Hello All,
I designed and assembled /almost completed a machine for industrial gas sampling. Main units of this machine is GLCD, 2 Electronic Cards (Controller card and Analog Measurement Card), Switch Mode Power Supply, Frequency Controller and 1KW asynchronous air pump. Analog electronic board/card contains 7 sensors (3 pcs pressure, 2 pcs temperature/LM35 and 2 pcs thermocouple). Controller Card contains 3 MCU (8722 x 2 and 4685). Frequency controller controls the pump, and MCU's controls the frequency controller. When the pump is off all the sensors works perfectly. But when the pump starts to operate thermocouple measurement results become unstable / when other sensors are still ok. The thermocouple ICs are AD594/595.
It is obvious that pump creates interference by its electromagnetic field and also by electrical connection lines where these interferences are aslo amplified by AD594/595. I've assembled choke coil between connection of analog card (where AD594/595. located) and frequency controller. Thermocouple results (AD594/595) become much more stable. Then to make signal much more stable I've add LC filter to both AD594/595 inputs and outputs. I've reduced the noise from input cables. Much more clear output but still have a problem.
Main problem is that. As I explained I've two twin Thermocouple IC AD595 and AD595. Both supply voltage is 12V DC and ground. I've shortcut the thermocouple inputs of these ICs (pin1 and pin14). When I look at the outputs one IC1 is something like 240mV (room temperature) but IC2 is something like 90mV. I thougt that there could be problem with IC2. Then I removed both and put each to the other's place. Now IC2 is 240mv and IC1 is 90mV. It seems that there is design problem over PCB.
Both ICs Pin1 is connected to the ground. But there is priority problem. when IC1 240mv and IC2 90mV, IC1's line from thermocouple first visits pin1 of IC1 and then connected to the ground. IC2's thermocouple first visits ground and then connected to pin1 of IC2.
When I connect the IC2's thermocouple first to the pin1 output voltages became equal / both are 240mV.
Before solving this the biggest problem was :
When pump is off, IC1 output was 240mv and IC2 output was 90mV. When pumps starts to operate IC1 output was something 500mv and IC2 output was something 600mV. Magnituded became higher. When pump was turned off IC1 comes to it previous value of 240mv but IC2 comes to unpredictible value. Sometimes 400mV or 450mv, sometimes 390mv, sometimes 100mv. Is there anyone can guess or explain something why this is happening?
For example then the pump stops and IC2 output is 450mv I shot down the system and also pulled off the electric cable from line. Wait for 3-4 mins. then started the system. IC2 output is still 450mV. In real it must be 240mV.
Anyone can guess what is happening or any idea? Could it be Ground Capacitance differentiation??
I know it is not easy to realize what is happening from my writtens. Circuit and all the system has to be inspected and experimented but may be someone inside you could experienced similar problem?
I designed and assembled /almost completed a machine for industrial gas sampling. Main units of this machine is GLCD, 2 Electronic Cards (Controller card and Analog Measurement Card), Switch Mode Power Supply, Frequency Controller and 1KW asynchronous air pump. Analog electronic board/card contains 7 sensors (3 pcs pressure, 2 pcs temperature/LM35 and 2 pcs thermocouple). Controller Card contains 3 MCU (8722 x 2 and 4685). Frequency controller controls the pump, and MCU's controls the frequency controller. When the pump is off all the sensors works perfectly. But when the pump starts to operate thermocouple measurement results become unstable / when other sensors are still ok. The thermocouple ICs are AD594/595.
It is obvious that pump creates interference by its electromagnetic field and also by electrical connection lines where these interferences are aslo amplified by AD594/595. I've assembled choke coil between connection of analog card (where AD594/595. located) and frequency controller. Thermocouple results (AD594/595) become much more stable. Then to make signal much more stable I've add LC filter to both AD594/595 inputs and outputs. I've reduced the noise from input cables. Much more clear output but still have a problem.
Main problem is that. As I explained I've two twin Thermocouple IC AD595 and AD595. Both supply voltage is 12V DC and ground. I've shortcut the thermocouple inputs of these ICs (pin1 and pin14). When I look at the outputs one IC1 is something like 240mV (room temperature) but IC2 is something like 90mV. I thougt that there could be problem with IC2. Then I removed both and put each to the other's place. Now IC2 is 240mv and IC1 is 90mV. It seems that there is design problem over PCB.
Both ICs Pin1 is connected to the ground. But there is priority problem. when IC1 240mv and IC2 90mV, IC1's line from thermocouple first visits pin1 of IC1 and then connected to the ground. IC2's thermocouple first visits ground and then connected to pin1 of IC2.
When I connect the IC2's thermocouple first to the pin1 output voltages became equal / both are 240mV.
Before solving this the biggest problem was :
When pump is off, IC1 output was 240mv and IC2 output was 90mV. When pumps starts to operate IC1 output was something 500mv and IC2 output was something 600mV. Magnituded became higher. When pump was turned off IC1 comes to it previous value of 240mv but IC2 comes to unpredictible value. Sometimes 400mV or 450mv, sometimes 390mv, sometimes 100mv. Is there anyone can guess or explain something why this is happening?
For example then the pump stops and IC2 output is 450mv I shot down the system and also pulled off the electric cable from line. Wait for 3-4 mins. then started the system. IC2 output is still 450mV. In real it must be 240mV.
Anyone can guess what is happening or any idea? Could it be Ground Capacitance differentiation??
I know it is not easy to realize what is happening from my writtens. Circuit and all the system has to be inspected and experimented but may be someone inside you could experienced similar problem?
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