Hello everyone,

I have a project that requires converting a 0-10VDC signal to a 10kOhm thermistor value.

Overview:
I have a device that is running a process based on a temperature reading from a thermistor. 10kOhm@77F.
Essentially, the process is running to achieve a set temperature. The set temperature variable can not be changed. I need a way to force the process to change as if the temperature setpoint was altered.

The goal(idea):
To intercept the thermistor value into a PLC, modify the temperature value by some offset value in the PLC, and then output the modified value back to original process.
(Example: Thermistor is reading a value of 40F. PLC offset value is set to -5F. PLC outputs 35F value to original process)

Paths considered and roadblocks:
The original process must be a 10kOhm@77F thermistor as an input.
The PLC is only capable of 0-10VDC output.
So, the modified temperature value can only be expressed as a 0-10VDC signal.
How can the modified temperature value that’s in the form of 0-10VDC be converted to a 10kOhm thermistor value to input back to the original process?

My first attempt at a solution was a motorized potentiometer.
A rather expensive solution, but I think it would work.
I could drive the motorized pot with the 0-10VDC, and it would rotate to the resistance required to simulate the thermistor reading I need.
I can’t help but think there’s an easier solution

I have searched for a few hours trying to find an easy solution without much luck.

Does anyone have any ideas? Please let me know if any clarification is needed.
Thank you.

 

Here is the motorized pot I was looking at.
https://www.micronor.com/products/files/MR267/MICRONOR_DS_MR267.pdf

 

How is the thermistor presently connected up so that its resistance value can be sensed? Can you post a schematic of the relevant part of the circuit?

 

How is the thermistor presently connected up so that its resistance value can be sensed? Can you post a schematic of the relevant part of the circuit?

It is simply a 2 wire sensor into a RTD analog input to the processes PLC.

 

hi guy,
Do you want to change the thermistor input to just a different fixed value or have it variable over a range of values.?

E

 

hi guy,
Do you want to change the thermistor input to just a different fixed value or have it variable over a range of values.?

E

No, it will need to be variable. I am actually using a standard 10kOhm mechanical pot as an input to the additional PLC, to allow the user to adjust the temperature offset value. The additional PLC will have min and max values that will be adjustable for the pot. (Example: Full CCW will set offset to -5F, full CW will set offset to +5F)

BTW, thank you for the responses.

 

Use an Arduino or any UP, with a table lookup ? And a DAC. And just
implement the equation that handles its non linearity .....


https://www.electronics-tutorials.ws/io/thermistors.html


Regards, Dana.

 

Use an Arduino or any UP, with a table lookup ? And a DAC. And just
implement the equation that handles its non linearity .....


https://www.electronics-tutorials.ws/io/thermistors.html


Regards, Dana.

I do see what you're saying. But I'm not sure how I can have the process PLC read the signal.
It is a 2 wire thermistor, that is being powered by the excitation voltage from the analog input. So, I can not simply put voltage from a different source to the input. Unless I'm missing something.

 

Replace the motorized pot with a digital pot.
If you know the model number of the thermistor you can look up its transfer function.
Use a microcontrol to read the 0-10VDC signal and set the digi-pot.

 

Replace the motorized pot with a digital pot.
If you know the model number of the thermistor you can look up its transfer function.
Use a microcontrol to read the 0-10VDC signal and set the digi-pot.

I did explore that option as well. I don't have any experience with them. It looked like all of them were incremental with a "count up" or "count down". And they appear to be made for PCBs. Ideally I would want screw terminals.
I have established that I need a variable resistor with values ranging from 800Ohm(or zero) to 100kOhm, with 100Ohm resolution, to mimic the thermistor range.
Do you think there is a digital pot to achieve this?

 

You will need a PCB for any electronics components. Add your desired screw terminals to the PCB.

For 100kΩ pot with 100Ω resolution you will need 1024 steps. These are available.
Here is one example.

https://www.analog.com/media/en/technical-documentation/data-sheets/AD5272_5274.pdf

 

No, it will need to be variable. I am actually using a standard 10kOhm mechanical pot as an input to the additional PLC, to allow the user to adjust the temperature offset value. The additional PLC will have min and max values that will be adjustable for the pot. (Example: Full CCW will set offset to -5F, full CW will set offset to +5F)

So, if I am understanding you, currently, the thermistor is only functioning as a set point for 25C/77F, that is at 10K. You want to be able to vary that set point (by user input) to the range of 72F to 82F. Currently, they use a 10K pot and that works, but lets them set out of band values.

A couple of thoughts occur to me, if, I understand you. These do not require a digital pot or even an analog switch, not that those are bad suggestions.

1. Replace the pot with a manual switch that connects one of five different thermisters that have different R@25C values, giving a range of set points.

2. Replace the thermistor with one that represents the lowest temp band (or highest temp band depending on NTC or PTC) and then the user selects settings on a manual switch that adds resistance by including a range of 1% fixed resistor to functionally change the R@25C.

3. Use one of those locking pots that has fine and coarse knobs, disallowing the coarse changes.

 

You will need a PCB for any electronics components. Add your desired screw terminals to the PCB.

For 100kΩ pot with 100Ω resolution you will need 1024 steps. These are available.
Here is one example.

https://www.analog.com/media/en/technical-documentation/data-sheets/AD5272_5274.pdf

Thank you for finding this.
Would you be able to help me understand how I can control the change in resistance with my PLC? The PLC has a 0-10VDC output, or just digital relays. I'm a little lost trying to figure out how I take a resistance value I want the pot to have, that the PLC knows, and make the pot go to that resistance.

