[time-nuts] Temperature sensors and bridge amps

[Perry Sandeen]

List,

My following comments are am exploratory thought process of which I don’t profess to know the answers. Perhaps in the future experiments will provide some. 

So here we start.

The Ni1000 SOT temperature sensor is a nickel based unit that has a basic resistance of 1K ohms at 20 degrees C and a 6+ ohms (approx) change per degree.

The sensitivity of a standard platinum 100 ohm sensor is a nominal 0.385 ohm/°C.

Wrote:< Nickel sensors are more stable than thermistors, but not as stable as platinum. The cost is more attractive than Pt, tho.

Agreed.  The platinum price I was able to find was about $30 each.  So the Ni1000 is one tenth the price.

Wrote: I'd consider staying analog with a DC bridge and a PID control op-amp. You don't need a highly accurate voltage source for the bridge because null is null, whatever the excitation voltage. Of course, you'll want a stable null for the op-amp, too.

I don’t know what a PID is but I agree about using a bridge circuit.

Wrote: <need a highly stable set of bridge resistors for a stable temperature. In the old days, precision, stable resistors were wound on ceramic forms by soldering a loop of e.g. constantan wire to the lead wires at each end of the form. Then you pull the loop at the center so that you can wind it on the core in a non-inductive manner. <Snipped>

I have a number of them salvage from older test equipment.  Using them in a small enclosed temperature control module is really impractical.  One can easily buy 50 PPM/ degree metal film resistors.  Probably sorting the two other branch resistors from a batch of 10 with a 4 1/2 digit or greater resolution DVM can provide an extremely well matched set.

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.

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?

Another question is are we over-engineering a regulating circuit for the crystal, as in how sharp is the turning point?  Will this be gold plating a Yugo?  I have no idea.  I’m bringing this up for discussion.

Wrote:< Don't even think of using any kind of variable resistor to adjust the bridge null. What you want is a stable temperature near the value that gives the least crystal tempco.

Agreed.  But I have a question.  

If one was using the Ni1000 couldn’t one use say a 20 turn 10 ohm ceramic trimpot swamped with a 10 ohm resistor or a low value Beckmen 10 turn pot to find the center of the turning point?

The last unknown for me is what type of op-amp does one use?

Answers or contrarian opinions welcomed.

Regards,

Perrier