JPH0710474Y2 - 2-wire transmitter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a two-wire type transmitter that receives a current supply through two transmission lines and also transmits a signal, and particularly this two-wire transmitter. The present invention relates to a two-wire transmitter improved so as to increase the current capacity of a stabilized power supply used inside the transmitter.

<Prior Art> FIG. 2 is a block diagram showing a configuration of a conventional two-wire transmitter.

The 2-wire transmitter 10 is a process variable such as pressure and flow rate.
Corresponding to PV, this is converted into an electric signal and its output terminal T ₁ ,
The current signal I _L corresponding to this process variable PV is output to T ₂ .

There are two transmission lines at the output terminals T ₁ and T ₂ of this 2-wire type transmitter 10.
Power supply 11 for energizing this 2-wire transmitter 10 via l ₁ and l ₂
And a receiver 12 for receiving and indicating the current signal I _L , for example, are connected in series.

This current signal I _L is 4 to 4 in the field of process control.
It is set within the unified current range of 20mA. Of these, the 4 mA current output when the process variable PV is zero is the minimum current required to cover the circuit power supply of the transmitter 10.

In this case, the circuit configuration in the vicinity of the output terminals T ₁ and T ₂ of the two-wire type transmitter 10 is as shown in FIG. 3, for example.

A diode D ₁ , an output transistor Q ₁ and a feedback resistor R _f are connected in series between the output terminals T ₁ and T ₂ in this order.
Between the collector and the emitter of this output transistor Q ₁ , a field effect transistor Q ₂ in which a resistor R ₁ is connected between the source and the gate functioning as a constant current circuit CC and a Zener diode DZ ₁ are connected in series. There is.

The base and collector of the transistor Q ₃ are connected between the gate and drain of the electric field transistor Q ₂ . Power supply terminal for biasing the control amplifier Q ₄ that controls the output transistor Q ₁ is connected between its emitter to the emitter of the output transistor Q _1.

The control amplifier Q ₄ processes the voltage corresponding to the process variable PV by a circuit (not shown) to control the current flowing through the output transistor Q ₁ . Then, the current I _{S1 of the} internal circuit is taken out from the anode of the Zener diode DZ ₁ .

In the above configuration, a part of the current I _L flowing through the output terminals T ₁ and T ₂ is taken into the constant current circuit CC as the constant current I _D , and a part of the current I _Z1 is passed through the Zener diode DZ _1. Generate a reference voltage E _Z1 across both ends. Then, the current I _S1 is supplied from both ends to the internal circuit requiring a stable circuit power supply.

<Problems to be Solved by the Invention> However, the power supply unit of the two-wire transmitter as described above has the following drawbacks.

The current I _D1 flowing in the constant current circuit CC is the sum of the current I _Z1 flowing in the Zener diode DZ ₁ and the current I _S1 flowing in the internal circuit. Then, when the current I _S1 is small, the current I _D1 is also small. In addition, a field effect transistor Q _{2 with} a small current I _D1
Generally has a small gate-source voltage V _GS1 . Therefore, the minimum operating voltage can be reduced while the current I _S1 is small, but the current I _D1 increases as the current I _S1 increases, and the voltage V _GS1 also increases accordingly, making the minimum operating voltage difficult. The problem occurs.

<Means for Solving the Problems> In order to solve the above-mentioned problems, the present invention provides a method of
A two-wire type transmitter that receives a power supply through a transmission line of a wire, converts a physical quantity to be measured into an electric signal, processes the signal, and transmits the electric signal to a load as a current signal through the transmission line. Constant current means for taking in a part of the constant current from one of the output ends of the two-wire type transmitter as a constant current, and a zener diode for flowing a part of the constant current to the other of the output ends and generating a predetermined reference voltage, Another part of the current flows as the power supply and the reference voltage is applied to one of the input terminals, and the other part of the current signal flows to the collector and the voltage of the emitter is the input terminal of the operational amplifier. A transistor whose base is controlled by the output of the operational amplifier and which is applied to the other of the two, and the emitter of this transistor supplies the internal current of the two-wire transmitter.

<Operation> A part of the current signal is taken as a constant current from one of the output ends of the two-wire type transmitter by the constant current means, and a part of this constant current is used to generate a predetermined reference voltage by the Zener diode.

