JPH0460279B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention converts the output signal of a sensor that detects physical quantities such as pressure and displacement into a corresponding current, and transmits the signal to a remote reception via a two-wire transmission line. The present invention relates to a two-wire transmitter for transmitting data to other parts.

<Prior art> As is well known, a two-wire transmitter converts the output signal of a sensor into a corresponding current and then supplies it to a two-wire transmission line.Since only two transmission lines are required, it is suitable for industrial measurement. Widely used in the field.

Although there are various types of conventional two-wire transmitters of this type, one example of the conventional two-wire transmitter that forms the basis of the improvement of the present invention is shown in FIG. 1, and will be described below.

The terminals L ₁ and L ₂ are connected to a power source E _b such as 24 VDC and a load L such as an indicator that receives current via transmission lines l ₁ and l ₂ . Terminal _L2 is connected to the common potential point COM of the internal circuit. A current adjustment circuit 1 is connected in series to the terminals L ₁ and L ₂ . The current adjustment circuit 1 includes compositely connected transistors Q ₁ , Q ₂ ,
A resistor connected to the emitter of transistor Q.
R ₁ , resistor connected to the base R ₂ , transistor
It consists of a resistor R ₃ connected to the emitter and base of Q ₂ , and a bridge circuit BR with one side extending between the base and emitter of transistor Q ₁ .

The bridge circuit BR includes resistors R ₄ , R ₅ , R ₆ and a side resistance between the base and emitter of the transistor. The input terminal of operational amplifier Q ₃ is connected between the connection point between resistor R ₄ and the base of transistor Q ₁ and the connection point between resistors R ₅ and R ₆ , and its output terminal is connected to the resistor
Connected to the connection point of R ₄ and R ₆ . With this configuration, operational amplifier _Q3 detects changes in the voltage between the base and emitter of transistor _Q1 due to temperature, and a current corresponding to this change is caused to flow through resistor _R2 via resistor _R4 . Transistor across ₂
A voltage corresponding to the voltage between the base and emitter of Q ₁ is generated in the opposite direction to equivalently transform the transistor Q ₁ .
A stable current adjustment circuit 1 is realized by avoiding the influence of base-emitter voltage fluctuations.

A constant current circuit 2 and a Zener diode Z _D are connected in series between terminals L ₁ and L ₂ .
An internal voltage V _C1 is created across Z _D. The operational amplifier section 3 includes an operational amplifier _Q4 powered by an internal voltage V _C1 ,
It consists of a resistor R ₆ connected between the output terminal of the constant current circuit 2 and the inverting input terminal of the operational amplifier Q ₄ , and a resistor R ₇ connected between the inverting input terminal and the output terminal.

The output end of the sensor circuit SC is connected to the non-inverting input end and the inverting input end of the operational amplifier _Q4 via resistors _R8 and _R9 , respectively. The sensor circuit SC is also energized with an internal voltage V _C1 .

Furthermore, the zero point adjustment circuit 4 is supplied with the internal voltage V _C1 and includes an operational amplifier forming a voltage follower.
Q ₅ , an internal voltage V _C1 is applied across the operational amplifier Q ₅
It consists of a variable resistor VR that supplies a voltage for zero point adjustment to the non-inverting input terminal of , and a resistor R ₁₀ connected between the output terminal of operational amplifier Q ₅ and the non-inverting input terminal of operational amplifier Q ₄ . There is.

The operational amplifier 3 receives and amplifies a voltage signal of, for example, 0 to 320 (mV) from the sensor circuit SC, and
It is supplied to the current adjustment circuit 1 as a voltage signal of 1.6 (V). The current adjustment circuit 1 transmits a current of 0 to 16 (mA) to the load L in response to this voltage signal.

On the other hand, the constant current circuit 2 has a sensor circuit SC, a Zener diode Z _D , and a constant current of 4 (mA), for example.
Load L via zero point adjustment circuit 4, operational amplifier 3, etc.
flow to.

Therefore, with this configuration, if the current value of the constant current circuit 2 is stable and the internal voltage V _C1 is a stable constant voltage, an accurate load current corresponding to the output voltage of the sensor circuit SC can be transmitted.

However, the circuit currents in the sensor circuit SC, operational amplifier 3, and current adjustment circuit 1 include some fluctuations, which causes fluctuations in the internal voltage V _C1 related to the fluctuations in the internal resistance of the Zener diode Z _D and the circuit current. ,
Fluctuations may occur in the output signal of the operational amplifier 3 or the sensor circuit SC.

Therefore, it is necessary for the constant current circuit 2 to control the load current flowing from the outside, the discharge voltage from the internal voltage V _C1 to the circuit, and the discharge current from the internal voltage V _C1 .

<Object of the Invention> In view of the above-mentioned prior art, an object of the present invention is to provide a two-wire transmission circuit that is not affected by fluctuations in internal current consumption and can supply a stable internal circuit energizing voltage and a stable transmission current. With the goal.

