JPS5925277B2 - 2 wire transmitter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a two-wire transmitter, for example, an automatic transmitter configured to transmit a detection signal obtained at a detection end of a plant installed at a site to an instrumentation panel located at a remote location. It can be applied to control systems.

Three-wire or four-wire transmission is generally used as a transmission method in this type of automatic control system.

This transmission system includes a power supply path from the instrumentation panel to the site, and an output current path that sends a detection signal as a current signal (referred to as an output current) to the instrumentation panel. For example, when transmitting the opening degree of a valve, as shown in FIG. is given by the power supply path 2, and on the other hand, the divided voltage obtained by the potentiometer 1 according to the opening degree of the valve is converted into an output current by the converter 3 disposed near the valve, and this converted current is applied to the detection output resistor RL provided in the instrumentation panel via the output current path 4, and the detection voltage obtained across the resistor RL is thus applied to the next-stage meter 5. By the way, recently, as measurement circuits have been integrated into ICs, power consumption has been reduced, and wiring costs between the field and instrumentation panels have been reduced. In order to satisfy these demands, two-wire transmitters, as shown in FIG. 2, have come into use in which the power supply current is directly varied by a detection signal.

However, in recent years, there has been a trend in industrial meters to unify and standardize the power supply voltage to 24 V DC, the output current to 4 to 20 mA, and the signal receiving voltage to 1 to 5 V.

Conventionally, two-wire transmitters having the configurations shown in FIGS. 3 to 5 have been proposed in accordance with this standardization. The transmitter shown in FIG. 3 (or FIG. 4) has a detection end configured as a potentiometer, and is equipped with a current feedback type (or voltage feedback type) converter 3. Further, the transmitter shown in FIG. 5 has a sensing end as a temperature measuring resistor, and is equipped with a voltage feedback type converter.
In any of the transmitters, the detection output of the detection terminal is given to the non-inverting input terminal of the operational amplifier 11, and by controlling the conversion transistor 12 by the output, 4 to 2
It is converted to a constant current of 0 mA, and this converted current is transmitted to the instrumentation panel as an output current. In this two-wire transmitter, firstly, a predetermined reference voltage is applied to the detection end, that is, a potentiometer and a resistance temperature sensor, and secondly, a constant current element 13 is provided to drive the circuit at a constant current. is an essential element.

Therefore, conventionally, a constant voltage diode DZ has been used as a reference voltage element that should satisfy the first condition.

However, voltage regulator diodes have the problem of temperature drift, and in order to compensate for this, a method is generally adopted in which the temperature drift of the junction of two diodes is canceled out. It was necessary to pass a current. For example, the zener current of a normal diode is about 10 mA, and even if a diode with a particularly small energizing current is manufactured, it is necessary to flow at least a current of about 2 mA. Therefore, in order to comply with the standard output current limit of 4 to 20 mA, an expensive constant voltage diode must be used, which is one of the purposes of 2-wire transmitters, which is to reduce costs. The result is not in accordance with the
It wasn't practical. Furthermore, if a converter 3 that requires a large number of constant voltage (reference voltage) circuits, such as a temperature converter with a linearizer, is used, the voltage regulator diodes of these constant voltage circuits may also be Since each requires a supply current comparable to that for the voltage regulator diode described above, the total supply current cannot be limited to 4 mA or less.
In principle, wire transmission was impossible. On the other hand, two-wire transmitters have been adopted for the purpose of reducing wiring costs as mentioned above, and are often used when transmission distances are long. As shown, it is necessary to provide a lightning arrester 15 on the transmission line to prevent accidents caused by lightning strikes.

Therefore, it is generally desirable for a transmitter to have a large load resistance. However, since conventional two-wire transmitters include a constant voltage diode DZ, the supply voltage must be at least as large as possible to stably operate the diode DZ at a constant voltage (usually 6 to 7 V). Therefore, due to the need to reduce the load fluctuation rate, it was inevitable that the load resistance that could be taken would be considerably small (about 300Q). Incidentally, since a series resistor R is actually connected to the constant voltage diode DZ, if the load resistance becomes large, it becomes impossible to apply a voltage higher than one Zener voltage to the diode DZ. Taking the above points into consideration, the present invention consumes only a low current that satisfies the output current limit, allows for a much larger load resistance than in the past, and achieves the following effects. Therefore, the present invention proposes a two-wire transmitter that does not require the structure of the converter 3 to be larger and more complicated than the conventional one, and has good temperature characteristics and power fluctuation characteristics.

An example of the present invention will be described below with reference to the drawings.
8 and 8 show embodiments in which the present invention is applied to current feedback type and voltage feedback type two-wire transmitters as shown in FIGS. 3 and 4, respectively. That is, in the present invention, a reference voltage circuit 21 for the potentiometer 1 as a detection terminal includes a field effect transistor 22 and an operational amplifier 23.

A drain current setting resistor 24 is connected between the north S and gate G of the field effect transistor 22, the drain is connected to the output side of the constant current element 13, and the gate connection side end of the resistor 24 is connected to the reference voltage output resistor 25. It is connected to the common line 26 of the transmitter through. The voltage across the resistor 25 is input to the non-inverting input terminal of an operational amplifier 23 with an amplification factor of 1, for example, and based on the current 1c flowing to the output terminal of the amplifier 23 between the drain and source of the field effect transistor 22 and through the resistor 35, EO=V7. -1cXR
The output power of c is output. Here, Rc is the resistance value of the resistor 25. In the above configuration, the field effect transistor 22 has a temperature change in which the temperature change in the electrical conductivity of the silicon in the channel portion and the temperature change in the junction contact potential in the gate junction portion are opposite in sign, and the temperature is T, ~T. As shown in FIG. 9, there is always a point d of the drain current at which the temperature drift becomes zero when the temperature changes.

