[time-nuts] Temperature sensors and bridge amps

J. Forster jfor at quik.com
Fri Nov 12 17:29:59 UTC 2010


Why mess with bridges, etc? Analog Devices, among others, make solid state
temperature sensors that are very good and put out linearized currents,
something like 1 mV / Degree K.

-John

===============


> 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
>
>
>
>
>
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