire transmitter having such a configuration is as follows.
The power supply voltage supplied to the power supply circuit 11 can be reduced. That is, the non-inverting input terminal (+ side) of the operational amplifier OP3 is connected to the sensor 1 of the signal processing circuit 17.
The signal voltage VS1 and the feedback voltage Vf1 generated corresponding to the physical quantity such as the vortex signal detected in step 8 are applied, and the output voltage is controlled to the base of the transistor Q2 so that the absolute values of these voltages become equal. To change the collector current. When the collector current changes, the base current of the transistor Q1 changes and changes its internal resistance.
The current flowing through the transistor Q1 increases, and the current not consumed by the insulating circuit 16 is bypassed through a series circuit including the transistor Q5 and the diode DZ2. In this way, only the transmission current Io corresponding to the signal voltage VS1 can be supplied to the load resistor 14. Here, when the value of the primary voltage V2 is small, the diode DZ2 does not conduct, so that no current is bypassed to the series circuit including the transistor Q5 and the diode DZ2. Thus, the insulation circuit 16 operates normally even when the value of the primary voltage V2 is small.

The relationship between the primary voltage V2 applied to the input terminals T7 and T8 of the insulating circuit 16 and the primary current I2 flowing into the input terminals T7 and T8 is an inverse functional relationship. For example, when the primary voltage V2 is 11 V , The primary current I2 is 3.2
mA, and when the primary voltage V2 is 7 V, the primary current I2 is 5.1 mA. When the primary voltage V2 is 7 V, the primary current I2 increases to 5.1 mA. However, at this time, no problem occurs because the maximum transmission current Io is 20 mA. In this way, by changing the primary voltage V2 in accordance with the transmission current Io, control is performed such that the operation is performed within the change range of the transmission current Io.

For example, when the voltage of the power supply 13 is 12 V, DZ1 =
9V, load resistance = 250Ω, feedback resistance = 50Ω, when the transmission current Io is the minimum of 4 mA, the supply voltage V02 = 10.8V, the collector of the transistor Q1
Communication is possible when the voltage VCE between the emitters is 1.8 V. When the transmission current Io is the maximum of 20 mA, the supply voltage V02 = 6 V and the voltage VCE between the collector and the emitter of the transistor Q1 is approximately 0 V, and communication cannot be performed.

Next, the second embodiment of the prior art will be described.
Next, the wire type transmitter will be described. As shown in FIG. 9, the two-wire transmitter is provided on the load side and transmits the transmission current Io to this load.
Power supply means for supplying power, that is, a power supply circuit 11,
Measurement control means for supplying the transmission current Io via the transmission lines L1 and L2 and supplying the transmission current Io to the load by controlling the transmission current Io based on a signal corresponding to the measured physical quantity, that is, the measurement control circuit 12A. ing.

The power supply circuit 11 forms a series circuit with a DC power supply 13 provided on the load side and a load resistor 14 connected in series to the DC power supply 13, and supplies a supply voltage V02 to the load resistor 14. I do. Terminals T1 and T2 at both ends of the power supply circuit 11 are connected to output terminals T1 'and T2' of the measurement control circuit 12A via transmission lines L1 and L2.

The transmission current I flowing through the transmission lines L1 and L2
o is configured to flow in the range of 4 mA to 20 mA.
Of these, the minimum required power supply for the measurement control circuit 12A is the minimum 4 mA transmission current Io.

The measurement control circuit 11A includes a feedback resistor R1 for generating a feedback voltage Vf1 proportional to the transmission current Io, and a feedback voltage Vf1 for the signal voltage VS1 corresponding to the measured physical quantity.
Current control means for controlling the transmission current Io so as to coincide with each other and generating a primary voltage V2 related to the transmission current Io;
That is, a signal processing means for supplying a current control circuit 15A and a primary voltage V2 to measure a physical quantity and generating a signal frequency SF corresponding to the measured physical quantity, that is, the signal processing circuit 1
7A and a signal voltage VS1 corresponding to the signal frequency SF.
F / V conversion means (frequency / voltage conversion means) for generating
That is, it comprises an f / V conversion circuit 26 and pulse signal output means for converting this signal voltage VS1 into a digital pulse signal and outputting it, that is, a pulse signal output circuit 25.

