NTC Project (Wheatstone Bridge + Instrumentation Amplifier) - Stuck!

Thread Starter

jonaas18

Joined Apr 4, 2019
67
hi,
I am currently drawing out your circuit, will post when tested.
I guess you know that the resistance response of a Thermistor is exp' versus temperature.
There are methods for adding resistors in an attempt linearise the resistance, but these are only much good over short temperature spans.
Some users connect the Vout of a IA into the ADC input of a micro-controller and linearise the readings in the program.

A thermistor would not be my choice for 55C thru 150C, a small device like a LM35 IC, which gives linear Vout would be my choice.

E
We are going to linearize the graph later on, that's why it doesn't matter beeing linear or not atm.
 

Thread Starter

jonaas18

Joined Apr 4, 2019
67
hi,
With a 12V bridge voltage the Vdiff signal is over driving the IA, reduce the bridge voltage to 5V and try again, see the 2nd image.
E

Note: R5 is now 1k5
Alright, I have sim'd your schematic for my resistance limits (182.6 and 2989) and, on the output of the I.A., I have either 0V or 5.16V accordingly. It's exactly what I wanted. My calcs are also right FINALLY (Image below).

A question tho, should I use the sensor model in my schematic or should I simply manually input values to my RT. I guess the sensor is more practical but I can't seem to find any useful information on how to set it up properly (for example, defining my temperature range etc ...). I'll have to analyze the circuit via Monte Carlo Sim so I guess having the sensor would make it easier.
 

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Thread Starter

jonaas18

Joined Apr 4, 2019
67
I was trying to figure out how to the simulation V = F(T(ºC)). I think I've found it (image below).

Can't exactly explain why the I.A. voltage output starts falling when the temperature rises above 140ºC. Still, this isn't really the ideal solution as I am changing the overall temperature of the circuit and not the sensor's temperature only.
 

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crutschow

Joined Mar 14, 2008
25,648
To change the resistance of the sensor only you can use a .param Rt list command for the resistance versus temperature (say for 5°C intervals) and run a .op simulation.
 

Thread Starter

jonaas18

Joined Apr 4, 2019
67
To change the resistance of the sensor only you can use a .param Rt list command for the resistance versus temperature (say for 5°C intervals) and run a .op simulation.
I've tried some solutions from a search on the internet but it doesn't seem to work properly. Any idea how to write the command properly?

From what I've seen, I can change the sensor's resistance value by x ammount but for y ammount of time, not temperature.
 

bertus

Joined Apr 5, 2008
20,877
Hello,

As NTC's are not linear, the calculation of the resistance versus temperature is given in this formula:

NTC_formula.png

I have also attched the PDF where I have taken the formula from.

Bertus
 

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Thread Starter

jonaas18

Joined Apr 4, 2019
67
hi jonaas,
Do you have this option on your simulator.?
ie: NTC step Tempr
E
I'm working on Cadence Orcad v 17.2.

I dont really think so? Writing it down doesn't change anything (although I'm checking out some posts from people who're searching for the same information).

Hello,

As NTC's are not linear, the calculation of the resistance versus temperature is given in this formula:

View attachment 174301

I have also attched the PDF where I have taken the formula from.

Bertus
I have that formula on my papers but in what does it help me on the simulation issue?
 

Thread Starter

jonaas18

Joined Apr 4, 2019
67
I figured out how to sim Vout = F{Rt(ohm)}, as shown in the image below.

Thing is, I also wanted to sim Vout = F{T(ºC)} so I could present both graphs. I attached the error window I get.
 

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Thread Starter

jonaas18

Joined Apr 4, 2019
67
hi j,
In your simulation there is no V(tmp) for the thermistor temperature change. Ref the marked up image.
Use a PWM 0sec > 1sec , 55 > 150 to generate that variable.

E
Thanks a lot. It works properly.

Really want to say thank you to everyone who helped. I was having trouble understanding simple things that I now understand properly. Everything went according to plan, my calcs and my simulation results are equal! Cheers everyone!!!
 

Thread Starter

jonaas18

Joined Apr 4, 2019
67
Of course!

Here are my final results. Diagram + Voltage in order of rising temperature graph + Voltage in order of rising resistance graph + Max and Min Monte Carlo analisis.
 

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