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op amp voltage scaling

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atomicspud

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Need help, trying to scale voltage from thermistor using opamp.
from temp range = -20 to +60degc
Thermistor = 3.014k to 68.237k
voltage divider = 2.17v to 10v
Voltage = 0 to 10vdc
Have been using simulator Circuit Simulator Applet (import op1.txt)
opamp does as it should unless i connect voltage dividers then goes wrong.

Help with circuit please.
 

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Here is my suggestion. Read the notes in the schematic.
 

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op amp scaling

Thanks for quick and accurate response looks like the exact thing i need.


For my benefit, can you explain how you calculated values of R1&3 and R5.
also, what did you use to simulate circuit?
 
LTSpice, free download at Linear.com

From previous experience, I know that the Voltage across the thermistor is more linear with resistance-change when the thermistor is fed with a constant current source. A constant-current source is approximated by a voltage source (12V) in series with a big resistor. However, the downside is that the ΔV across the thermistor decreases, but we can make up for that by raising the gain in the opamp.

I have done this so many times, that I can just about estimate the required gain in my head, which gives me a starting value for R5 . Note that the zero output requires that the bridge is balanced, which occurs when R1 = R3, and R2 = RT. With a running sim, I can just iterate on the values of R2 and R5 until the line hits zero and 10V respectively.
 
So, just so ive got this clear.
1. R1&R3 as long as they are the same can be raised to a larger value, initial value was approximated. The trade off bieng smaller voltage range accross thermistor which is offset by the gain.
2. R2= min thermistor resistance for the range of temp i require which zeros bridge.
3. R5 found by setting R4 to max required range and then entering circuit into simulator and adjusting R5 until 10v out.
 
You got it. I could have said it better this way: Set up the four-arm bridge so it is initially balanced when R4 = ~3K.

Note that the gain of the opamp is approx. [1+R5/R2] (ignoring R1, which you can do because R1>>R2).
 
Thanks for your help so far,youve saved me hours of searching through the net.
Couple more questions:
1. Im trying to invert the circuit but cant get the gain right, must be doing something basic wrong gain always ends up too small.
2. Ive downloaded ltspice, and run a simulation by inserting different values for the thermistor ok. How do you get a graph like the one you generated from 3k to 68.237k ?
 
By "invert", do you mean put the thermistor on the top half of the bridge, or do you just want the opamp output to swing the other way?

All Spices can do several types of analyses. In the example I posted, I am doing serial .DC solutions using the .Step Param RT ... statement, where the thermistor resistance is varied as in a FOR LOOP:

Code:
For Rt = 3000 Step 1000 Until 70,000

      Simulate the circuit using .DC 
      Plot the selected node voltages/branch currents at this RT

Next Rt

Read the HELP file on Parameter substitution, which happens when {curly braces} enclose a parameter.
 
Last edited:
swing the other way, so to speak. As it is the higher the voltage the lower the temp. I wish to invert this to match voltage to temp. Also there is more voltage span at the lower temp so i wish to turn this around also. Please see attachment.

I dont know wether its possible to have an even voltage span for each degree of temp?
 

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...

I dont know wether its possible to have an even voltage span for each degree of temp?

Not with a common thermistor. Try a PT100 or PT1000 instead.
 
This comes close: the first way is how not to do it: the second comes closer.
 

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