The measurement control circuit 11 having such a configuration
The connection state at A is that the terminals T1 'and T2'
Emitter-base of transistor Q1, transistor Q
2 constitutes a circuit in which the collector, base, emitter, resistor R2 and feedback resistor R1 are connected in series. A current control circuit 15A is connected in parallel between the collector of the transistor Q1 and the connection point of the resistor R2 and the feedback resistor R1 in the circuit connected in series to generate a primary voltage V2.
Also, a series circuit including the transistor Q1 and terminals T1 'and T1'
2 ', a pulse signal output circuit 25 for generating a pulse signal PLS is connected in parallel.

The pulse signal output circuit 25 generates a pulse signal PLS under predetermined conditions. One power supply VCC is connected to a terminal T1 'via a jumper J3, and the other power supply VEE is connected to a terminal T2. 'Connected. The set pulse signal SPLS, which is the input signal, is connected to the jumper J
6, and is input from the signal processing circuit 17A.

The current control circuit 15A includes an operational amplifier OP4 for controlling the transmission current Io supplied to the load resistor 14 by the feedback voltage Vf1, and a non-inverting input terminal (+ side) of the operational amplifier OP4 jumpers the reference voltage VREF. J5 and the resistor R5, the feedback voltage Vf1 is connected via the resistor R6, and the signal voltage VS1 from the signal processing circuit 17A is connected via the resistor R4. The inverting input terminal (− side) of the operational amplifier OP4 is a resistor R
2 and the feedback resistor R1.

The signal processing circuit 17A has a sensor 18 for extracting the number of Karman vortices generated corresponding to the measured flow rate, that is, a physical quantity that can be measured. Based on the physical quantity extracted by the sensor 18, The signal frequency SF is input to the f / V conversion circuit 26 via the jumper J2,
The signal voltage VS converted by the f / V conversion circuit 26
1 becomes a partial input of the non-inverting input terminal (+ side) of the operational amplifier OP4 via the resistor R4. At the same time, the signal frequency SF
Are connected to input the set pulse signal SPLS to the pulse signal output circuit 25 via the jumper J6.

The receiving communication circuit 27 has a jumper J1 interposed between the signal processing circuit 17A and the terminal T1 '(point a), and a resistor between the signal processing circuit 17A and the feedback resistor R2. The transmission communication circuit 28 is connected via R7 and the capacitor C1. Also, the transmission / reception communication circuit 27,
In 28, the side other than the signal processing circuit 17A side is connected to the terminal T4 via the jumper J4.

In such a configuration, when an analog output is obtained and when a pulse output is obtained, jumpers J1 to J1 are used.
This is performed by switching J6.

To obtain an analog output, use jumper J
1. Short-circuit J2. Then, the signal frequency SF from the signal processing circuit 17A becomes a signal of 1 Khz to 5 Khz. This is changed by the f / V conversion circuit 26 to the signal voltage VS1 proportional to the frequency. The operational amplifier OP4 has a signal voltage VS1 / resistance R4 + feedback voltage Vf1 / resistance R6
= 0, so that the transmission current Io is controlled. Further, since communication is performed from the transmission lines L1 and L2, communication is enabled by short-circuiting the jumper J1 and connecting the communication signal to the communication circuit 27 for reception.

On the other hand, when obtaining a pulse output, the jumpers J3 to J6 are short-circuited. Then, the signal frequency SF from the signal processing circuit 17A can output a pulse signal weighted such as 1 m ³ per pulse signal. Since the jumper J2 is open, the signal voltage VS1 on the output side of the f / V conversion circuit 26 is zero volt. Since the jumper J5 is short-circuited, the transmission current Io is VS1 / R4 (VS1 is zero volt) + reference voltage VRE
F / resistance R5 + feedback voltage Vf1 / resistance R6 = 0.

In this way, a current for stably operating the two-wire transmission circuit, that is, about 10 mA in actuality, can be secured. In the case where the pulse output is set, if the jumper J1 is short-circuited, the signal from the pulse signal output circuit 25, that is, the signal between the transistor Q1 and the terminal T1 '(point a) becomes noise. Communication becomes impossible. Accordingly, the jumper J4 is short-circuited and communication is performed from the terminal T4.