<Configuration of the Invention> The configuration of the present invention that achieves this object relates to a two-wire transmitter that receives load current through a pair of transmission lines in which a power source and a load are connected in series. constant current means that partially maintains a substantially constant constant current;
A part of the constant current is supplied to a series circuit of a resistor and a reference power supply, and an internal voltage means is inserted between the constant current means and the series circuit to create an internal voltage across this series circuit, and a part of the constant current is supplied to a series circuit of a resistor and a reference power supply. A detection resistor is connected between the transmission line and the load is connected to a current adjusting means for controlling the current; a sensor circuit to which an internal voltage is supplied and which outputs an electrical signal corresponding to the physical quantity to be measured; The present invention is characterized by comprising an operational amplifying means for controlling the adjusting means, and a current compensating means for controlling the current adjusting means via the operational amplifying means so as to cancel the voltage change in the detection resistor.

<Example> Hereinafter, an example of the present invention will be described in detail based on the drawings. Note that parts having the same functions as those in the prior art are given the same numbers, and redundant explanations will be omitted.

FIG. 2 is a circuit diagram showing one embodiment of the present invention.

The constant current circuit 2 is composed of, for example, a field effect transistor, and has a detection resistor R ₁₁ , a resistor R ₁₂ and a Zener diode Z _D connected in series between its output terminal and a common potential point COM. The voltage generated across the series circuit of resistor R ₁₂ and Zener diode Z _D is used as internal voltage V _C2 and used as the power supply voltage for each internal circuit.

The voltage at the connection point of resistor R ₁₂ and the Zener diode Z _D that constitutes the reference power supply and the internal voltage V _C2 are determined by resistor R ₁₃ ,
The voltage divided by R ₁₄ is input to the operational amplifier Q ₆ energized by the internal voltage, and its output is used as the constant current circuit 2.
The gate of the field effect transistor is controlled to keep the current flowing through the Zener diode Z _D constant. These components constitute the current control circuit 5.

A resistor _R6 is inserted between the connection point of the detection resistor _R11 and the constant current circuit 2 and the inverting input terminal of the operational amplifier _Q4 ,
Changes in the output current of the constant current circuit are detected by operational amplifier _Q4 .

In the above configuration, the detection resistor R ₁₁ , the resistor R ₁₂ ,
If the shunt current to the Zener diode Z _D is controlled to be constant by the current control circuit 5, the internal voltage V _C2 will inevitably become a constant voltage. However, although the circuit current flowing toward the common potential point due to the internal voltage V _C2 is approximately constant, it flows with some fluctuations due to circuit operation. This fluctuation causes a slight fluctuation in the adjustment current of the constant current circuit 2 because the shunt current to the resistor R ₁₂ is controlled to a constant value by the current control circuit 5, creating an error factor in the load current.

Therefore, the detection resistor _R11 detects a slight fluctuation in the adjusted current of the constant current circuit 2, and feeds it back to the operational amplifier _Q4 via the resistor _R6 , changing its output and changing the base of the transistor _Q1 . through the resistor _R1 to compensate for the error introduced in the load current. For this we need a sensing resistor R ₁₁ , a resistor
The relationship among R ₆ , R ₇ , and R ₁ may be selected so that R ₁₁ · R ₇ = R ₆ · R ₁ .

<Effects of the Invention> As described above in detail with the embodiments, according to the present invention, by controlling the shunt current to the reference power supply to a constant value by the current control means, the internal voltage can be maintained at a constant value, and at the same time. Since the change in load current that changes along with this is compensated by the current compensation means, the load corresponds to the output voltage of the accurate sensor circuit, which is not affected by slight fluctuations included in the energizing current to the internal circuit. Current can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional two-wire transmitter, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. 1...Current adjustment circuit, 2...Constant current circuit, 3...
... operational amplifier section, 4 ... zero point adjustment circuit, 5 ... current control circuit, Q ₁ , Q ₂ ... transistor, Q ₃ , Q ₄ ,
Q ₅ , Q ₆ ... operational amplifier, BR ... bridge circuit,
SC...sensor circuit, E _b ...power supply, L...load,
l ₁ , l ₂ ...transmission line, R ₁₁ ...detection resistor.

Claims (1)

[Claims] 1. In a two-wire transmitter that receives load current through a pair of transmission lines in which a power source and a load are connected in series, a constant current means that makes a part of the load current a substantially constant constant current; A part of the constant current is supplied to a series circuit of a resistor and a reference power supply, and an internal voltage means is inserted between the constant current means and the series circuit to generate an internal voltage across the series circuit, and a detection resistor that detects a change as a voltage change; and a current control means that detects a change in the internal voltage and the voltage of the reference power source and feeds it back to the constant current means to control the current flowing to the reference power source to be constant. , a current adjusting means connected between the transmission lines and controlling the load current; a sensor circuit to which the internal voltage is supplied and outputs an electrical signal corresponding to the physical quantity to be measured; and a load corresponding to the output of the sensor circuit. operational amplification means to which the internal voltage is supplied to control the current adjustment means such that the internal voltage becomes a current; and a current that controls the current adjustment means via the operational amplification means so as to cancel the voltage change at the detection resistor. A two-wire transmitter, comprising compensation means.