Hi guys,

I've been trying to design a conductivity meter based on the tutorial in this link , The oscillator works perfectly in simulation but the second part of the circuit always outputs a constant ~10.5V DC instead of it being proportional to the resistance of the solution (I replaced the conductive solution by a resistor in simulation), no matter what value resistance I put.

Here are the circuit and simulation

 

hi nadir,
Please post your LTSpice asc file for the circuit, we can then run a test check.
E

 

hi nadir,
Please post your LTSpice asc file for the circuit, we can then run a test check.
E

Hi there,

sorry here they are, you'll need the libraries for the opamp and diodes used as well

 

hi nadir,
You have U7 upside down.
Check this out.
E
Note: for test ONLY I shortened the TRAN time.

 

hi nadir,
You have U7 upside down.
Check this out.
E
Note: for test ONLY I shortened the TRAN time.
View attachment 228284

knew it was something stupid, will check more thoroughly next time, thanks a lot

 

hi nadir,
You have U7 upside down.
Check this out.
E
Note: for test ONLY I shortened the TRAN time.
View attachment 228284

Quick follow up, i don't know if it's my ltspice bugging out but whenever I try to raise the transient beyond 160ms (I tried seeing when it reaches a steady state), I get a very weird output as you can see using 200ms transient

 

hi,
This is what I get with Rprobe step 100R to 500R, looks OK, also tried with 200mTRAN.

E

Added asc ver3..

 

hi,
This is what I get with Rprobe step 100R to 500R, looks OK, also tried with 200mTRAN.

E

Added asc ver3..

View attachment 228287

Yep turns out it was because of the initial operating point check, learned something today, thanks again

 

hi,
This is a 500mS run. ok.
e