Here, IDSS is the saturated drain current, and V is the pinch-off voltage. Therefore, in this embodiment, the value of the drain current setting resistor 24 is selected in advance so that the drain current becomes Id. Note that point 1d is IDS
It can be determined by measuring S and with a measuring instrument such as a curve tracer, and substituting the measurement result of p into equation (1).

Further, the drain current ID of the field effect transistor 22 is the first
As shown in Figure 0, it exhibits constant current characteristics, and therefore the voltage z (=IcXRO) across the resistor 25 also exhibits constant voltage characteristics.
On the other hand, the operational amplifier 23 functions as a buffer for the constant voltage output to the potentiometer 1, and its output impedance is approximately OΩ.

In this way, the field effect transistor 22 has the function of constant voltage against power supply fluctuations and the function of improving temperature characteristics against temperature changes, while the operational amplifier 23 has the function of making the output impedance approximately OΩ. The reference voltage circuit 21 exhibits constant voltage operation with good temperature characteristics and power supply fluctuation characteristics. However, as the field effect transistor 22, an ordinary field effect transistor (for example, N channel, Vm-, 2m0, IDss-2 to 3mA) is used.
), the value 1d is approximately 200 to 300t.
On the other hand, if a linear IC is used as the operational amplifier 23, an operating current of about 10 μA can be obtained.

Therefore, since the current consumption of the reference voltage circuit 21 is mainly the sum of the drain current of the field effect transistor 22 and the operating current of the amplifier 23, the current consumption as a whole is a sufficiently small value of about several hundred μA. As described above, according to the present invention, as the reference voltage circuit 21 necessary for the converter installed at the site,
Compared to the conventional voltage regulator diode configuration, it is possible to obtain a device with significantly lower current consumption and lower operating voltage, which satisfies the limiting conditions for the output current and makes it easier to use the voltage regulator diode. Since the load resistance values that can be used, which were previously limited, can be expanded, a two-wire transmitter that can be used for long-distance transmission can be obtained.

In the reference voltage circuit 21 of FIGS. 7 and 8, a drain current setting resistor 24 is provided in order to operate the field effect transistor 22 with a drain current that causes zero temperature drift. It is also possible to form a resistance equivalent to this resistance 24 internally during the manufacturing process (in this case, a field effect transistor without temperature drift can be obtained); in this case,
The resistor 24 may be omitted. In this case amplifier 23R1+
At the output terminal of R2, EO=DXRs×? The output R1 voltage is obtained.

Here, R1 is the resistance value of the input resistor 30, and R2 is the resistance value of the feedback resistor 31. Furthermore, if the output resistor 25 in the reference voltage circuit 21 of FIGS. 7 and 8 is configured with a variable resistor or the amplification degree of the amplifier 23 is made variable, the value of the reference voltage to the detection terminal can be changed as required. It can be arbitrarily changed depending on the situation.

Furthermore, in the above description, the operational amplifier 23 of the reference voltage circuit 21
Although an example has been described in which a non-inverting type is applied, the same effect as in the above case can be obtained by applying an inverting type instead, as shown in FIG.

In this case, at the output terminal of the amplifier 23, -EO-1D><R
An output voltage of f is obtained. Here, Rf is the resistance value of the feedback resistor 35.

[Brief explanation of the drawing]

Fig. 1 is a system diagram showing a 3-wire transmitter, Fig. 2 is a system diagram showing a 2-wire transmitter to which the present invention can be applied, and Figs. 3 to 5 are system diagrams showing a conventional 2-wire transmitter. FIG. 6 is a system diagram showing an application example of a two-wire transmitter, FIGS. 7 and 8 are systematic connection diagrams showing an example of a two-wire transmitter according to the present invention, and FIG. and FIG. 10 are characteristic curve diagrams for explaining the same.
FIG. 11 is a connection diagram showing the main parts of another example of the present invention. 1...Detection end (potentiometer, resistance temperature detector), 3...Converter, 5...Meter, 11
...... operational amplifier, 12... constant current conversion transistor, 13... constant current element, 15.
... Lightning arrester, 21 ... Reference voltage circuit, 2
2... Field effect transistor, 23...
・Operation amplifier, 25...Resistance for reference voltage output,
26... Common line, DZ... Constant voltage diode, Es... Power supply, RL...
・Resistance for detection output.

Claims (1)

[Claims] 1. In a two-wire transmitter that converts a detection signal from a detection end into a current by a converter installed near the detection end, and transmits this current to a remote location as an output current. , the above-mentioned converter has a field effect transistor equipped with a resistance for temperature compensation between the source and gate, and the current flowing through the source and drain of this field effect transistor is caused to flow through the reference voltage output resistor. A reference voltage circuit is provided that supplies the voltage generated across the reference voltage output resistor to an operational amplifier, outputs a highly stable voltage obtained from the operational amplifier, and supplies this output voltage to the detection terminal as the reference voltage. A two-wire transmitter characterized by being designed to give