# Creating the proper Vref for a thermistor

I have a 10kΩ @ 25°C NTC thermistor for temperature measurement. Problem is, I only have 3.3 V on the board and the ADC inside my micro-controller has a Vref of 1.6 V.

I tried to make a voltage divider circuit (with two 10 kΩ resistors), then have the 2nd divider circuit with the thermistor and another 10 kΩ resistor to ground:

3.3 -------R1--+--(Therm)--+---ADC pin
|           |
R2           R3
|           |
GND         GND


R1 = R2 + R3 = 10 kΩ

This didn't work. Is there a way to do what I need with resistors alone? Seems like there should be.

edit
Thanks to Steven, I was able to get the solution. I'm editing my original question so others can see the complete solution.

So I actually wanted the limit to be 122°F/50°C, and that (via lookup table for my thermistor) = 3500 Ω. Solving for R3 in the traditional voltage divider equation shows 3300 Ω. Now the issue is how to measure the resistance of the thermistor so the Steinhart Hart equation can be used to calculate the temperature. Here is my code (excuse the python)

Vcc = 3.3
Rdiv = 3300
To = 298.15     # K = 25C
beta = 3950
r25 = 10000

V = float(adc) / 1024 * 1.6
R = (Rdiv*Vcc)/V - Rdiv
r_inf = r25 * math.exp(-beta / To)
tempK= beta / math.log(R / r_inf)

• Do you have access to the 1.6V reference? If so, just make a divider of the thermistor and a ~10K resistor, running off the 1.6V. Aug 12 '12 at 22:48
• Are you sure about that 3500 Ω? It's a high value for 122 °C, and my calculations for a β of 3950 say 386 Ω. I get 3500 Ω at 50 °C. Aug 13 '12 at 11:18
• Doh, I'm an idiot. I meant to say 122°F. I've changed the question to fix this blunder. Aug 14 '12 at 6:15
• "Doh, I'm an idiot." Well, I don't know about that, so I'm not going to comment on it :-). But 122 °F = 50 °C, so everything looks OK! Aug 14 '12 at 6:18

Calculate or lookup the NTC's resistance for the highest expected temperature, for instance 40 °C. Let's say your NTC's $\beta$ is 4310, then the resistance at 40°C will be 5 kΩ. Now remove R1 and R2, so that you're left with a simple divider NTC/R3. The 5 kΩ is your lowest resistance, and will give you the highest output voltage. That's your 1.6 V. Then
$\dfrac{R3}{R3 + 5 k\Omega} \cdot 3.3V = 1.6 V$