> One can easily buy 50 PPM/ degree metal film resistors. You can do much better than that. I don't know how much it matters if the bridge resistors are mounted near (temperature wise) the sensor.
I poked 10K into Digikey, scrolled down to Resistors, selected through-hole, and picked 10K (there were a lot of false hits) and then 1, 2, and 5 ppm/C 1ppm/C, 0.01% is $18. 2ppm/C, 0.01% is $14. 5ppm/C, 0.05% is $2.75 ea, min qty is 10 5ppm/C, 0.1% is $1.41 ea, min qty is 10 I didn't find any 1 ppm/C in surface mount. For the ones I checked, prices are in the same ballpark. I'm not sure what values they already stock and/or which values they will order if nobody has asked for that value yet. (I assume they order a whole reel if somebody orders 10.) > I donât know what a PID is but I agree about using a bridge circuit. PID is Proportional/Integral/Derivative It's the magic in a large class of feedback loops. http://en.wikipedia.org/wiki/PID_controller > Letâs assume for example we want 80 C. for our oscillator. > The Ni1000 is rated as 1482.5 ohms at 80C and 1489.1 ohms at 81C resulting > in a change of about.0066 ohms per milli-degree. > As stated earlier, the standard platinum 100 ohm sensor is a nominal 0.385 > ohm/°C. or .000385 ohms per milli-degree. I assume you would pick handy values for 2 of the resistors, then adjust the 3rd resistor so the bridge is ballanced at the target temperature. By "adjust" I was thinking of starting with a good guess and adding a series/parallel resistor for the fine print. The idea is that you are aiming for a null. You adjust the target temperature by tweaking the resister balancing the sensor. > The last unknown for me is what type of op-amp does one use? Look up Instrumentation Amplifier. One of the classic applications is reading a bridge. The typical package is 3 op-amps, 2 unity gain buffers as isolation stages feeding a 3rd op-amp that does the real work. Usually the package contains a few well matched resistors. Sometimes you need to add one more resistor to set the gain, sometimes you set the gain by tying a few pins high/low. > Although the platinum sensor is superior can such a low value of change be > used practically in a bridge circuit made by us time-nuts? How much gain do you need? Instrumentation amplifiers work fine when setup for gains like 1K. Rick Karlquist and crew have written a wonderful paper on this area. http://www.karlquist.com/oven.pdf I think things will be a lot easier if you can use a lot of volume for insulation. -- These are my opinions, not necessarily my employer's. I hate spam. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.