 

So, if I am understanding you, currently, the thermistor is only functioning as a set point for 25C/77F, that is at 10K. You want to be able to vary that set point (by user input) to the range of 72F to 82F. Currently, they use a 10K pot and that works, but lets them set out of band values.

A couple of thoughts occur to me, if, I understand you. These do not require a digital pot or even an analog switch, not that those are bad suggestions.

1. Replace the pot with a manual switch that connects one of five different thermisters that have different R@25C values, giving a range of set points.

2. Replace the thermistor with one that represents the lowest temp band (or highest temp band depending on NTC or PTC) and then the user selects settings on a manual switch that adds resistance by including a range of 1% fixed resistor to functionally change the R@25C.

3. Use one of those locking pots that has fine and coarse knobs, disallowing the coarse changes.

I think there is a little misunderstanding. I'll try to clarify.
The thermistor is actual measuring a discharge air temperature from a process. The process has a temperature setpoint of 35F. This temperature setpoint is not adjustable. So the process runs to maintain the thermistor reading to 35F. (The thermistor is a 10kOhm@77F)
I am trying to alter the thermistors measured temperature value before it's read into the process PLC, to force the process to run the maintain a warmer or cooler discharge air temperature. Currently there are no potentiometers of any kind in the process.

 

I think there is a little misunderstanding. I'll try to clarify.
The thermistor is actual measuring a discharge air temperature from a process. The process has a temperature setpoint of 35F. This temperature setpoint is not adjustable. So the process runs to maintain the thermistor reading to 35F. (The thermistor is a 10kOhm@77F)
I am trying to alter the thermistors measured temperature value before it's read into the process PLC, to force the process to run the maintain a warmer or cooler discharge air temperature. Currently there are no potentiometers of any kind in the process.

I guess I am misunderstanding....when you said

I am actually using a standard 10kOhm mechanical pot as an input to the additional PLC, to allow the user to adjust the temperature offset value.

but now "Currently there are no potentiometers of any kind in the process"

That discrepancy is what is confusing me.

 

You need to find the model number of the thermistor. The manufacturer will be able to provide you with a table relating resistance versus temperature. You plug these numbers into a spread sheet program so that you can determine the resistance for any given temperature.

I don't know where your PLC is getting its information. Nevertheless, you measure the 0-10VDC output using a microcontroller (MCU).
The MCU either calculates or uses a look-up table to determine the desired thermistor resistance. The MCU steps the digital pot to the desired resistance. (The digital pot does the same function as your idea of a motorized potentiometer.)

 

You need to find the model number of the thermistor. The manufacturer will be able to provide you with a table relating resistance versus temperature. You plug these numbers into a spread sheet program so that you can determine the resistance for any given temperature.

I don't know where your PLC is getting its information. Nevertheless, you measure the 0-10VDC output using a microcontroller (MCU).
The MCU either calculates or uses a look-up table to determine the desired thermistor resistance. The MCU steps the digital pot to the desired resistance. (The digital pot does the same function as your idea of a motorized potentiometer.)

In principle, but digital pots can not handle much current and certainly not what that motorized pot posted can. Would not that be a potential limitation?

 

In principle, but digital pots can not handle much current and certainly not what that motorized pot posted can. Would not that be a potential limitation?

I am not certain of what the TS is trying to do. As I understand it, TS wants to mimic the thermistor so as to "fool" the system into reading a different temperature. The nominal thermistor resistance is 10kΩ @ 25°C. With 5V supply that will draw 0.5mA. Thermistors should be operated at low currents in order to prevent self-heating. The digi-pot listed above is good for 2mA.

 

I guess I am misunderstanding....when you said


but now "Currently there are no potentiometers of any kind in the process"

That discrepancy is what is confusing me.

Ah, I see. My apologies. I was speaking about a potentiometer I plan to add to this modification I'm working on. My plan was to have this potentiometer be the means for the end user to select the new temperature setpoint the process will run to. (The "offset" I have been describing). This would be an input to the additional PLC I am adding. Totally separate from the thermistor reading.

 

You need to find the model number of the thermistor. The manufacturer will be able to provide you with a table relating resistance versus temperature. You plug these numbers into a spread sheet program so that you can determine the resistance for any given temperature.

I don't know where your PLC is getting its information. Nevertheless, you measure the 0-10VDC output using a microcontroller (MCU).
The MCU either calculates or uses a look-up table to determine the desired thermistor resistance. The MCU steps the digital pot to the desired resistance. (The digital pot does the same function as your idea of a motorized potentiometer.)

I am following you. I'm trying to see if there is a way I can do this using just the PLC I'll already have in place for this mod, instead of adding another component(MCU).

The 0-10VDC output from the PLC doesn't need to be used. I just have that available from the PLC. So my original goal was to convert the 0-10VDC signal to the variable resistance value(fake thermistor). But if I can use a PLC output to control the digital pot to achieve the resistance value, perfect!

In summary:
(For talking purposes I will call the PLC that's already in place and running, the "Process PLC". I will call the new PLC I plan to add the "Mod PLC")
Thermistor is read into Mod PLC.
Mod PLC converts thermistor resistance value into a temperature value.
Mod PLC adjusts temperature value based on end user selected offset.
Mod PLC controls Digital Pot to thermistor resistance value equal to adjusted temperature value.
Digital Pot resistance value is read into Process PLC.

If this is the route I end up taking, I need to figure out how the Mod PLC will control the Digital Pot.