[0028]

However, in the two-wire transmitter according to the first embodiment of the prior art, a capacitor is connected to the normal primary voltage V2 for stabilization. Therefore, when the transistor Q1 is in a conductive state, the impedance of the alternating current between the terminals T1 ′ and T2 ′ as viewed from the power supply side decreases, and when communication is attempted, the waveform is distorted or reduced, and communication can be performed normally. There is a problem of disappearing.

In the two-wire transmitter according to the second embodiment described above, it is necessary to connect a large number of jumpers to switch between analog output and pulse output, so that the operation is complicated and the hardware becomes complicated. There's a problem.

Therefore, even if the primary current for controlling the transmission current supplied to the load resistor becomes large, the measurement control means can operate normally and communication can be performed, and switching between analog output and pulse output can be easily performed. There is a problem that must be solved by a circuit configuration that can be used.

[0031]

To solve the above-mentioned problems, a two-wire transmitter according to the present invention comprises a power supply means provided on a load side for supplying a transmission current to the load, and a power supply means for supplying the transmission current to the load. A two-wire transmitter, comprising: a measurement control unit that controls the transmission current based on a signal corresponding to the measured physical quantity and supplies the transmission current to the load via a transmission line. A feedback resistor that generates a feedback voltage proportional to the current, and a current control unit that can control the transmission current so that the feedback voltage matches a signal voltage corresponding to the measured physical quantity and generate a primary voltage. An insulating means for linearly converting the primary voltage into a constant secondary voltage while maintaining an inverse functional relationship with the flowing primary current, supplying the secondary voltage, measuring a physical quantity, and measuring the measured physical quantity. Base Signal processing means for generating a have signal voltage, when the primary voltage has been lowered, a limiter means for limiting the primary voltage to a voltage value where the current control means can operate, consists.

Further, another two-wire transmitter is provided on the load side and supplies power to the load, and supplies power to the load, and supplies the transmission current via the transmission line and responds to the measured physical quantity. A two-wire transmitter having measurement control means for controlling the transmission current based on a signal to supply the load to the load, wherein the measurement control means generates a feedback voltage proportional to the transmission current; Current control means for controlling the transmission current so that the feedback voltage coincides with the signal voltage corresponding to the measured physical quantity to generate a primary voltage related to the transmission current; and Signal processing means for measuring and generating a signal frequency corresponding to the measured physical quantity; f / V converting means for generating a signal voltage corresponding to the signal frequency; and outputting the signal voltage as a pulse signal And the pulse signal output means, and connector and switching means for switching the signal frequency to an analog output or pulse output, consisting of.

As described above, since the limiter for limiting the primary voltage is provided on the measurement control means even if the primary current becomes large, communication cannot be performed even if the AC impedance seen from the power supply means becomes small. Can be avoided, and stable power supply can be performed.

Further, since the switching between the analog output and the pulse output is performed by the connector, the switching can be performed by one touch in one circuit.

[0035]

Next, an embodiment of a two-wire transmitter according to the present invention will be described with reference to the drawings. The same components as those described in the related art will be described with the same reference numerals.

As shown in FIG. 1, the two-wire transmitter according to the first embodiment of the present invention is provided on the load side and is a power supply means for supplying a transmission current Io to the load, ie, a power supply. A circuit 11 and measurement control means for supplying the transmission current Io via the transmission lines L1 and L2 and controlling the transmission current Io to a load by controlling the transmission current Io based on a signal corresponding to the measured physical quantity, that is, a measurement control circuit 12B It is composed of

The power supply circuit 11 having such a configuration
A DC power supply 13 provided on the load side and a load resistor 14 connected in series to the DC power supply 13 form a series circuit. Terminals T1 and T2 at both ends of the series circuit are connected to output terminals T of the measurement control circuit 12B via transmission lines L1 and L2.
1 ', T2'. Also, terminals T1, T2
The communication device 30 is connected between them.

The transmission current I flowing through the transmission lines L1 and L2
o is flowed in the range of 4 mA to 20 mA. Among them, the minimum transmission current Io of 4 mA is the minimum power supply required for the measurement control circuit 12B.

