JPH0710475Y2 - 2-wire transmitter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention receives a supply of an external power source from a load side through two transmission lines, converts a physical quantity to be measured into an electric signal, and processes the signal. The present invention relates to a two-wire transmitter that transmits a current signal to a load via a transmission line, and in particular, it is the sum of the minimum operating voltage required for the operation of the circuit of the two-wire transmitter, the load resistance, and the voltage drop of the transmission line. The present invention relates to a two-wire type transmitter improved so as not to cause an abnormal operation such as hunting of output current or oscillation even when the external power supply voltage becomes smaller than the above voltage.

<Prior Art> FIG. 3 is a configuration diagram showing an overall configuration of a two-wire transmitter.

The two-wire transmitter TSR is supplied with power from an external power supply Es to its terminals T1 and T2 via a load RL and two transmission lines 1 and 12. Then, the transmitter TSR processes the process variable detected as an electric signal by the sensor and controls the current signal IL according to the fluctuation of the process variable via the terminals T1 and T2 to load the load.
The process variable is detected by receiving the voltage drop of RL.

That is, a system in which two transmission lines are shared for transmission of a process variable to a load and power supply of a two-wire type transmitter is adopted.

FIG. 4 shows the circuit structure of the output part of the inside of the two-wire type transmitter.

Output transistor TR1 and feedback resistor RO are connected between terminals T1 and T2.
And are connected in series, and the output transistor TR
The connection point between the emitter of 1 and the feedback resistor RO is connected to the common potential point COM.

The constant current circuit CC is connected between the terminal T1 and the common potential point COM.
And Zener diode VR are connected in series.

A Zener diode is connected to the inverting input terminal (-) of the error amplifier Q1.
Voltage across VR is divided by the resistors R1 and R2 divided voltage V ₁ ^-
Is applied to the non-inverting input terminal (+) of which the process voltage Vs, which is the process variable converted into an electric signal by the sensor, is applied via the resistor R3 and the feedback voltage generated across the feedback resistor R0 to the resistor R4. Are connected via addition. The output terminal of the error amplifier Q1 is connected to the base of the transistor TR1.

In the above configuration, the current signal IL flows between the terminals T1 and T2 and generates the terminal voltage VC between these terminals.

A part of this current signal IL flows to the Zener diode VR via the constant current circuit CC, and a constant voltage VK is generated across the Zener diode VR. The constant voltage VK was divided by resistors R1 and R2 divided voltage V ₁ ^-
Is applied to the inverting input terminal (-) of the error amplifier Q1. ^{_{Therefore, V 1 - = VK · R2}} / (R1 + R2) ... is (1).

Further, the voltage V ₁ ⁺ generated at the non-inverting input terminal (+) of the error amplifier Q1 is (Vs−V ₁ ⁺ ) / R3 = V ₁ ⁺ ＋ IL ・ R0) / R4, so V ₁ ⁺ is V ₁ ⁺ = (Vs / R3-IL ・ R0 / R4) / (1 / R3 ＋ 1 / R4) = (Vs ・ R4-IL ・ R0 ・ R3) / (R3 + R4) (2) In the steady state V ₁ ⁺ = V ₁ ^- and so as to the error amplifier
Since Q1 controls, IL = [Vs-R4-VK-R2- (R3 + R4) / (R1 + R2)] / R0-R3) (3), and the current signal IL proportional to the process voltage Vs is output.

On the other hand, the internal power supply voltage EC of the 2-wire type transmitter TSR is given by EC = Es-IL.RL + IL.R0 (4).

<Problems to be Solved by the Invention> However, such a two-wire transmitter has the following problems.

When the external power supply voltage Es is low, the load resistance RL is high, and the current signal IL is high, for example, Es = 24V, RL = 1KΩ, R0
= 50Ω, IL = 20mA, the internal power supply voltage EC becomes 4V.

Therefore, the minimum operating voltage E required for the circuit to operate is
It is not possible to secure m, eg 10V. However, even in this case, if the current signal IL drops to 4 mA, EC = 19V and the circuit can operate normally.

In other words, the internal power supply voltage EC is the external power supply voltage Es, the load resistance R
Depending on L, feedback resistor R0, and current signal IL, normal operation is possible when current signal IL is small even when external power supply voltage Es is small and load resistance RL is large.
If IL becomes large, it will show unstable behavior such as inoperability.

By the way, when a microprocessor is installed like a recent two-wire transmitter, the internal power supply voltage EC is monitored so as not to cause an abnormality in signal processing, and this internal power supply voltage EC is the minimum operating voltage. When it drops below Em, a reset signal is sent to the processor to reset it.

In such a case, when the above condition is satisfied, the current signal LL is small at 4 mA when the microprocessor is started, so the internal power supply voltage EC is larger than the minimum operating voltage Em, and therefore the normal operation and damping are performed. The current signal IL rises according to the time constant, but when the current signal IL increases, the internal power supply voltage EC decreases and the microcomputer is reset.

Therefore, the microcomputer will be restarted and it will rise from 4mA again.
Will be reset in the same way as becomes larger.

By repeating this, a problem occurs that indicates the limit cycle of the oscillation state.

