CS238708B1 - Evaluation circuit, especially for conductivity sensor with galvanic isolation - Google Patents

Abstract

The solution is to connect the evaluation circuit of the conductivity sensor, especially for sensing the level of electrically conductive liquids. The circuit is also suitable for sensor and conductivity sensors. The evaluation circuit essentially consists of a limiting element, a pair of complementary transistors, a control and isolation element and a galvanically isolated output. In the above connection, an alternating electric current flows through the conductivity sensor circuit, which does not exceed a safe value. There is no polarization of the electrodes and therefore no coating is created due to the electric current. The application of the subject invention is suitable in automation and control technology.

Description

(54) Evaluation circuit, especially for conductivity sensor with galvanic isolation

The solution is to connect the evaluation circuit of the conductivity sensor, especially for sensing the level of electrically conductive liquids. The circuit is also suitable for sensor and conductivity sensors.

The evaluation circuit consists essentially of a limiting element, a pair of complete transistors, a regulating and decoupling element, and a galvanically isolated output.

In this connection, an alternating electric current that does not exceed a safe value flows through the conductivity sensor circuit. There is no polarization of the electrodes, and hence the coating is not formed by the electric current.

The application of the subject invention is suitable in automation and control technology.

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GIANT. 1

The invention solves an evaluation circuit, in particular for a conductivity headquarters and a galvanic isolation using a transformer Elen, a protective resistor, two transistors, a rectifier Sien and a terminal

Sien.

So far, thyristor evaluation circuits are used in connection with a conductivity seat. Another type of connection is the evaluation circuit with a DC transistor.

When using an optocoupler evaluation circuit, the isolation Siens are usually fed from the transformer via a bridge rectifier.

The disadvantage of wiring with thyristors and transistors is that the current passes through the sensor in only one half-wave of the supply current, thereby polarizing the electrodes. When using an optocoupler, the entire current of the optron input excitation passes through the conductive sensor. For most optrons, a safe current passing through a conductivity sensor, which in this case must be less than 10 milliamps, is not sufficient to excite.

These drawbacks are eliminated by the connection of the evaluation circuit, in particular for a conductivity sensor with galvanic isolation using a trans-form Si, a protective resistor, two transistors, a rectifier, a decoupling member and an end member according to the invention. It is based on the fact that a second diode cathode, an anode of the first diode and a conductivity sensor are connected to the first output of the transformer member, which is connected via a first protective resistor. This sensor has been wired for a longer time via a second protective resistor based on a first NIN transistor and a second PNP transistor. The bases of these transistors are connected via a control resistor to the emitter of the first NPN transistor and the emitter of the second PNP transistor and the first rectifier input. The second input of the rectifier member is connected to the second output of the transformer member. The cathode of the first diode is connected to the collector of the first NPN transistor. The first output of the rectifier member is coupled via a limiting resistor to a first input of the decoupling member, the second input of which is connected to the second output of the rectifier member. The output of the separating member is connected to an input of an end member that is provided with first and second outlets.

In particular, the circuit according to the invention has the advantage that only a safe electrical current flows through the conductivity sensor, there is no polarization of the electrodes and thus a coating on the electrodes. By means of the arrangement of the individual functional elements, current amplification is achieved at the output, with the possibility of inclusion in automation technology, regulation, etc.

An exemplary circuit arrangement according to the invention is schematically shown in the accompanying drawing.

The circuitry consists of a transformer member, first and second protective resistors. first and second transistors ££, ££, rectifier member ££, decoupling member ££, and terminal member 22 ·

The cathode of the second diode 32 and the anode of the first diode 20 are connected to the first output 83 of the transformer member and, through the first protective resistor 70, a conductivity sensor 3, which is longer connected via the second protective resistor 73 and via a control resistor 110 to the emitter of the first transistor £ and the emitter of the second transistor £ and the first input of the rectifier member, the second input of which is connected to the second output of the transformer, the cathode of the first diode. is coupled to the collector of the first transistor 12 and the anode of the second diode 30 is coupled to the collector of the second transistor 48, while the first output 83 of the rectifier member 54 is coupled via a limiting resistor 52 and the first input 81 of the separator 80. 82, the second output 48 of the rectifier member 60 is connected, and the output of the separator member is coupled to the input of the end member 22 ' It is provided with a first outlet 22 and a second outlet 21.

If, in said circuit according to the invention, it does not pass through the conductivity sensor 3, only a negligible residual current of the complementary transistors 80, 80, flows into the inlet of the separating member 80. In the case of current passing through the conductivity sensor 3, the current in both halfwaves flows through the protective resistors £, £ 3. The value of these resistors is chosen so as to provide a safe current coming alternating into the transistor bases. meet also one of the protective resistors 22> 23 included in the conductivity of its sensor. In this connection, each transistor conducts a current in one half-wave. The sensitivity of the evaluation circuit can be adjusted by a control resistor .110. The current passing through the conductivity sensor may be much smaller than the collector current of transistor 40. 90 · The diode bridge acts as a polarity switch. The limiting resistor 100 limits the input current of the separating circuit 80 - e.g. an optocoupler. The above mentioned wiring leads to el. current through the conductivity sensor in both half-waves of the alternating current and longer its considerable limitation in the conductivity sensor circuit to a safe value.

The application of the invention is useful in providing level sensing for electrically conductive liquids.

Claims (1)

SUBJECT OF THE INVENTION An evaluation circuit, especially for a conductivity sensor with galvanic isolation using a transformer, a protective resistor, two transistors, a rectifier, a decoupler and a terminator, characterized in that a second diode cathode is connected to the first output (13) of the transformer (10). (30), an anode of the first diode (20) and, via a first protective resistor (70), a conductivity sensor (3), which is further coupled via a second protective resistor (73) based on the first NPN transistor (40) and second transistor (50) of a PNP and through a control resistor (110) to the emitter of the first NPN transistor (40) and the emitter of the second PNP transistor (50) and to the first input (61) of the rectifier (60), the second input (60) of which is connected to the second an output (14) of the transformer (10), the cathode of the first diode (20) is connected to the collector of the first NPN-type transistor (40) and the anode of the second diode (30) is connected to the collector of the second PN-type transistor (50) P. While the first output (63) of the rectifier member (60) is connected via a limiting resistor (100) to the first input (81) of the separator member (80), to the second input (82) of which the second output (64) of the rectifier member (60) is connected. and further, the outlet of the separating member (80) is coupled to an inlet of an end member (90) having a first outlet (93) and a second outlet (94)