# PT100 to 0-10V circuit question

#### alianaslan

Joined Sep 9, 2016
4
I'm working on a signal conditioner circuit for PT100. I found this awesome app note.
There is an example circuit which provides outputs of 0.97V at -100°C and 2.97V at 200°C

[Source]

My Temp range is -20'C to 150'C where PT100 is 92.16 ohms to 157.32 ohms respectively.
How can I adjust this output to 0-10V level? Any help would be really useful.

#### crutschow

Joined Mar 14, 2008
29,786
What power supply voltages do you have available?

#### alianaslan

Joined Sep 9, 2016
4
What power supply voltages do you have available?
I have 5V 12V 24V.

Joined Jan 15, 2015
6,585
You may just want to use an off the shelf turn key solution like Uxcell 24V DC 4-20mA -50C to 150C PT100 Temperature Sensor Transmitter this being just one example of dozens out there like this. With this unit and a 24 VDC supply you place a good precision 1% 500 Ohm resistor in the output current loop so as it is -50 C to 150 C has a temperature span of 200 C. The 4 to 20 mA with a resistor in the loop will give you 2 to 10 Volts for an 8 Volt span. That is 40 mV per degree C. I have used similar programmable versions from Inor which cost more but offer the programmable feature. I just compensated for the 2 to 10 volts rather than 0 to 10 volts in my code. The Uxcell is hard to beat at about $14 USD. Anyway, something you may want to think about. Ron #### tcmtech Joined Nov 4, 2013 2,867 You may just want to use an off the shelf turn key solution like Uxcell 24V DC 4-20mA -50C to 150C PT100 Temperature Sensor Transmitter this being just one example of dozens out there like this. With this unit and a 24 VDC supply you place a good precision 1% 500 Ohm resistor in the output current loop so as it is -50 C to 150 C has a temperature span of 200 C. The 4 to 20 mA with a resistor in the loop will give you 2 to 10 Volts for an 8 Volt span. That is 40 mV per degree C. I have used similar programmable versions from Inor which cost more but offer the programmable feature. I just compensated for the 2 to 10 volts rather than 0 to 10 volts in my code. The Uxcell is hard to beat at about$14 USD. Anyway, something you may want to think about.

Ron
That's basically what I use for the same PT100 to 4- 20 mA/ quasi 0 - 10 volt signal conversion processes

If you mess with the two adjustment pots for Gain and Span it's pretty easy to get them to put out a ~4 - 24 mA signal that when ran through a ~420 ohm resistor then reduced with three diodes in series to get a ~2 volt forward drop they can put out a nice neat 0 - 10 volt reference voltage. Or at least one that close enough to that to work for any basic temperature to voltage reference conversion work I have never needed.

#### crutschow

Joined Mar 14, 2008
29,786
The gain is adjusted to give a 10V span and offset adjusted to start at 0V.
The gain adjustment will affect the offset, but the offset adjustment will not affect the gain (span).

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#### alianaslan

Joined Sep 9, 2016
4
The gain is adjusted to give a 10V span and offset adjusted to start at 0V.
The gain adjustment will affect the offset, but the offset adjustment will not affect the gain (span).

View attachment 135930
Thank you so much for the effort. Appreciate your help! I have a question, I would like to design this kind of circuits on my own. For example, what was the reason for picking that OpAmp, what was special about it? Would you mind explaining stages a little bit so I can move on with learning the subject more in details by myself.

#### crutschow

Joined Mar 14, 2008
29,786
what was the reason for picking that OpAmp
The main criteria here is that you want to go to zero output, so you need an op amp whose output will go to zero with a single positive supply (which standard op amps can't do), since you have no negative supply.
Those are called single-supply op amps.
The one I used is a single-supply, rail-rail type where both the input and output can go from the positive to the negative supply voltages (rails).
You also want an op amp with low offset and offset drift since you are operating at a high gain, and the offset is amplified by the gain.
The LMC6484 values are shown here:

which are quite good.

The max4236 op amp shown in your posted circuit also has good offset specs, but its maximum supply voltage is 6V so it couldn't be used to generate your desired 10V output.

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#### alianaslan

Joined Sep 9, 2016
4
The main criteria here is that you want to go to zero output, so you need an op amp whose output will go to zero with a single positive supply (which standard op amps can't do), since you have no negative supply.
Those are called single-supply op amps.
The one I used is a single-supply, rail-rail type where both the input and output can go from the positive to the negative supply voltages (rails).
You also want an op amp with low offset and offset drift since you are operating at a high gain, and the offset is amplified by the gain.
The LMC6484 values are shown here:
View attachment 136075
which are quite good.

The max4236 op amp shown in your posted circuit also has good offset specs, but its maximum supply voltage is 6V so it couldn't be used to generate your desired 10V output.
Thank you sir! I am sending you all the good karma.

#### Picbuster

Joined Dec 2, 2013
1,038
I'm working on a signal conditioner circuit for PT100. I found this awesome app note.
There is an example circuit which provides outputs of 0.97V at -100°C and 2.97V at 200°C

View attachment 135886
[Source]

My Temp range is -20'C to 150'C where PT100 is 92.16 ohms to 157.32 ohms respectively.
How can I adjust this output to 0-10V level? Any help would be really useful.
I do see a problem with 5V and 3.01K + Pt100 value you could run into self heating Pt100 resulting in erroneous measurement.

Picbuster

#### crutschow

Joined Mar 14, 2008
29,786
I do see a problem with 5V and 3.01K + Pt100 value you could run into self heating Pt100 resulting in erroneous measurement.
That's why I increased the value to 7.15k in my simulation schematic, to give the same current through the Pt100..