Tempertature Measurement with NTC Thermistors

While working on my solder reflow oven project, I found extremely difficult to measure temperature with a good level of precision over a large temperature range.

The problem occurs because of the NTC thermistor’s exponential resistance response curve as temperature changes. When read with a linear ADC, it is difficult to detect the correct temperature with any level of accuracy, especially at extremes.

There is however a very simple, yet effective way to flatten this curve by placing the NTC thermistor (Rt) in series with another resistor (Rs) as follows. Adding yet another parallel resistor (Rp) to the circuit, flattens this curve even more. 

The voltage output from this voltage divider is much more accurate when read with a microcontroller’s ADC. The resulting voltage curve then becomes pretty flat over the entire temperature range.

Since the response is still not linear, a lookup table has to be used to get the correct temperature values, but this approach makes the temperature reading problem very inexpensive and quite accurate. 

This is one of the pages I lost recently. I have some additional information including a spreadsheet that will generate a Voltage -> Temperature lookup table based on an NTC thermistor of a given specification and the two additional resistors (Rs) and (Rp).

This information  will be added here as soon as I locate it.