# Try to determine the resistance of an NTC thermistor

• Lessnan
In summary, a PID controller can be used to control the setpoint of a thermistor in conjunction with a TEC, but a digital potentiometer is more accurate and less noisy than an analog potentiometer. The PID controller should stay in the oven to eliminate thermal differences and drift.
Lessnan
I am currently using a Wheatstone bridge to determine the resistance of a NTC thermistor(Futher information about thermistor: http://www.apogeeweb.net/article/38.html) and feed the difference to the setpoint (analog potentiometer) as the error signal to a PID controller, giving me a way to stabilize the temperature of a sample in conjunction with a TEC to a very good degree.

Now, I'd like to have a way to instead control the setpoint digitally using a microcontroller. The first way I thought of was to simply replace the analog potentiometer by a digital one:

However, the selection of suitable digital potentiometers (able to have 5V at one terminal and possibly a small, negative one at the other) is quite limited and the best one I could find (AD5292) has a temperature coefficient of ~30ppm/K, which is very high for the required accuracy. Additionally, the Digipot gives me a setpoint resolution of ~0.02°C, which I'd like to push down to 0.01°C.

What do you guys think of instead replacing the potentiometer by a fixed resistor, and comparing the voltage after the INA with the output of a DAC and using this difference as the error signal for the PID loop? Would this give me the best performance? (Noise/Stability of this circuit is required to be on the order of milli-Kelvins or even less)

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Lessnan said:
(Noise/Stability of this circuit is required to be on the order of milli-Kelvins or even less)
Over how long a time span?

1. You will need to place your entire analogue circuit and PID controller in the stabilised oven to eliminate thermal difference voltages. That includes the final voltage regulator for the analogue supply voltages.

2. A "digital pot" is a ratiometric potential divider, not a reference resistor. Use it as a divider, not as a resistor. Keep it in the oven.

3. If the microcontroller is in the loop, use a PWM output to control the TED.

4. If you use the controller's A to D converter you should keep the controller in the oven also, or amplify the error signal before it exits the oven.

5. There will be a long term drift. If possible monitor the drift rather than trying to correct the temperature setting. Any disturbance of settings will add confusion to the knowledge of the stability.

## 1. What is an NTC thermistor?

An NTC thermistor is a type of temperature sensor that has a negative temperature coefficient, meaning its resistance decreases as the temperature increases. It is made of a ceramic or polymer material that has been doped with impurities to make it more conductive.

## 2. How does an NTC thermistor work?

As the temperature increases, the atoms in the thermistor material vibrate more, causing more collisions between the free electrons and the atoms. This results in a decrease in resistance, allowing more current to flow through the thermistor. Conversely, when the temperature decreases, the resistance increases and less current can flow.

## 3. How do you determine the resistance of an NTC thermistor?

To determine the resistance of an NTC thermistor, you can use a multimeter set to the resistance or ohms mode. Connect the thermistor to the multimeter and measure the resistance at the desired temperature. Alternatively, you can use a circuit with a known voltage and measure the current through the thermistor to calculate the resistance using Ohm's law (R = V / I).

## 4. What are the applications of NTC thermistors?

NTC thermistors are commonly used in temperature sensing and control applications, such as in thermostats, ovens, and refrigerators. They are also used in electronic devices to compensate for changes in temperature that can affect their performance.

## 5. How accurate are NTC thermistors?

The accuracy of an NTC thermistor depends on several factors, including the quality of the manufacturing process, the stability of the materials used, and the calibration of the device. Generally, NTC thermistors have an accuracy of around ±1°C, but this can vary depending on the specific thermistor and its operating conditions.

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