The measurement control circuit 12B has terminals T1 ', T
2 ', the emitter-base of the transistor Q1, the collector-base-emitter of the transistor Q2, the resistor R
2, a circuit in which the feedback resistors R2 are connected in series, and supplies the load resistor 14 with the supply voltage V02. Between the collector of the transistor Q1 of the circuit connected in series and the connection point (point b) between the resistor R2 and the feedback resistor R1,
Current control means, that is, a current control circuit 15B is connected in parallel to generate a primary voltage V2.

The current control circuit 15B includes a transistor Q1
The emitter-base of the transistor Q2, the collector-base-emitter of the transistor Q2, the collector-base-emitter of the transistor Q2, the resistor R2, and the feedback resistor R1 are connected in series with the emitter-base-collector of the transistor Q3 and the collector-base-emitter of the transistor Q4. The series circuit thus configured is connected in parallel. A regulator 31 for generating a reference voltage VREF based on the primary voltage V2 is connected to the base of the transistor Q3 via a resistor R6.
The output terminal of the operational amplifier OP2 is connected to the base of the transistor Q4 via a resistor R5. The output terminal of the operational amplifier OP1 is connected to the base of the transistor Q2. A divided voltage VC obtained by dividing the reference voltage VREF of the regulator 31 by the resistors R10 and R11 is input to an inverting input terminal (− side) of the operational amplifier OP2.
A voltage obtained by dividing the primary voltage V2 by the resistors R8 and R9 is input to the non-inverting input terminal (+ side). Here, the regulator 31, the resistor R6, and the transistor Q3 constitute limiter means, and have a function of limiting the voltage to a predetermined value when the primary voltage V2 decreases.

The non-inverting input terminal (+
Side), the feedback voltage Vf1 generated in the feedback resistor R1 is connected via the resistor R12, and the signal voltage VS1 generated in the signal processing circuit 17 is connected via the resistor R7, and the sum of the two is applied. Is applied. On the other hand, a connection point (point b) between the resistor R2 and the feedback resistor R1 is connected to the inverting input terminal (− side).

The primary voltage V2 is applied to the terminals T7 and T8 of the insulating circuit 16, and the signal is isolated by a transformer (not shown). Then, a constant voltage Vd1 which is a secondary voltage is output to the terminals T7 'and T8', and is supplied as a power supply voltage of the signal processing circuit 17 via the terminals T9 and T10.

This insulating circuit 16 has terminals T7 ', T8'
A constant voltage Vd1 is output as a secondary voltage, and in this case, the relationship between the primary voltage V2 and the primary current (output current) I2 of the input terminals T7 and T8 is configured to be an inverse functional relationship. . This will be described later.

The signal processing circuit 17 is provided with a sensor 18 for detecting a physical quantity to be measured, for example, the number of Karman vortices and converting the signal into an electric signal. The signal voltage VS1 converted into the electric signal has a resistance R7. The voltage is applied to the non-inverting input terminal (+ side) of the operational amplifier OP1 via the input terminal. Also, the signal processing circuit 1
The receiving communication circuit 19 is connected between the terminal 7 and the terminal T1 '. Further, a transmission communication circuit 2 is provided between the signal processing circuit 17 and the non-inverting input terminal (+ side) of the operational amplifier OP1.
0 is connected.

Next, the operation of the measurement control circuit 12B configured as described above will be described. A signal voltage VS1 corresponding to a physical quantity such as a vortex signal detected by the sensor 18 of the signal processing circuit 17 and a feedback voltage Vf1 are added and applied to the non-inverting input terminal (+ side) of the operational amplifier OP1. The output voltage VC1 is applied to the base of the transistor Q2 so that the voltages become equal, and the collector current is changed. The collector current of the transistor Q2 changes the base current of the transistor Q1 and changes its internal resistance, thereby controlling the primary voltage V2 and the primary current I2 and supplying the same to the insulating circuit 16.

That is, the output current I2 supplied to the insulating circuit 16 is controlled by the operational amplifier OP1 so that (signal voltage VS1 + feedback voltage Vf1 = 0).

