Thermister Circuit Questions

Hello, we've had some troubles with measuring the resistance of thermister for a while. We use 2252 NTC Thermistors. For the PLC side, we use AB CompactLogx and the module connecting to thermisters is 1769-IF16V which read analog voltage input. From the resistance versus temperature for thermistors table, the reasonable resistance range is between 1740 to 9600 ohm. We designed the simple voltage divider circuit with 24V applied to it. Basically it's just adding a resistor in serial with a thermistor. We tried using 18K resistor. Some thermistors read a reasonable voltage, some don't (at first read fine, but later the voltage drop to about half the value as before). We suspect it may because of self heat. So we tried using resistor with higher resistance, like 50k and 100k but the problem persist. Am I missing something here? How will you connect the PLC to thermistors?

Thank you everyone!

Use an RTD module?

Use an RTD module?

Click to expand...

The OP has a thermistor, different animal. Or replace the thermistor with a RTD.

Even if you got your voltage divider to work the response not linear.

A transmitter would be your best option.

http://www.robertoweninc.com/produc...ng/4-20ma/resistance/thermistor/hardware.html

http://www.advindsys.com/Products/Transmitters.htm

http://www.google.com/#hl=en&sclien...iWXy_43zGQ&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.&

Mickey said:

The OP has a thermistor, different animal. Or replace the thermistor with a RTD.

A transmitter would be your best option.

http://www.robertoweninc.com/produc...ng/4-20ma/resistance/thermistor/hardware.html

http://www.advindsys.com/Products/Transmitters.htm

http://www.google.com/#hl=en&sclien...iWXy_43zGQ&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.&

Click to expand...

Thanks Mickey! I checked the links. Do those transmitters output 4-20mA only? The PLC module we have now only read voltage input (0-5V, 1-5V or 0-10V)

Mickey said:

The OP has a thermistor, different animal. Or replace the thermistor with a RTD.

Even if you got your voltage divider to work the response not linear.

A transmitter would be your best option.

http://www.robertoweninc.com/produc...ng/4-20ma/resistance/thermistor/hardware.html

http://www.advindsys.com/Products/Transmitters.htm

http://www.google.com/#hl=en&sclien...iWXy_43zGQ&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.&

Click to expand...

Oops, misread the questions

One thing you did not state is the temperature range for the stated resistance range. Thermistors are very sensitive to body heat from the fingers. I am using a 500 Kohm thermistor that varies between 75F and 400F for a 480Kohm change in a voltage divider connected to a Siemens S7-200 PLC. I have no problems with the voltage divider circuit .

KAJ said:

One thing you did not state is the temperature range for the stated resistance range. Thermistors are very sensitive to body heat from the fingers. I am using a 500 Kohm thermistor that varies between 75F and 400F for a 480Kohm change in a voltage divider connected to a Siemens S7-200 PLC. I have no problems with the voltage divider circuit .

Click to expand...

Hi, thanks for the reply. These thermistors were placed down the ground for geothermal reasons. From the table, the resistance of the thermistors change from 1160 ohm to 76000 ohm. If your thermistor has 500 Kohm, then it could be quite different from ours.

colorofwind said:

Thanks Mickey! I checked the links. Do those transmitters output 4-20mA only? The PLC module we have now only read voltage input (0-5V, 1-5V or 0-10V)

Click to expand...

Then put a resistor across the card's input terminals.

250 ohms give you 1-5 volts
500 ohms gives you 2-10 volts.

Then just do the engineering units scaling in the PLC.

Mickey said:

Then put a resistor across the card's input terminals.

250 ohms give you 1-5 volts
500 ohms gives you 2-10 volts.

Then just do the engineering units scaling in the PLC.

Click to expand...

Oops, how dumb I am to forget this!

I think we'll try fixing the voltage divider circuit first before buying these transmitters for economical reasons. But it's good to know that these options are available.

colorofwind said:

Oops, how dumb I am to forget this!

I think we'll try fixing the voltage divider circuit first before buying these transmitters for economical reasons. But it's good to know that these options are available.

Click to expand...

As stated above

Even if you got your voltage divider to work the response is not linear.

Click to expand...

Mickey said:

As stated above

Click to expand...

Mickey you are right. We convert the resistance to temperature in PLC software using a second order equation we got from Excel. It showed a 99.95% accuracy rate with the lookup table.