CN203572885U

CN203572885U - Conductivity/frequency converting circuit

Info

Publication number: CN203572885U
Application number: CN201320670623.0U
Authority: CN
Inventors: 高玉琴
Original assignee: Individual
Current assignee: Individual
Priority date: 2013-10-29
Filing date: 2013-10-29
Publication date: 2014-04-30
Anticipated expiration: 2023-10-29

Abstract

The utility model relates to a conductivity/frequency converting circuit, which is used to detect the solution conductivity. A comparator A1, an analogue switch IC, resistors R1-R3, a capacitance C, and a conductivity electrode DDJ form a square-wave generator; which is characterized in that: the capacitance C and a solution resistance between the conductivity electrode DDJ form a RC recharging and charging circuit of the square-wave generator; and then the current, which goes through the conductivity electrode, is converted into an alternating current. The circuit structure is simple; the conductivity of the solution can be converted into corresponding frequency, then frequency is output; the frequency of the current, which goes through the solution resistor, can be automatically adjusted according to the resistivity of the solution, and the parallel capacitance effect error and the polarization error of the conductance cell can be automatically compensated.

Description

A kind of conductivity/freq converting circuit

Technical field

The utility model relates to a kind of conductivity/freq converting circuit, for detection of the conductivity of solution.

Background technology

The measuring method of existing electrical conductivity of solution is, will a conductance electrode inserts in solution and form a conductance cell, and the electric conductivity of solution can be with the solution resistance R between conductance electrode _x inverse electricity lead 1/R _xrepresent, by solution resistance R _xafter connecting with a reference resistance R, at the two ends of this series circuit, apply on the interface that an alternating voltage U(alternating voltage can not make electrode contact with solution and polarize, guarantee the accuracy of measuring), then measure the voltage U on solution resistance _x, according to Ohm law U _x=U × R _x/ (R _x+ R) known, voltage U _xwith solution resistance R _xbecome a corresponding relation.But the structure of the measurement mechanism that this measuring method is used is comparatively complicated: 1, an oscillator device need to be set and produce alternating voltage U, an amplifying circuit also will be set to measured signal voltage U _xamplify, to the signal voltage U after amplifying _xalso need to carry out rectification and convert thereof into direct-flow signal voltage; 2, oscillator being had to higher requirement is that the amplitude of the output signal voltage of oscillator must guarantee to stablize.Moreover need to automatically control the conductivity of solution in many occasions, this need to convert the direct current simulation signal of conductivity to digital signal, for computer control system, process, converting direct current simulation signal to digital signal also needs an A/D conversion equipment.

In addition when measuring highly concentrated solution conductivity, because the current density on conductance electrode is larger, can form electrostatic double layer at electrode surface and produce chemical polarization, or make near electrode the generation concentration polarization that changes of the concentration of solution, make the conductivity of measuring lower than standard value; And measure low conductivity solutions during as pure water because conductance electrode exists an equivalent shunt capacitance, make the conductivity of measuring higher than standard value; For reducing polarisation error and shunt capacitance effect errors, when measuring, to adopt the frequency of different alternating voltages, make the design of alternating-current voltage source more complicated.

Utility model content

The technical problems to be solved in the utility model is to provide a kind of conductivity/freq converting circuit, and by the conductivity inversion frequency signal output of solution, this converting circuit structure is simple, and polarisation error and the shunt capacitance effect errors of energy auto-compensation conductance cell.

The technical solution of the utility model is its new circuit structure, it comprises comparer A1, analog switch IC, resistance R 1-R3, capacitor C, it is characterized in that, resistance R 1, R2, one end of R3 is connected with the in-phase input end of comparer A1, another termination power vd D of resistance R 1, the output terminal of another termination comparer of resistance R 2, the other end ground connection of resistance R 3, the inverting input of comparer is by capacitor C ground connection, between the inverting input of comparer and common port 5 pin of analog switch IC, be connected to conductance electrode DDJ, normal-closed end 6 pin of analog switch IC meet power vd D, the Chang Kaiduan 4 pin ground connection of analog switch IC, the output terminal of phase inverter is exported its frequency of a square-wave signal and is become a proportionate relationship with solution resistance in conductance electrode.

Described resistance R 1, the resistance of R2, R3 are 10K ohm, and the capacity of described capacitor C is 0.1 microfarad, and the electrode constant of described conductance electrode is 1.0.