Here, of the primary voltage V2 supplied to the insulation circuit 16, the current component not consumed by the insulation circuit 16 is:
The signal is bypassed through a series circuit formed by a transistor Q3 controlled by the regulator 31 and a transistor Q4 controlled by the operational amplifier OP2.

The operational amplifier OP2 controls the transistor Q4 such that ((primary voltage V2-divided voltage VC) / resistance R8) = ((signal voltage VS1-divided voltage VC) / resistance R9). The primary voltage V2 to be bypassed is controlled.

This is because the primary current I2 is large and the load resistance 1
When the supply voltage V02 supplied to the power supply 4 decreases, the primary voltage V2 also decreases. That is, the primary voltage V2 and the primary current I2
Is configured to be an inverse function relationship. As a result, a potential difference can be left between the collector and the emitter of the transistor Q1.

On the other hand, even if the operational amplifier OP2 tries to bypass the current controlled by the transistor Q4, the limiter means composed of the transistor Q3 and the resistor R6 can be obtained by the following equation (primary voltage V2 = reference voltage VREF + transistor Q3
When the condition of (base-emitter voltage VBE) is satisfied, the transistor Q3 is turned off to prevent and reduce the primary voltage V2. Therefore, the primary voltage V2 does not fall below (reference voltage VREF + base-emitter voltage VBE of transistor Q3). That is, the primary voltage V2 is not unnecessarily reduced, and the operation of the circuit on the power supply side of the insulating circuit 16, specifically, the operation of the operational amplifiers OP1 and OP2 can be ensured.

Considering this point in detail,
As shown in FIG. 2, the relationship between the output current I2 and the primary voltage V2 will be described by comparing the presence or absence of limiter means (transistor Q3 and resistor R6). As a condition, the power supply voltage is 12
V, the feedback resistor R1 is 50Ω, and the reference voltage VREF is 3.4.
V, the load resistance 14 is 250Ω, the primary voltage V2 is 9 V when the output current I2 is 4 mA, and the base-emitter voltage VBE of the transistor Q3 is 0.6 V.

First, when no limiter means is provided,
As shown in the figure, the primary current I2 is increased to 16 mA,
Even when the current reaches about 20 mA, the primary voltage V2 remains at this primary current I
It decreases linearly in inverse proportion to the increase of 2. On the contrary, when the limiter unit according to the present invention is provided, first,
As shown in (1), when the primary current I2 exceeds 16 mA, the voltage between the collector and the emitter of the transistor Q1 is limited by the limiter means including the resistor R6 and the transistor Q3, the transistor Q3 is turned off, and the bypass is prevented. Therefore, the operational amplifier OP1 operates to balance the feedback voltage Vf1 via the feedback resistor R1, the internal resistance between the collector and the emitter of the transistor Q2 increases, and the internal resistance between the emitter and the collector of the transistor Q1 decreases. Less. That is, as shown in (1), when the primary current I2 exceeds 16 mA, the bypass is prevented by the limiter means of the resistor R6 and the transistor Q3 and the conduction state of the transistor Q1 is improved, so that the primary voltage is plotted by the limiter means. When the primary current I2 exceeds about 16 mA, the voltage becomes constant, and its value is about 4V. In this way, the measurement control circuit 1 can be used without making the primary voltage V2 unnecessarily too small.
The operation of the circuit such as the operational amplifiers OP1 and OP2 of the power supply 2B, particularly on the power supply side, can be ensured.

On the other hand, when the primary voltage V2 is maintained at a constant voltage by the limiter means, the insulation circuit 1
3, the primary current I2 is approximately 4 mA when the primary voltage V2 is approximately 9 V, as shown in FIG. 3, and the primary voltage V2 decreases linearly until the primary current I2 becomes approximately 16 mA. When the primary voltage V2 is approximately 4 V or more, the primary current I2
Keeps 4V even if increases. When the primary current I2 and the primary voltage V2 having such a relationship are supplied to the insulating circuit 16, when the primary voltage V2 is linearly supplied to the insulating circuit 16 from about 9 V as shown in FIG. 16 increases from 4 mA to 8 mA, and the primary voltage V
When 2 becomes a constant value of approximately 4 V, the current consumption can be maintained at approximately 8 mA, and the operation of the insulating circuit 16 can be ensured without the primary voltage V2 being unnecessarily too small.

