JPS6343598Y2 - - Google Patents

Description

[Detailed description of the invention] This invention relates to an improvement to a two-wire signal transmitter which is generally used in the industrial measurement field, has an input side and an output side which are DC-isolated, outputs the input signal as a current, and uses the transmission line to obtain operating power.

In industrial measurement, in order to transmit an electric signal corresponding to a weak measurement quantity generated at a detection end to a measurement location, it is often converted into a unified current signal of, for example, 1 to 20 mA, and then transmitted. In many conventional transmitters, the detection end circuit and the signal conversion/transmission circuit are not electrically insulated within the transmitter, which often causes various problems. For this reason, signals are sent to other instruments by insulating them inside a power supply box or distributor housing the transmitter drive power source and receiving instruments such as indicators and recorders. On the other hand, the transmitter is preferably of an input/output isolated type in which the input signal and the transmitted signal are isolated internally. The advantages are, for example: (a) Installation work becomes easier.

(b) The influence of external noise is reduced, increasing the reliability of measurements.

(c) Safety in use is increased because grounding can be done arbitrarily.

etc. can be mentioned.

However, in reality, there are many technical problems in meeting these conditions for the transmitter in connection with the power supply and insulating it internally, and these are difficult to solve, so it is not possible to put it into widespread practical use. has not been reached yet.

This idea solves these technical problems,
This satisfies the requirements for an inexpensive input/output isolated transmitter.

First, a conventional input/output isolated type transmitter will be explained with reference to FIG. The input signals applied to the input terminals 1 and 2 are converted into current signals and output to the output terminals 5 and 6 via the primary circuit 3 and the secondary circuit 4 which is DC-insulated from the primary circuit 3, and then transmitted. I get a signal.
That is, a signal from a detection terminal applied between input terminals 1 and 2 is zero-point adjusted by a zero-point adjustment circuit 7 in the primary side circuit 3, and then amplified by an amplifier 8. The amplified output is converted into an alternating signal by the converter 9, DC-insulated by the isolation transformer 11, and supplied to the secondary circuit 4. In the secondary circuit 4, the output of the isolation transformer 11 is again converted into a DC signal by the synchronous switch 12. The DC signal is added to the feedback signal in the adder circuit 13, and the amplifier 14
The amplified output is supplied to the base of the voltage-current conversion transistor 15. The collector of the transistor 15 is connected to the output terminal 5, and the emitter is connected to the output terminal 6 through a resistor 16 and a current detection resistor 17. The connection point of resistors 16 and 17 is connected to a common potential point. Output terminal 5,
6 is connected to both ends of a power supply box (or distributor) 20 through two-wire transmission lines 18 and 19. In the power supply box 20, the transmission lines 18 and 19 are connected to each other through a load 21 such as an indicator or a recorder and a drive power source 22. The feedback voltage obtained at the current detection resistor 17 is fed back to the addition circuit 13 through the feedback resistor 23.

The output terminal 5 is branch-connected to the voltage stabilizing circuit 24, and the output terminal 5 is connected to the amplifier 14 by the output of the voltage stabilizing circuit 10.
A positive operating voltage is applied to DC−
It is supplied to the input circuit 26 of the DC converter 25. The alternating output of the input circuit 26 is supplied to an isolation transformer 27 and also to a rectifying and smoothing circuit 28 . The output of the rectifying and smoothing circuit 28 is supplied to the amplifier 14 as a negative operating power supply voltage. The output of the isolation transformer 27 is supplied to a rectifying and smoothing circuit 29 in the primary circuit 3, and its positive and negative outputs are supplied to the amplifier 5 as an operating power supply voltage. The converter 9 and the synchronous switch 1 are connected by the alternating signal of the DC-DC converter 25.
2 are synchronously controlled.

With this configuration, 1 to 100% of the input is converted to a current of, for example, 4 to 20 mA, and the transistor 1
5 and is transmitted to the load 21, and the operating current of the primary circuit 3 and secondary circuit 4 is 4 mA or less and is obtained from the drive power supply 22 through the transmission lines 18 and 19.

In the conventional circuit as described above, the power supply to the primary circuit 3 and the negative power supply to the secondary circuit 4 are shorter than the positive power supply to the secondary circuit 4 due to the startup time and rectification smoothing of the DC-DC converter 25. The operation is delayed by the time constant of the circuits 28 and 29. For this reason, a positive voltage is generated on the base side of the transistor 15, causing an excessive current to flow during startup, and the externally connected load 21
In addition, because the voltage drop caused by the current detection resistor 17 makes it impossible to obtain the required voltage of the transmitter, there is a risk that stable startup will not be possible.

