JPH0364919B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention is directed to a two-wire transmission device that converts a physical quantity to be measured into an electrical signal and transmits current to a remote load via a two-wire line. In particular, the present invention relates to the effective use of power source energy obtained via a two-wire line.

<Conventional technology> Figure 4 shows a conventional two-wire transmission device developed in 1983.
-6434 "Title of the Invention: Transmitter" (corresponding U.S. Pat. No. 4,084,155) and will be described.

10 is a sensor such as a vortex flow meter, which outputs a frequency signal corresponding to the flow rate. This frequency signal is amplified by the amplifier 11 and sent to the Schmitt circuit 12.
is applied to Schmitt circuit 12 generates square wave pulses P _S whose repetition rate corresponds to the frequency of the flow signal. This frequency is converted into an analog voltage _{V a} corresponding to the flow rate by a frequency/voltage converter 13, and this analog voltage V _a is converted by a conversion circuit 14.
For example, it is converted into a unified current of 4 to 20 mA.

The conversion circuit 14 includes a load 15 such as an indicator, and a terminal.
A current is supplied from the DC power supply 16 via two-wire lines L ₁ and L ₂ that connect T ₁₀ and T ₁₁ and terminals T ₂₀ and T ₂₁ , respectively, and the conversion circuit 14 outputs a current corresponding to the flow rate, for example. control.

A diode 17, a current control transistor 18, and a Zener diode 1 are connected between terminals T ₁₁ and T ₂₁ .
9, a resistor 20 is connected in series, and a battery 21 is connected in parallel to both ends of the Zener diode 19.

The voltages +V and -V generated across the series connection of the Zener diode 19 and the resistor 20 are applied to both ends of the series circuit of the constant current diode 22 and the Zener diode 23, and the connection between the constant current diode 22 and the Zener diode 23 is completed. A reference voltage V _f is obtained at the point.

This reference voltage V _f is applied to the positive input terminal of the differential amplifier 24, and a constant voltage V ₀ is obtained at its output terminal.
This constant voltage V ₀ is voltage-divided and sent to a differential amplifier 25.
A predetermined bias voltage is applied to the positive input terminal of the An analog voltage V _a corresponding to the output signal of the sensor 10 is applied from the output terminal of the frequency/voltage converter 13 to the negative input terminal, and the current control transistor 18 is connected to the corresponding voltage generated at the output terminal.
control the transmission current flowing through the

26 is a counter circuit whose power supply +V _b , -
_Vb is supplied from a battery 21 independent of the power supplies +V and -V of the conversion circuit, and the pulses obtained by dividing the square wave pulse _Ps of the Schmitt circuit 12 by a frequency dividing circuit 27 are counted.

The transmission current flowing through the load 16 is, for example, 4~
It is assumed that the unified current is 20 mA, but in this case, when the output from the sensor 10 is zero, the counter circuit 26
The operating current of the circuit is selected such that it consumes less than 4 mA, excluding, for example, maintaining the circuit power supply at 3 mA. Therefore, the battery 21 is charged with 1 mA to 17 mA. The current consumption at the counter 26 is the sensor 10
increases as the output increases, but at the same time the transmission current also increases, so the discharging rate and charging rate per unit time in the battery 21 are balanced, the energy utilization rate is averaged, and the operation of the counter 26 substantially increases the The transmission current will not be affected.

The power supplies of the circuits except the counter 26 are voltages +V, -
Supplied by V.

<Problems to be Solved by the Invention> However, in such a conventional two-wire transmission device, when the battery 21 is discharged, both ends of the Zener diode 19 are short-circuited, causing damage to the differential amplifiers 24, 25, etc. Power supply voltage is not applied and 4~
A problem arises in that the 20mA transmission current is not transmitted.

