Conductivity Meter Digital Display Unstable Troubleshooting

The conductivity meter is a digital display instrument that measures the conductivity of aqueous solutions. It appeared in use to show the phenomenon of unstable word jump, this phenomenon occurred in the spring rainy season in the south.

We know that the input stage circuit of digital display instruments is small because of the collected signals, so the stability of its work is very important. The above-mentioned phenomenon is estimated to be related to this stage of the circuit. The instrument does not provide circuit diagrams. It is learned from the actual object that the input stage of the instrument consists of two op amp ICs (LM324N, CA3140E) and peripheral components. Both ICs and the printed circuit board are connected by IC sockets. This type of socket connection is installed. Convenient features, but the biggest drawback is that the contact resistance and thermoelectric potential formed by its plug terminals will change with changes in the ambient temperature, humidity, dust, plug pressure, etc., and this change is random, the general situation Under the circumstances, it is impossible to completely compensate. This will inevitably affect the stability of the system and create an additional output of "temperature drift" and "time drift." In addition, during the signal transmission process, the flexible contact points of the IC socket will always form an “unreliable factor”. After considering the above conditions, the IC socket is removed first, and then the two op-amp ICs are directly soldered on the printed circuit board. In order to prevent stray interference signals from being introduced from the input side of the instrument and affect the stable operation of the input stage, the input (the platinum black electrode used for measurement) to the input stage of the instrument is replaced with a shielded layer and a printed circuit. The "public areas" of the board are connected.

After the above measures were taken, the instrument's work was stable and reliable, completely eliminating the previously unpredictable jump phenomenon.