[time-nuts] Frequency referenced temperature regulator

[Bill Hawkins]

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

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.

You do 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. Inductance doesn't matter, but you want
to finish the winding with the center bent double and sticking
out a few inches from the core. Attach the lead wires to a
measuring device with sufficient precision and accuracy, and
hope that the winding has slightly more resistance than you want.

Now take a razor blade and short the center wires closer to the
winding. The resistance should go down. When you find the spot
that gives the right resistance, remove the insulation and solder
the wires together.

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.

Yes, this is also how to make a meter shunt, but you'll be using
much finer wire. The best thing to do might be to find an antique
precision resistance bridge. It will have many such resistors in
it, and you might be able to avoid winding altogether.

Please write for details.

Bill Hawkins


-----Original Message-----
From: Perry Sandeen
Sent: Wednesday, November 10, 2010 11:03 PM

List,

Looking for a stable temperature sensor I first went to YSI.  They have sold
their sensor products to.  Measurement Specialties, Inc.

Perusing their site I came upon a Ni1000 SOT temperature sensor.  It is a
nickel based unit that has a basic resistance of 1K ohms at 20 degrees C and
rising to 1482 ohms at 80 degrees C. .  It has close to a 6 ohm change per
degree.  I tried to find one of their distributors without success.

Entering the part number in Google, I found it is also made by ZETEX.  ZETEX
calls it an  IC TEMP SENSOR NI1000 SOT23-3.  The Digi-Key catalog as part
number is ZNI1000CT ND.  They are $2.77 each.  

The ZETEX data sheet has a nice circuit for a digital thermometer.  

Perhaps a LM 331 precision voltage-to-frequency IC or using a change in a
bridge circuit to a varactor on a VCXO might provide the lack of aging
problems that exist with a thermistor when precisely trying to obtain a
temperature-to-frequency conversion.