JPS58205865A - Two-electrode type digital display conductivity measuring circuit - Google Patents

Abstract

PURPOSE:To exclude the reception of influence due to external disturbance noise, by a method wherein a current detecting resistor is provided to a conductivity cell in series and voltages at both ends of said resistor are inputted in an A/D converter while voltages at both ends of a conductive cell are connected to the reference input terminal of the A/D converter. CONSTITUTION:Voltage difference e1-e2 at both ends of a current detecting resistor R is multiplied by a differential amplifier 14 and rectified by a rectifying circuit 15 to be inputted in an A/D converter 17. On the other hand, the voltage e2 at both ends of a conductivity cell 13 is rectified by a rectifying circuit 16 to be applied to the reference input terminal of the A/D converter 17 and e4 is divided by e5 in the A/D converter 17 to be displayed by a digital display 18. By this constitution, because one end of the conductivity cell 13 can be connected to a common terminal, a conductivity measuring circuit becomes strong against disturbance noise and, because cell applied voltage e1 does not impart influence to the display value at all, it is unnecessary to provide a buffer circuit even if an unstable power source is used.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an Fi bipolar digital display conductivity measuring circuit.

Conventionally, a circuit shown in FIG. 1 has been used as such a measuring circuit. E1 in the figure. E! is a reference voltage, l is a 11 current voltage-AC voltage converter (hereinafter referred to as a DA converter), 2 is a buffer circuit for keeping the output voltage et' constant, 3
is a conductivity cell, 4d amplifier, 5 is a rectifier circuit, 6 is an analog-to-digital converter (hereinafter referred to as an A-D converter), 7
#i Digital display.

The principle by which conductivity can be measured using this measuring circuit is as follows. That is, the voltage applied to the conductivity cell 3 is el
If the output voltage of the s amplifier 4 is C2, and the equivalent resistance of the conductivity cell is γ, then el is f e one - C1... (1)
It can be expressed as γ. Since e is considered to be a constant voltage as mentioned above, e! is equal to 1, that is, it is proportional to the conductivity. Since this measured specific conductivity is an analog quantity, the rectifier circuit 5, A-
The signal is converted into a digital quantity through a D converter 61 and displayed digitally on a display 7.

However, the above measurement circuit has the following drawbacks in actual measurement.

■ Both ends of the conductivity cell are floating from the common terminal, so it is susceptible to disturbance nozzles, which causes measurement errors.

■ Fluctuations in the cell applied voltage e directly affect measurement accuracy. For this reason e! Therefore, the buffer circuit 2 becomes an essential configuration. However, in order to provide this buffer circuit, the circuit configuration on the cell applied voltage source side becomes extremely complicated.

The present invention provides a novel two-electrode digital display conductivity measurement circuit that can eliminate the above-mentioned drawbacks of conventional measurement circuits. A current detection resistor is provided, the voltage difference across the resistor is connected to the input of an analog-to-digital converter, and the voltage across the conductivity cell is connected to the reference input terminal of the converter.

Embodiments of the present invention will be described below based on the drawings.

FIG. 2 shows a two-pole digital display conductivity measurement circuit as an embodiment of the present invention, in which the EIFi reference voltage, 11
is a D- converter, 12 is an oscillator that drives the converter,
A conductivity cell 13 is connected in series with a current detection resistor R and connected to the output terminal of the DA converter 11. -m of the conductivity cell 13 is connected to the common terminal. I4 is a differential amplifier, 15.16 is a rectifier circuit, 17 A
-D converter, 18 is a digital display.

According to this configuration, the voltage difference (e+e2) between both ends of the current detection resistor R is amplified by the differential amplifier 14, and the rectifier circuit 15
The voltage e! across the conductivity cell 13 is rectified and input to the A-D converter 17. is rectified by the rectifier circuit 16 and applied to the reference input terminal of the A-D converter 170. Then, the A-0 converter 17 divides the input voltage e, t of the former by the reference input voltage e5 of the latter in the A-D conversion process, and the method of division is displayed on the digital display 1B.

The digital display 18 can display the conductivity K (=-) of the conductivity cell 13 by the following operation.The following is an explanation using all mathematical formulas. When the current I flowing through the conductivity cell 13 is expressed using Re eH, C2, the output voltage e3 of the differential amplifier 14 and the output voltage e4 of the rectifier circuit 15 are C3"" eHC2... ...(3)C4
= a x C3= a (e+-e,) +++++++
+...+ --- (4) However, it is a conversion coefficient of the aperture rectifier circuit 15. When formula (3) (<) is substituted into formula (2) and rearranged, the relationship between e and I becomes as follows. Alternatively, it can also be written as follows.

C4=aXRXI ・・・・・・・・・・・・
・ (6) - 10,000, the relationship between the current ■ and the voltage e2 across the conductivity cell 13 is as follows: If the conversion coefficient of the rectifier circuit 16 is b, its output voltage e
5t/'i, the output voltage of the A-D converter 17, that is, the digital display 1
The displayed value A of 8 becomes C50 from equations (6) and (8), and as a result, the digital display will display all values proportional to the conductivity K. And in this case, "la
If XR is adjusted to 1, it can be displayed in the conductivity.

In the above configuration, one end of the conductivity cell 13 can be connected to the common terminal, making it resistant to disturbance nozzles, and as can be seen from equation (9), the cell applied voltage e1 has no effect on the displayed value. Therefore, it can be used even with power sources with high output impedance or unstable power sources that are affected by ambient conditions, such as batteries. Moreover, even if such unstable power supply noise is used, there is no need to provide a buffer circuit as in conventional circuits.

Hi, in Figure @2, the differential amplifier 14 amplifies the difference in voltage across the power supply detection resistor R and then rectifies it, but conversely, the voltages at both ends are first rectified separately. The difference voltage may be amplified. The circuit in that case is shown in Figure gIIJ3.

Furthermore, it is advantageous to use an A-D converter with a built-in differential amplifier, since the number of circuit elements can be reduced.

The circuit shown in Figure 4 uses an A-reverse converter 17 with a built-in differential amplifier, and uses synchronous rectifier circuits as the rectifier circuits 15 and 16.

This circuit is equivalent to the circuit shown in FIG. but,
If implemented using the circuit shown in Fig. 4, there will be fewer circuit elements than the circuit shown in Fig. 2.3, and the overall structure will be simplified, resulting in cost reduction, and additional advantages include lower power consumption. have.

Since the bipolar digital display conductivity measuring circuit according to the present invention is constructed as described above, it has the following effects.

■ Since the power source can be detected with a resistor connected in series with the conductivity cell, one end of the conductivity cell can be dropped to the common or common terminal, making it resistant to disturbance nozzles.

- Since the cell applied voltage does not affect the indicated value of the digital display, an unstable power source such as a battery can be used as the cell applied power source, and a buffer circuit for voltage stabilization is not required. Therefore, this has the effect of simplifying the power supply side circuit and reducing costs by one person.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional two-pole digital display conductivity measurement circuit, FIG. 2 is a circuit diagram as an embodiment of the present invention, and FIGS. 3 and 4 are respectively other embodiments of the present invention. FIG. 2 is an example circuit diagram. 13... Conductivity cell, 17... A-D converter,
R...Resistance for current detection.

Claims (1)

[Claims] A current detection resistor is provided in series with the conductivity cell, and the voltage difference between both ends of this resistor is connected to the input of an analog-to-digital converter, and the voltage across the conductivity cell is converted as described above. A two-electrode digital display conductivity measuring circuit characterized in that it can be connected to the reference input terminal of a device.