JPS6161159B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a two-wire DC transmission device that converts industrial process variables into standard DC current signals for industrial instruments and simultaneously supplies power and transmits signals over two wires.

In recent years, there has been a growing demand for process control, and in order to centrally control it, methods have been used to convert process variables such as temperature, pressure, flow rate, etc. into electrical quantities and transmit them to distant locations. To transmit these amounts of electricity, a method is usually adopted in which alternating current power is supplied from the control room and electrical signals are sent to the control room. However, as the distance between the control room and the measurement site increases, the cost of electrical wires and wiring becomes very high, so in some cases DC power is supplied from the control room through the load and converted according to process variables. A method is used to transmit signals by changing the DC resistance of the device. In this case, there is an advantage that only two wires are needed instead of four wires, but the signal for the internal loss of the converter always has a base component, and at least the internal loss/base current voltage is the load. must be added to the voltage. For example, when a thermocouple is used to measure temperature,
It is difficult to reduce internal loss in order to amplify minute voltages, and adopting a two-wire DC feeding system requires a large voltage or large signal base current, while instruments that receive electrical signals do not require a large amount of electricity. does not require. In addition, increasing the base current ratio is undesirable because it causes errors when subtracting the base amount on the receiving instrument side.
On the other hand, it is disadvantageous to use a two-wire system when the signal current is large. Furthermore, there are models that use a 3-wire DC power supply system, which has the advantage of not having the disadvantages of the 2-wire system described above in that internal loss and signal transmission are carried out on separate lines, regardless of the presence or absence of an electrical signal base. However, this method is not necessarily satisfactory in that it requires three lines for transmission. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to eliminate the above-mentioned drawbacks and inconveniences, and to provide a two-wire DC transmission device that has a simple configuration and a wide operating range.

This invention will be explained below.

In FIG. 1 showing an embodiment of the present invention, 1
is a Zener diode for operating the differential amplifier 5 and others at a constant voltage, 2 is a field effect transistor for operating the circuit with a constant current, and 3 is a temperature coefficient using the temperature coefficient of the voltage between the base and emitter of the transistor. Transistors that make up the sensor,
4 is a field effect transistor for operating this temperature sensor 3 at a constant level, and 5 is the voltage shown in the figure.
A differential amplifier that amplifies the difference between V1 and V2, 6 is a variable resistor for span adjustment, 7 and 9 are fixed resistors forming a feedback circuit, and 8 is a variable resistor inserted between them for zero point adjustment. 10 is an output transistor that converts the output of the differential amplifier 5 into a current;
Reference numerals 11 and 12 denote a load such as a receiving instrument connected to the sensor side via transmission lines 13 and 14, and a power source for supplying power to this device. The feedback circuit is connected between the emitter of the output transistor 10 and the connection point of the Zener diode 1 and the field effect transistor 2, and the negative terminal input (V2) of the differential amplifier 5 is connected to the sliding terminal of the variable resistor 8. , the positive terminal input (V1) is input to the emitter of transistor 3, respectively.

In this configuration, the temperature sensor 3 operates with a constant current by utilizing the constant current characteristics of the field effect transistor 4, so the voltage V1 generated across the temperature sensor 3 is proportional only to the temperature change. and change. Therefore, the differential amplifier 5 amplifies the difference between the voltage V1 from the temperature sensor 3 and the voltage V2 from the variable resistor 8 of the feedback circuit to increase or decrease the current flowing through the output transistor 10.
The voltage generated in the variable resistor 6 by this current is fed back through a circuit composed of a fixed resistor 7, a variable resistor 8, and a fixed resistor 9, and the voltage is
V2 is controlled to be equal to V1. Therefore, by changing the resistance value of the variable resistor 6, the coefficient of current-voltage conversion changes, so the span can be adjusted. It has a variable zero point adjustment function, and since these also serve as feedback circuits, the number of components configuring the circuit can be reduced. Further, by making the low resistance value of the variable resistor 6 much smaller than the low resistance values of the fixed resistors 7 and 9, the influence on the zero point when span adjustment is performed can be ignored.

By the way, in a two-wire transmission device, the power for operating the circuit, that is, the current, needs to be changed in proportion to the output signal or kept at a constant value. Here, in this invention, the constant current characteristics of field effect transistors are used, and by using a type with good temperature characteristics under these usage conditions, the operating current of the circuit is maintained, and the constant current circuit using other Zener diodes and transistors In addition to being able to reduce the number of parts, the low minimum operating voltage allows the transmission equipment to have a wide operating voltage range, is less susceptible to power supply voltage fluctuations, and can be operated with a large load resistance. It has advantages.

Note that FIG. 2 shows another embodiment of the present invention corresponding to FIG. 1, in which two transistors 3 and 3' are used as temperature sensors, and the reference potentials are reversed. . Furthermore, it is also possible to use three or more transistors or a thermistor as a temperature sensor.
On the other hand, this device can be connected to any detection section that can detect changes in potential due to other process variables such as pressure and humidity in addition to temperature.

The present invention has been variously explained above with reference to preferred embodiments, but the present invention is not limited to these embodiments, and it goes without saying that many modifications can be made without departing from the spirit of the invention. That's true.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing one embodiment of the present invention, and FIG. 2 is a circuit diagram showing another embodiment. 1... Zener diode, 2... Field effect transistor, 3, 3'... Transistor, temperature sensor,
4... Field effect transistor, 5... Differential amplifier,
6, 8... Variable resistor, 7, 9... Fixed resistor, 10
...output transistor, 11...load, 12...power supply,
13, 14...Transmission line.

Claims (1)

[Claims] 1 In a DC transmission device using a two-wire connection method that converts process variables into unified electrical signals for industrial instruments and serves both as a power supply line and a signal transmission line, a difference in which a change in potential is used as an input variable and is compared and amplified. a dynamic amplifier, a fixed resistor and a variable resistor constituting a feedback circuit including zero point adjustment and span adjustment functions, an output transistor for converting the output of the comparison amplifier into a current and supplying the current to the load, and the comparison amplifier. A first field effect transistor for operating the amplifier at a constant level, a Zener diode for operating the circuit system at a constant potential, and a second field effect transistor connected to the feedback circuit for operating the circuit system at a constant current. A two-wire DC transmission device characterized by comprising a transistor.