Next, a two-wire transmitter according to a second embodiment of the present invention will be described with reference to FIG.

The two-wire transmitter according to the second embodiment is similar to that of FIG.
As shown in the figure, the transmission current I
power supply means for supplying the power o, that is, the power supply circuit 11
And the transmission current Io is supplied via the transmission lines L1 and L2 and based on a signal corresponding to the measured physical quantity.
And a measurement control means for supplying the load to the load, that is, a measurement control circuit 12C.

In such a configuration, the power supply circuit 11
Forms a series circuit with a DC power supply 13 provided on the load side and a load resistor 14 connected in series to the DC power supply 13, and supplies a supply voltage V02 to the load resistor 14. Terminals T1 and T2 at both ends of the series circuit are connected to transmission lines L1 and L2, respectively.
2 are connected to terminals T1 'and T2' of the measurement control circuit 12C.

The transmission current I flowing through the transmission lines L1, L2
o is flowed in the range of 4 mA to 20 mA. Among them, the minimum transmission current Io of 4 mA is the minimum power supply necessary for the measurement control circuit 12C.

The measurement control circuit 12C has terminals T1 ', T
2 ′, the emitter-base of the transistor Q1, the collector-base-emitter of the transistor Q2, and the resistor R
2. Between the circuit in which the feedback resistors R1 are connected in series, respectively, and the connection point (point b) between the collector of the transistor Q1 and the resistor R2 and the feedback resistor R1 in the circuit connected in series.
Current control means connected in parallel, that is, a current control circuit 15B for generating a primary voltage V2, a signal processing circuit 17A having a sensor 18 for measuring Karman vortex and the like, and a signal frequency SF from the signal processing circuit 17A F / V conversion circuit 26 for converting to voltage VS1, and switching means for switching to analog / digital signal by connector 35, that is,
And a switching circuit 36.

A pulse signal output circuit 25 for generating a pulse signal PLS is connected via a connector 35 between the series circuit constituted by the transistor Q1 and the terminals T1 'and T2'.

The pulse signal output circuit 25 generates a pulse signal PLS which is a digital signal under a predetermined condition. One of the power supply terminals is a pin 1 and a pin 2 (in the case of analog) of the connector 35, or Pin 2 and Pin 3
The power supply voltage VCC is supplied so that the power supply voltage VCC can be connected to the terminal T1 ′ via (in the case of a pulse). The other power supply terminal VEE is connected to the terminal T2 '. or,
The pulse signal PLS is output to the terminal T3 'based on the signal frequency SF generated by the signal processing circuit 17A. As shown in FIG. 7, the pulse signal output circuit 25 includes a transistor Q between the power supply VCC and VEE.
20, 21 are connected in series, and this transistor Q20,
The resistor R22 and the collector, base and emitter of the transistor Q22 are connected in parallel to Q21. Resistance R22
Between the collector of the transistor Q22 and the resistor 21
Through the transistors Q20 and Q21. The set pulse signal SPLS is input to the base of the transistor Q22, and an intermediate point between the emitters of the transistors Q20 and Q21 becomes the output of the pulse signal PLS.

The operation of the pulse signal output circuit 25 having such a configuration is as follows.
When input is made at GH, the transistor Q22 turns on. When the transistor Q22 is turned on, a current flows from the power supply VCC in the VEE direction, and the base potential of the transistor Q20 decreases, so that the transistor Q21 is turned on, the transistor Q20 is turned off, and the pulse signal PLS becomes LOW. When the set pulse signal SPLS is input LOW, the transistor Q22 turns off, the base potentials of the transistors Q20 and Q21 rise, turning on the transistor Q20 and turning off the transistor Q21. Transistor Q
20 is turned on, the power supply VCC becomes pulse signal P
It flows to the LS side and goes HIGH. Thus, when the set pulse signal SPLS is HIGH, the pulse signal P
LS can be controlled to be LOW, and when the set pulse signal SPLS is LOW, the pulse signal PLS can be controlled to be HIGH.