Further, the reference voltage is applied to one of the input terminals to which another part of the constant current is supplied as a power source, and the emitter voltage of the transistor whose other part of the current signal is applied to the collector is applied to the other input terminal. The operational amplifier controls the potential of the base of the transistor to supply a power supply current from this emitter to an internal circuit requiring a stable power supply.

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

Constant current circuit CC between the collector and the emitter of the output transistor Q _1, a diode D ₂ to D _4, Zener diode DZ ₁
And are connected in series.

The operational amplifier Q ₅ has a power supply terminal to which the sum of the voltages of the diodes D _{2 to} D ₄ and the Zener diode DZ ₁ is applied, and its non-inverting input terminal (+) to the Zener diode DZ ₁
The zener voltage is applied. This diode D ₂ ~
D ₄ is connected to ensure the operating voltage range of operational amplifier Q ₅ .

The collector of the transistor Q ₃ is located at the connection point between the diode D ₁ and the constant current circuit CC, and the operational amplifier Q ₅ is located at the base of the collector.
The output terminals of are connected to each other. Then, the power supply current I _S2 is supplied from the emitter of the transistor Q ₃ to an internal circuit (not shown) that requires a stable power supply.

Furthermore, the voltage available at this emitter is also used to energize the control amplifier Q ₄ .

Next, the operation of the embodiment configured as described above will be described.

A part of the current signal I _L is taken into the constant current circuit CC as a constant current I _D2 , and this constant current I _D2 is the current I _Z2 flowing through the Zener diode DZ ₁ and the consumption current I flowing through the power supply terminal of the operational amplifier Q _5.
It is equal to the added current with _P.

The operational amplifier Q ₅ is a transistor so that the reference voltage E _Z2 generated across the Zener diode DZ ₁ applied to the non-inverting input terminal (+) and the voltage at the inverting input terminal (−) are the same.
It controls the base of Q ₃ and draws the power supply current I _S2 from its emitter. That is, a stable and large power supply current I _S2 is obtained by buffering the reference voltage E _Z2 with the operational amplifier Q ₅ .

In this case, the constant current I flowing through the field effect transistor Q _2
The power supply current I _S2 does not flow through _{D2, and} it is a small current because it is only the sum of the current I _Z1 of the zener diode _{D Z 1} and the consumption current I _P flowing at the power supply end of the operational amplifier Q ₅ , and is therefore a small current. Gate-source voltage of field effect transistor Q ₂ V _GS1
And the minimum operating voltage can be reduced.

Incidentally, tools may be used Ena diode or resistor in place of the diode the diode D ₂ to D ₄ in the embodiment shown in Figure 1.

<Effect of Device> As described above in detail with the embodiments, according to the present invention, the current taken into the constant current circuit is configured to be small. Therefore, the voltage effect in this constant current circuit should be small. Therefore, the minimum operating voltage can be reduced.
Further, since the reference voltage is buffered by the operational amplifier and is output via the transistor, a stable and large consumption current can be supplied.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of one embodiment of the present invention, FIG. 2 is a block diagram showing a configuration of a conventional 2-wire type transmitter, and FIG. 3 is a 2-wire type transmitter shown in FIG. It is a circuit diagram showing an internal configuration on the output end side. 10 ... two-wire transmitter, 11 ... power supply, 12 ... receiver, Q ₁ ... output transistor, Q ₄ ... control amplifier, Q ₅ ... operational amplifier, CC ... constant current circuit.

Claims (1)

[Scope of utility model registration request] 1. A physical quantity to be measured, which is supplied with power from a load side through two transmission lines, is converted into an electric signal, and the electric signal is processed as a current signal to the load through the transmission line. In a two-wire type transmitter for transmitting, a part of the current signal is taken as a constant current from one of the output ends of the two-wire type transmitter, and a part of the constant current is sent to the other of the output ends. A Zener diode that flows and generates a predetermined reference voltage, an operational amplifier in which another part of the constant current flows as a power supply, and the reference voltage is applied to one of the input ends, and another of the current signals. A transistor, a part of which is applied to the collector, the voltage of the emitter of which is applied to the other of the input terminals of the operational amplifier, the base of which is controlled by the output of the operational amplifier. 2-wire transmitter which is characterized in that so as to provide an internal current.