<Means for Solving the Problems> In order to solve the above problems, the present invention includes a load, a two-wire line, a first constant voltage element, a current control means for controlling the transmission current flowing through the load, and a battery. are connected in series to a DC power source to form a transmission line, and a second constant voltage element is connected in parallel to a series circuit in which at least the current control means and the first constant voltage are connected in series via a constant current circuit. Then, a constant voltage is applied across the second constant voltage element as a circuit power supply for a signal conversion circuit that converts the physical quantity to be measured into an electric signal corresponding to the physical quantity, and when this battery completes charging, it is connected in parallel with the battery. Connect a third constant voltage element that allows current to flow,
The voltage across the first constant voltage element is selected to be greater than the sum of the lowest voltage that does not impair the constant current characteristics of the constant current circuit and the constant voltage of the second constant voltage element, and the battery power source is connected to the circuit power source. The circuit is connected as a power source for another auxiliary circuit independent of the circuit, and the current control means is controlled by the output of the signal conversion circuit.

<Example> Hereinafter, an example of the present invention will be described based on the drawings. FIG. 1 is a block diagram showing one embodiment of the present invention. Note that parts having the same functions as those of the two-wire transmission device shown in FIG. 4 are designated by the same reference numerals, and explanations thereof will be omitted as appropriate.

A Zener diode 28, a current control transistor 18, a Zener diode 19, and a feedback resistor 29 are connected in series between terminals _T11 and _T21 .

Furthermore, a series circuit of a constant current circuit 30 and a Zener diode 31 is connected in parallel to both ends of the series-connected circuit of the Zener diode 28, the current control transistor, and the Zener diode 19.

Voltage across the Zener diode 31 +V _c , -V _c
is used as a power supply voltage for the sensor circuit 32, digital processing circuit 33, analog circuit 34, etc.

A series circuit of a diode 35 and a battery 21 is connected in parallel to both ends of the Zener diode 19. Voltage across battery 21 +V _b , -V _b
is used as a power source for the auxiliary circuit 36. The auxiliary circuit 36 is connected to the digital processing circuit 33 via line 3.
Information is exchanged and controlled via 7. The auxiliary circuit 36 includes, for example, a liquid crystal indicator and a voice device.

Further, the voltage across the resistor 29 is applied to the analog circuit 3 via a line 38 to a voltage proportional to the transmission current I _L.
Return to 4.

The sensor circuit 32 converts a physical quantity, such as flow rate or pressure, into an analog voltage. The converted analog voltage is input to the digital processing circuit 33.

The digital processing circuit 33 includes, for example, an analog/digital converter, a microprocessor, a memory, and a digital/analog converter, digitizes the input analog voltage, performs necessary digital operations, converts it into an analog voltage, and converts it into an analog voltage. circuit 34
Output to. The digital processing circuit 33 also instructs the auxiliary circuit 36 to operate, for example, a liquid crystal indicator in the auxiliary circuit 36, as necessary.

The analog circuit 34 controls the current control transistor 18 so that the voltage difference between the output voltage of the digital processing circuit 33 and the feedback voltage generated across the resistor 29 becomes zero.

The transmission current I _L controlled in this way is
The value corresponding to the physical quantity detected by the sensor circuit 32 can be determined by the voltage generated across the load 15.

By the way, if the current consumption I _C in the constant current circuit 30 is, for example, 3.8 mA, a current of (I _L -3.8) flows through the current control transistor 18, the battery 21 is charged with this current, and the battery 21 is further charged with this energy. Drive circuit 36. When charging of the battery 21 is completed, the signal current controlled by the current control transistor 18 is shunted by the Zener diode 19.

A diode 35 inserted in series with the battery 21 is for preventing discharge from the battery 21 when the DC power supply 16 is not connected.

The Zener diode 28 is for ensuring the minimum voltage at which the constant current circuit 30 exhibits constant current characteristics, and even if the battery 21 is discharged, the constant current circuit 30 is supplied with a voltage of, for example, I _C =3.8 mA. The constant voltage value of the Zener diode 28 is selected so that the current flows. For example, if the constant voltage value of the Zener diode 31 is 6.4 (V) and the constant current circuit 30 exhibits constant current characteristics at 1 V or more, the constant voltage value of the Zener diode 28 is set to a value of 7.4 (V) or more. Note that the Zener diode 31 has a normal voltage +V _C , - when the current consumption in the constant current circuit 30 is 4 mA or less, for example 3.8 mA.
Selected to be V _C.