The current control circuit 15B includes an operational amplifier OP4 for controlling the feedback voltage Vf1 and the primary voltage V2, and a non-inverting input terminal (+ side) of the operational amplifier OP4.
Are the pulse mode SP which is a pulse output via the non-inverting buffer U5 and the resistor R5, the feedback voltage Vf1 via the resistor R6 and the feedback resistor R1, and the signal voltage VS1 from the signal processing circuit 17A via the resistor R4. Each is added and input. The inverting input terminal (− side) of the operational amplifier OP4 is connected to the connection point (b) between the resistor R2 and the feedback resistor R1.
Point).

The signal processing circuit 17A is provided with a sensor 18 for converting a voltage signal corresponding to a measured flow rate such as the number of Karman vortices. The signal frequency SF on the output side of the sensor 18 is f / V for converting a predetermined frequency into a voltage. It is input to the conversion circuit 26. The signal voltage VS1 generated by the f / V conversion circuit 26 is supplied to the non-inverting input terminal (+
Side). In addition, the signal frequency SF inputs the set pulse signal SPLS to the pulse signal output circuit 25 via the gate when the analog output SA is HIGH.
The signal frequency SF is input to the f / V conversion circuit 26 via the gate U4 when the pulse output SP is LOW, and outputs a signal voltage VS1 converted to a predetermined voltage.

Here, the pulse output SP and the analog output SA generated by the switching circuit 36 become HIGH / LOW depending on the connection state of the pins 1, 2, and 3 of the connector 35. In the case of the analog output SA, By shorting pins 1 and 2 of the connector 35, the transistor Q6
Is turned on, the analog output SA goes high, and the pulse output SP goes low. In the case of pulse output, by shorting the pins 2 and 3 of the connector 35, the transistor Q6 is turned off, the analog output SA becomes LOW level, and the pulse output SP becomes HI.
GH level.

The receiving communication circuit 27 is provided between the signal processing circuit 17A and the terminal T1 'with the pins 1 and 2 of the connector 35 interposed therebetween, and is connected between the signal processing circuit 17A and the feedback resistor R1. The transmission communication circuit 28 is connected via the resistor R6, the capacitor C1, and the resistor R7. The other side of the transmission / reception communication circuits 27 and 28 not connected to the signal processing circuit 17A is connected to the terminal T4.

The measurement control circuit 12 having such a configuration
C can be performed by changing the connection state of the connector 35 with one touch between the analog output and the pulse output.

First, in the case of an analog output, as shown in FIGS. 4 and 5, the output of the gate U3 is fixed at a low level, and no signal is sent to the pulse signal output circuit 25. Gate U4 outputs signal voltage VS1 equal to signal frequency SF. At this time, the f / V conversion circuit 26 outputs a signal voltage VS1 proportional to the frequency. Gate U5 is LOW
Since the level is output, the operational amplifier OP4 outputs the signal VS
The primary voltage V2 and the primary current I2 are controlled so that 1 / resistance R4 + feedback voltage Vf1 / resistance R6 = 0. Further, since the pins 1 and 2 of the connector 35 are short-circuited, they are connected to the receiving communication circuit 27, so that communication can be performed from the transmission lines L1 and L2. Therefore,
As shown in FIG. 5, a signal from the signal processing circuit 17A is input to terminals T1 ′ and T2 ′, and can be supplied to a load via transmission lines L1 and L2.

In the case of pulse output, as shown in FIGS. 4 and 6, the gate U3 outputs a signal equal to the signal frequency SF. The gate U4 fixes and outputs the HIGH level, and the output VS1 of the f / V conversion circuit 26 is 0. Since the output of the gate U5 is HIGH, the primary current I2 is (reference voltage VREF / resistance R5 + feedback voltage Vf1 / resistance R6
= 0), a current for stably operating the circuit can be secured. Therefore, FIG.
As shown in the figure, the signal from the signal processing circuit 17A is
The signals can be supplied to the transmission lines L1 and L2 via 1 'and T2' and output to the counter 37 connected to the terminals T1 and T3 via the terminal T3 '.

In the case of pulse output, communication is performed at terminal T4.
Do from. Since the pins 1 and 2 of the connector 35 are open, there is no influence of noise from the pulse signal output circuit 25, so that communication is not disabled.