<Means for Solving the Problems> In order to solve the above-mentioned problems, the present invention provides a method of
A two-wire transmitter that receives the supply of an external power source through a book transmission line, converts the physical quantity to be measured into an electric signal, processes the signal, and transmits it as a current signal to the load through the transmission line. A first error amplifier that controls the output transistor so that the electric signal and the current signal are proportional to each other when the internal power supply voltage generated from the signal is equal to or higher than the minimum operating voltage; and the internal power supply voltage is monitored and this internal power supply voltage is the minimum operating voltage. And a second error amplifier connected to the output transistor to reduce the current signal when:

<Operation> When the internal power supply voltage generated from the current signal by the first error amplifier is equal to or higher than the minimum operating voltage, the output transistor is controlled so that the electric signal and the current signal are proportional, and the second error amplifier monitors the internal power supply voltage. Then, when the internal power supply voltage becomes equal to or lower than the minimum operating voltage, the current signal to the load is decreased to raise the internal power supply voltage.

In this way, the operation of the circuit is stabilized by keeping the internal power supply voltage constant.

<Embodiment> An embodiment of the present invention will be described below with reference to the drawings.
FIG. 1 is a circuit diagram of essential parts showing one embodiment of the present invention. The parts having the same functions as those of the circuits shown in FIGS. 3 and 4 are designated by the same reference numerals, and the description thereof will be appropriately omitted.

The inverting input terminal (-) of the error amplifier Q2 connects the constant voltage VK to the resistor R7.
A divided voltage V ₂ ⁻ divided by R8 is applied, and a divided voltage V ₂ ^{+ obtained} by dividing the internal power supply voltage EC by resistors R5 and R6 is applied to its non-inverting input terminal (+). Its output is connected through a resistor R9 to one end to the terminal T1 is connected to the cathode of the diode D _2, the anode of the diode D2 is connected to the base of the output transistor TR1. The base of the output transistor TR1 is the anode of the diode D1.
The cathodes are respectively connected to the output terminals of the error amplifier Q1.

Then, the outputs of the error amplifiers Q1 and Q2 are selected by the diodes D1 and D2 that function as diode switches, and the outputs of the error amplifiers Q1 and Q2 having the smaller output voltage are applied to the transistor TR1.

The divided voltage V ₂ ⁺ applied to the non-inverting input terminal (+) of the error amplifier Q2 is V ₂ ⁺ = ECR6 / (R5 + R6) (5) and is applied to its inverting input terminal (-). Divided voltage V ₂
^- The V ₂ ^- = a VK · R8 / (R7 + R8 ) ... (6).

Therefore, when the divided voltage V ₂ ⁺ related to the internal power supply voltage EC is larger than the divided voltage V ₂ ⁻ corresponding to the minimum operating voltage Em (V ₂ ⁺ > V ₂ ⁻ ), the output of the error amplifier Q2 is the positive side. The output is not affected because the diode D2 is turned off and the diode D2 is turned off.

However, if the divided voltage V ₂ ⁺ related to the internal power supply voltage EC is smaller than the divided voltage V ₂ ⁻ corresponding to the minimum operating voltage Em (V ₂ ⁺
<V ₂ ^-) is the output of the error amplifier Q2 is scaled out to the negative side, the diode D2 are turned on, the output of the error amplifier Q1 is diode D1 in a state in which fully swings positive side is turned off.

In this case, the current signal IL is controlled so that the internal power supply voltage EC becomes a constant value of EC = VK. [R8. (R5 + R6)] / [R6. (R7 + R8)] (7). The signal IL is given by IL = (Es-EC) / (RL + R0) (8).

FIG. 2 is a characteristic diagram showing the above operating conditions.

In the range shown by the straight line A1 where the external power supply voltage Es is small, while the internal power supply voltage EC shown by the straight line B1 is higher than the minimum operating voltage Em shown by the straight line C, the process voltage Vs and the current signal shown by the straight line D1.
Although IL is proportional, when the process voltage Vs increases and the current signal IL increases and the internal power supply voltage EC reaches the minimum operating voltage Em, the current signal IL becomes constant as indicated by the straight line D2.

In this case, if the external power supply voltage Es is large as shown by the straight line A2, the current signal IL has a proportional range expanded as shown by the dotted line D2, and a constant current state does not appear, and normal operation is performed.

<Effects of the Invention> As described above in detail with the embodiments, according to the present invention, the internal power supply voltage of the internal circuit is kept constant regardless of the relationship between the external power supply voltage and the magnitude of the current signal. Therefore, the operation of the circuit becomes stable, and the output of the 2-wire type transmitter equipped with the microprocessor does not exhibit oscillation operation such as limit cycle.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of one embodiment of the present invention, FIG. 2 is a characteristic for explaining the operation of the embodiment shown in FIG. 1, and FIG. 3 is a system configuration of a conventional two-wire transmitter. Block diagram showing,
FIG. 4 is a circuit diagram showing a configuration of an output part inside the two-wire type transmitter shown in FIG. Es ... External power supply voltage, TSR ... Two-wire transmitter, RL ... Load resistance, EC ... Internal power supply voltage, CC ... Constant current circuit, VK ... Constant voltage,
Vs ... Process voltage, Q1, Q2 ... Error amplifier, IL ... Current signal, Em ... Minimum operating voltage.

Claims (1)

[Scope of utility model registration request] 1. A physical quantity to be measured is converted into an electric signal by receiving an external power supply from the load side through two transmission lines, and the electric signal is processed by the electric signal to a current signal to the load via the transmission line. A two-wire transmitter for transmitting as a signal, the first error amplifier controlling an output transistor so that the electric signal and the current signal are proportional to each other when an internal power supply voltage generated from the current signal is equal to or higher than a minimum operating voltage, A second error amplifier connected to the output transistor to monitor the internal power supply voltage and reduce the current signal when the internal power supply voltage falls below the minimum operating voltage. 2-wire transmitter.