To prevent this phenomenon, a limiting resistor or the like is used to prevent current from flowing into the transistor 15 from the amplifier 14 beyond a certain level, but the DC-DC converter 2
In order for 5 to operate stably at startup, it is necessary to lower the supply voltage from the voltage stabilizing circuit 10 as much as possible.
- The DC converter 25 must output a high voltage with a low input voltage, and the required current when viewed from terminals 5 and 6 increases, for example, in the case of 4 to 20 mA transmission, the lower limit current exceeds 4 mA. There is a risk that the product may become damaged. Moreover, doing so reduces the positive power supply voltage of the amplifier 14, which reduces its dynamic range, impairs linearity, and makes it more susceptible to noise.

As described above, conventional input/output isolated transmitters have not been able to provide sufficient performance.

This invention improves the various drawbacks mentioned above by obtaining both the positive and negative operating power supply voltages required for the secondary circuit from a DC-DC converter.

For example, as shown in FIG. 2 with the same reference numerals attached to parts corresponding to those in FIG.
A rectifying and smoothing circuit 32 is connected to the next winding 31, and positive and negative power supplies are obtained from the rectifying and smoothing circuit 32 and applied to the amplifier 14 as operating voltages.

Therefore, if the DC-DC converter 25 does not operate,
Since no current flows through the transistor 15, the transistor 15 does not operate until sufficient voltage is supplied to the primary side circuit 3 and the secondary side circuit 4 at startup. Furthermore, since the voltage supplied to the secondary side amplifier 14 can be arbitrarily designed within the allowable range of power consumption, the linearity and noise characteristics are also significantly improved compared to the conventional ones.

If excessive input is applied during operation,
When this is removed, there should be no abnormality in the operation of the circuit. That is, it is necessary to prevent excessive current from flowing through the transistor 15 due to an excessive input signal. For this purpose, a resistor 33 is inserted in series with the base of transistor 15. This resistor 33 can also be inserted on the emitter side of the transistor 15, but since the voltage drop caused by this resistor enters the transmission current circuit in series during steady operation,
It is necessary to reduce the allowable load resistance by that amount, but as shown in Figure 2, there is a resistor 3 on the base side.
3, the allowable load resistance can be increased.

In addition, as a transmitter for various detector signals, the output of the synchronous switch 12 must be able to handle positive and negative signals, and for this purpose, the signal fed back from the current detection resistor 17 through the feedback resistor 23 is required. A bias signal for cancellation is required. In the circuit of FIG. 2, the output of the voltage stabilizing circuit 24 is transferred to the stabilizing circuit 3.
4, the feedback signal is stabilized again, and a bias signal is added to the feedback signal by the adjuster 35, so that positive and negative signals can be set only by setting the bias signal. This bias signal may be obtained from the positive output or negative output of the rectifying and smoothing circuit 32 in addition to the output of the voltage stabilizing circuit 24.

As mentioned above, with this invention, an isolated two-wire transmitter with excellent characteristics can be easily and inexpensively realized, and it can also be used in combination with various detection terminals, so it can be used in a wide range of industrial measurement fields. It can be used for.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the conventional input/output isolated type transmitter, and Figure 2 is the input/output isolated type 2 according to this invention.
FIG. 2 is a block diagram showing an example of the configuration of a line transmitter. 1, 2...Input terminal, 3...Primary side circuit, 4...
...Secondary side circuit, 5, 6... Output terminal, 8, 14...
...Amplifier, 15...Voltage-current conversion transistor, 18, 19...2-wire transmission line, 25...DC
-DC converter, 29, 32... rectifier smoothing circuit.

Claims (1)

[Scope of utility model registration request] The input signal is supplied to the primary circuit, the output of the primary circuit is galvanically isolated and supplied to the secondary circuit, and the output terminal of the secondary circuit outputs a current corresponding to the input signal. In an input/output isolated transmitter that supplies power to a two-wire transmission line and obtains operating power from the power from the two-wire transmission line,
a DC-DC converter to which power is supplied from the output terminal; and a DC-DC converter connected to the DC-DC converter;
An input/output isolated two-wire transmitter comprising a rectifying and smoothing circuit that obtains positive and negative operating voltages in the next circuit.