[0070]

As described above, in the two-wire transmitter according to the present invention, the primary voltage is limited to a predetermined value in order to ensure the operation of the current control means for controlling the transmission current supplied to the load. Due to the provision of the limiter means, even if the primary current becomes large, the terminals T1 'to
There is an effect that communication can be normally maintained by preventing the impedance of T2 'from decreasing.

Also, by providing the limiter means, the primary voltage can be limited to a constant value even when the primary current is minimum (4 mA), so that the degree of freedom in design can be increased. There is also an effect.

Further, since the analog output / pulse output is switched by one connector, the analog output / pulse output using the same circuit can be switched by one touch, and the operability can be improved. There is an effect that can be.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram of a two-wire transmitter according to a first embodiment of the present invention.

FIG. 2 is a graph showing a relationship between a primary voltage V2 and a primary current I2 when the limiter is provided and when the limiter is not provided.

FIG. 3 is a graph showing a relationship between a primary voltage V2 and current consumption in an insulation circuit by the limiter unit.

FIG. 4 is a schematic block diagram of a two-wire transmitter according to a second embodiment of the present invention.

FIG. 5 is a circuit diagram schematically showing a connection state in the case of the analog output.

FIG. 6 is a circuit diagram schematically showing a connection state in the case of the digital output.

FIG. 7 is a circuit diagram schematically showing a connection state of the pulse output circuit.

FIG. 8 is a circuit diagram of a two-wire transmitter according to the prior art, which corresponds to the first embodiment.

FIG. 9 is a circuit diagram of a two-wire transmitter according to the related art, which corresponds to the second embodiment.

[Explanation of symbols]

11; power supply circuit (power supply means); 12; measurement control circuit (measurement control means); 12A; measurement control circuit (measurement control means); 12B; measurement control circuit (measurement control means);
3; power supply (DC power supply); 14; load resistance; 15; current control circuit (current control means); 15A; current control circuit (current control means); 15B; current control circuit (current control means);
16; insulation circuit (insulation means), 17; signal processing circuit (signal processing means), 17A; signal processing circuit (signal processing means), 18; sensor, 19; reception communication circuit, 20; transmission communication circuit, 25 Pulse signal output circuit (pulse signal output means), 26; f / V conversion circuit (f / V conversion means), 27; reception communication circuit, 28; transmission communication circuit,
Reference numeral 30: communication device, 31; regulator, 35; connector, 36; switching circuit (switching means), 37;

Claims (2)

[Claims] 1. A power supply means provided on a load side for supplying a transmission current to the load, and supplying the transmission current via a transmission line and controlling the transmission current based on a signal corresponding to a measured physical quantity. And a measurement control means for supplying the load to the load, wherein the measurement control means includes a feedback resistor for generating a feedback voltage proportional to a transmission current, and a signal voltage corresponding to the measured physical quantity. A current control means capable of controlling the transmission current so that the feedback voltage matches to generate a primary voltage; and a linear control means for maintaining the inverse function relationship with the primary current flowing into the primary voltage. Insulating means for converting to a secondary voltage, measuring the physical quantity by supplying the secondary voltage, and signal processing means for generating a signal voltage based on the measured physical quantity, and when the primary voltage has decreased Two-wire transmitter, characterized by comprising a limiter means for limiting the primary voltage to a voltage value where the current control means can operate. 2. A power supply means provided on a load side for supplying a transmission current to the load, and supplying the transmission current via a transmission line and controlling the transmission current based on a signal corresponding to a measured physical quantity. And a measurement control means for supplying the load to the load, wherein the measurement control means comprises: a feedback resistor for generating a feedback voltage proportional to the transmission current; and a signal voltage corresponding to the measured physical quantity. Current control means for controlling the transmission current so that the feedback voltage coincides to generate a primary voltage related to the transmission current, supplying the primary voltage and measuring a physical quantity, and according to the measured physical quantity. Signal processing means for generating a signal frequency, a frequency / voltage converting means for generating a signal voltage corresponding to the signal frequency, and a pulse signal output means for converting the signal voltage into a pulse signal and outputting the signal voltage When, two-wire transmitter, characterized by comprising a connector and switching means for switching the analog output or pulse output the signal frequency.