Saturday, January 19, 2008


I spent the afternoon measuring thermistors. I got 5 100K thermistors from DigiKey. They were the 5% ones ($0.80 each). Here's the technique I used:
1. Wrap the thermistor with PTFE tape and attach to the tip of a cooking thermometer. The one I used is good up to 230C.
2. Attach wires to the thermistor, keeping them separate. If the wires cross, they might short out when the insulation heats up.
3. Attach a multimeter to the wires to read the resistance.
4. Let the whole thing sit for a while to bring it to room temperature. Note the temperature and resistance. Use this as T0 and R0.
5. Put the thermometer and thermistor into a toaster oven set to 225C.
6. Wait for it to heat up and note the temperature and resistance. Use this as T and R.
7. Plug the values into the calculator and get beta and Rz.

The betas came out around 3900 for all five, with Rz around 336000. I put a 0.22μF electrolytic cap in for C3. As far as I can tell, that'll give me 1 degree increments up to about 280C.

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