[time-nuts] Temperature sensors and bridge amps

Hal Murray hmurray at megapathdsl.net
Fri Nov 12 04:15:32 EST 2010


> 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 don’t 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


> Let’s 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.




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