Further, the current consumption from the battery 21 is selected to be zero when the control signal from the digital signal processing circuit 33 to the auxiliary circuit 36 is zero.

According to such a configuration, even if the battery 21 is completely discharged and the Zener diode 19 is short-circuited, a transmission current I _L of 4 to 20 mA corresponding to the output of the sensor circuit 32 is transmitted to the load 15. can be supplied.

FIG. 2 is a partial block diagram showing another embodiment of the invention. In this case, the Zener diode 19
is inserted into the collector side of the current control transistor 18, and a Zener diode 28 for maintaining the circuit voltage when the battery 21 is completely discharged is placed on the emitter side of the current control transistor 18. A level shift circuit 39 for changing the operating level of the auxiliary circuit 36 is inserted on the control input side of the auxiliary circuit 36. Even with this configuration, even if the battery 21 is completely discharged, the basic functions as a two-wire transmission device will continue.
20mA current transmission is ensured.

In addition to the basic configuration shown in FIG. 1, the embodiment shown in _FIG .
0 is inserted, and a diode 41 and a battery 42 are connected in series to both ends thereof, and the power supply voltages of the auxiliary circuit 36 are set as +V _b and -V _b . In this case also the second
As in the embodiment shown, a level shift circuit 43 is inserted on the control input side of the auxiliary circuit 36.

Even with such a configuration, the object of the present invention can be achieved.

In each embodiment, the current control transistor 18 is not limited to a bipolar transistor.
You can also use MOSFET.

Further, when this two-wire transmission device is installed in a hazardous location, a resistor is connected in series to the battery 21 to limit energy release.

<Effects of the Invention> As specifically explained above in conjunction with the embodiments, according to the present invention, even in a two-wire transmission device, which is limited in electrical energy, the consumption of a unified current of, for example, 4 to 20 mA or more can be achieved. In addition to being able to operate in auxiliary circuits that require current, it also has the basic function of transmitting a current of 4 to 20 mA, which corresponds to the physical quantity in the sensor circuit, even if the battery 21 is discharged and there is no electrical energy. can be secured.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention;
FIG. 2 is a partial block diagram showing a first modified embodiment of the present invention, FIG. 3 is a partial block diagram showing a second modified embodiment of the present invention, and FIG. 4 is a partial block diagram of a conventional two-wire transmission device. FIG. 2 is a block diagram showing the configuration. 10...Sensor, 12...Schmitt circuit, 1
3... Frequency/voltage converter, 14... Conversion circuit,
15...Load, 16...DC power supply, 18...Current control transistor, 19, 28, 31, 40...
Zener diode, 21, 42...battery, 2
6... Counter circuit, 30... Constant current circuit, 32
...Sensor circuit, 33...Digital processing circuit, 3
4...analog circuit, 36...auxiliary circuit, 39,
43...Level shift circuit.

Claims (1)

[Claims] 1. A load, a two-wire line, a first constant voltage element, a current control means for controlling the transmission current flowing through the load, and a battery are connected in series to a DC power source to form a transmission path, and the current control means and the A signal conversion circuit that connects a second constant voltage element in parallel to a series circuit in which the first constant voltage is connected at least in series through a constant current circuit, and converts a physical quantity to be measured into an electrical signal corresponding to the series circuit. A constant voltage is applied to both ends of the second constant voltage element as a circuit power source, and a third constant voltage element is connected in parallel with the battery to allow current to flow when the battery completes charging. The voltage across the first constant voltage element is selected so as to be greater than the sum of the lowest voltage that does not impair the constant current characteristics and the constant voltage of the second constant voltage element, and the power source of the battery is connected to the circuit power source. A two-wire transmission device, characterized in that it is connected as a power source for another auxiliary circuit independent of the signal conversion circuit, and the current control means is controlled by the output of the signal conversion circuit.