The beneficial effects of the utility model are: 1, adopt simple circuit structure, frequency signal corresponding to the resistivity of solution between conductance electrode convert to; 2, this circuit can flow through with the big or small auto-changing of solution resistance rate the power frequency of solution resistance, shunt capacitance effect errors and the polarisation error of auto-compensation conductance cell.

Accompanying drawing explanation

Fig. 1 is circuit theory diagrams of the present utility model.

Fig. 2 is the working waveform figure of Fig. 1.

Embodiment

Now contrast accompanying drawing embodiment of the present utility model is described.

Circuit structure of the present utility model is shown in Fig. 1, it comprises comparer A1, analog switch IC, resistance R 1-R3, capacitor C, conductance electrode DDJ, its interconnected relationship is, resistance R 1, R2, one end of R3 is connected with the in-phase input end of comparer A1, another termination power vd D of resistance R 1, the output terminal of another termination comparer of resistance R 2, the other end ground connection of resistance R 3, the inverting input of comparer is by capacitor C ground connection, between the inverting input of comparer and common port 5 pin of analog switch IC, be connected to conductance electrode DDJ, normal-closed end 6 pin of analog switch IC meet power vd D, the Chang Kaiduan 4 pin ground connection of analog switch IC, control end 1 pin of analog switch IC is connected with the output terminal of comparer, this circuit forms a square-wave generator, the output terminal of phase inverter is exported a square-wave signal, its frequency becomes a proportionate relationship with solution resistance in conductance electrode.

The model of described comparer is LM311 or other intimate models, and the model of described analog switch is MAX4729, is hilted broadsword/double throw switch, and its conducting resistance is less than 3 ohm.

Described resistance R 1, R2, R3 form a positive-feedback circuit, level signal on comparator output terminal is fed back to the in-phase input end of comparer, the resistance of resistance R 1, R2, R3 is 10K ohm, when comparator output terminal is high level, the voltage of comparer in-phase input end is about 2/3 of supply voltage, when comparator output terminal is low level, the voltage of comparer in-phase input end is about 1/3 of supply voltage; Change the resistance of resistance R 1, R2, R3, can change the voltage of comparer in-phase input end.

Described capacitor C is the tantalum electric capacity of 0.1 microfarad, and the voltage of power vd D is+5V, for comparer, analog switch provide working power.

The electrode constant (also claiming cell constant of conductometric vessel) of described conductance electrode DDJ is 1.0.

Principle of work of the present utility model is, conductance electrode inserts in solution, solution resistance Rx between conductance electrode and capacitor C and analog switch form a RC charge-discharge circuit, terminal voltage Uc in capacitor C is input to the inverting input of comparer, when the voltage of inverting input is greater than the voltage of in-phase input end, comparator output terminal is low level " 0 ", and when the voltage of inverting input is less than the voltage of in-phase input end, comparator output terminal is high level " 1 "; Described charge-discharge circuit is controlled by the level state of inverter output, when comparator output terminal is high level " 1 ", 5 pin of analog switch communicate with 6 pin, capacitor C is charged by solution resistance Rx, when comparator output terminal is low level " 0 ", 5 pin of analog switch communicate with 4 pin, and capacitor C is discharged by solution resistance Rx, in solution resistance Rx, form alternating current thus, the frequency of this alternating current becomes a proportionate relationship with the size of solution resistance Rx.

When after power on circuitry of the present utility model, because the voltage uc of capacitor C can not suddenly change, voltage uc is 0, and the voltage of comparer inverting input is less than the voltage of in-phase input end, comparator output terminal is high level " 1 ", 5 pin of analog switch communicate with 6 pin, and capacitor C is charged by solution resistance Rx, and the terminal voltage of capacitor C is index and rises, referring to 0-t1 section waveform in Fig. 2, the waveform that in Fig. 2, Vo is comparator output terminal, uc is the waveform of capacitor C terminal voltage, I is the current waveform in solution resistance Rx.

When the terminal voltage of capacitor C rises to the voltage that is more than or equal to comparer in-phase input end, comparator output terminal is low level " 0 ", and 5 pin of analog switch communicate with 4 pin, and capacitor C is discharged by solution resistance Rx, the terminal voltage of capacitor C is index decreased, referring to t1-t2 section waveform in Fig. 2; When the terminal voltage of capacitor C drops to the voltage that is less than or equal to comparer in-phase input end, comparator output terminal is high level " 1 ", and 5 pin of analog switch communicate with 6 pin, and capacitor C is charged by solution resistance Rx, referring to t2-t3 section waveform in Fig. 2; So go round and begin again and form vibration, comparer is exported a square-wave signal, and the frequency of square-wave signal is

f = 1.443／（2Ro’＋2Rx’）C’ (1)

The resistance of the conducting resistance that in formula 1, Ro ' is analog switch, Rx ' is the resistance of the solution resistance between conductance electrode, the electric capacity that C ' is electric capacity;

Due to the resistance very little (resistance of solution resistance can convert by the conductance electrode of selecting Different electrodes constant) of solution resistance between the resistance Relative electro-conductivity electrode of the conducting resistance of analog switch, can ignore, therefore formula 1 can be rewritten as

f = 0.72／Rx’C’ (2)

The inverse of solution resistance is solution conductivity, can find out signal frequency and the electrical conductivity of solution relation in direct ratio of comparer output from formula 2.

In circuit of the present utility model, to the path of capacitor C charge circuit, be analog switch → solution resistance → electric capacity, the path in capacitor discharge loop is electric capacity → solution resistance → analog switch, therefore the electric current that flows through solution resistance is the alternating current of positive and negative half cycle symmetry, not containing DC component, when measuring electrical conductivity of solution, polarization phenomena are smaller.

Another feature of the utility model is can flow through with the big or small auto-changing of electrical conductivity of solution the power frequency of solution resistance, shunt capacitance effect errors and the polarisation error of auto-compensation conductance cell.Compensation principle is described as follows:

The electric capacity that described shunt capacitance forms for the migration of electrode for the mutual migration because of electric charge and electric charge, two pole pieces of it and conductance electrode are in parallel, and its capacity is about tens picofarads.Large (being greater than 100K ohm) of (as distilled water, pure water) its resistance when electrolysis of solutions matter concentration is lower, due to the impact of shunt capacitance, can make measured conductivity be greater than standard value, for reducing the impact of shunt capacitance, conventionally adopt lower power frequency (lower than 100HZ);

Its resistance less (being less than 100 ohm) when the electrolyte concentration of solution is higher, current density on conductance electrode is crossed conference and is caused polarization, show as at electrode surface and form electrostatic double layer or near the concentration of electrolytic solution conductance electrode is changed, this just makes equivalent solution resistance increase, and produces measuring error.According to electrochemical theory, can release the Wu Cha ⊿ R causing due to polarization,

⊿R＝E ²/fRx ² (3)

In formula 3, E is that polarization potential is relevant with the voltage swing that is applied to the alternating current on solution resistance with the electrolyte concentration of solution, and f is alternating current voltage frequency, and Rx is the solution resistance between conductance electrode;

From formula 3, can find out and adopt high ac power frequency can reduce polarisation error.

The utility model is measuring during conductivity and can, with the automatic change frequency of the size of solution resistance, can find out from formula 2, and when solution resistance, hour to flow through power frequency wherein higher, flows through power frequency wherein higher when solution resistance is larger.The capacity of getting capacitor C is 0.1 microfarad, and when solution resistance is 100 ohm-100K ohm, the power frequency that flows through solution resistance is 72KHZ-72HZ, shunt capacitance effect errors and polarisation error that therefore can auto-compensation conductance cell.

The utility model output frequency signal is equivalent to a digital signal, can save A/D conversion equipment and be directly connected with the input end of single-chip microcomputer or other data processing chips, after data processing for controlling or showing.

Claims

1. a conductivity/freq converting circuit, it comprises comparer A1, analog switch IC, resistance R 1-R3, capacitor C, it is characterized in that, resistance R 1, R2, one end of R3 is connected with the in-phase input end of comparer A1, another termination power vd D of resistance R 1, the output terminal of another termination comparer of resistance R 2, the other end ground connection of resistance R 3, the inverting input of comparer is by capacitor C ground connection, between the inverting input of comparer and common port 5 pin of analog switch IC, be connected to conductance electrode DDJ, normal-closed end 6 pin of analog switch IC meet power vd D, the Chang Kaiduan 4 pin ground connection of analog switch IC, the output terminal of phase inverter is exported its frequency of a square-wave signal and is become a proportionate relationship with solution resistance in conductance electrode.

2. conductivity/freq converting circuit according to claim 1, is characterized in that, described resistance R 1, the resistance of R2, R3 are 10K ohm, and the capacity of described capacitor C is 0.1 microfarad, and the electrode constant of described conductance electrode is 1.0.