stable trimmed resistor

[Bernhard Kuemel]

Hi sed!

I want to build a basal body temperature thermometer which should
measure temperature in the 36-38C range to about 0,01C precision
(accuracy not needed). If I made a wheatstone bridge with the NTC in the
left leg and adjusted the right voltage divider with a potentiometer to
shift the voltage to the desired temperature, would the potentiometer be
stable enough? Or might it drift or change its value due to
thermomechanical motion? Then I would have to use fixed value resistors
instead which might require quite a few of them to get the needed
resistance value. Hmm, or maybe use few fixed value resistors and a low
value potentiometer in series (or a high value one in paralell) for fine
tuning.

The voltage range in the NTC leg is about 100mV across the 2C
temperature range, so 0.01C is about 0.5mV. Vs=5V, so 0.5mV is 1/10000
or 0.01%. What are the chances of a potentiometer changing by 0.01%?
What are reliable stabilities of potentiometers?

Thanks, Bernhard

 

[Bernhard Kuemel]

On Thu, 18 Jun 2009 14:21:38 +0200, Bernhard Kuemel


You can set a single-turn or multiturn trimpot - they're about the
same, really - to about 0.1% before it gets really annoying. They are
stable to a bit better if not shocked. 0.01% might be pushing your
luck.

Actually 0.01% is a little better than necessary, but it should not be
worse than 0.05%. Well, actually I mean 0.01C and 0.05C, so maybe the
resistors should really be stable to 0.01% so there is room for other
errors.

I don't think thermal deviations are a problem as long as they are
precise (repeatable). I would keep the whole bridge with the sensor so
it will be subjected to the temperature to be measured.

What I'm worried about is that the slider of the pot might change
position irreversibly due to thermomechanical motion. Also, the
thermometer (including the pot in a casing) will likely be put on a hard
surface thus experiencing shock or it will possibly be dropped a few
centimeters. And really it would be good if it survived somewhat higher
drops without decalibrating.

It's better to extend the range with fixed resistors...



--------+---------+
| |
R1 |
| |
| P
------+--R3---->O
| T
| |
R2 |
| |
--------+---------+

Hmm, I don't understand your circuit. I thought of something like this:

5V---------+
| |
NTC R2
| |
| |
|--ADC-+------+
| | |
R1 | POT
| R3 |
| | |
0V---------+------+

Bernhard

 

[Bernhard Kuemel]

Hmm, I don't understand your circuit.

It is getting clearer ...

Bernhard

 

[James Arthur]

Opps, I made a mistake on my previous post. (Just ignore it.)

I use a circuit like this for temperature control of laser diodes.
I've put the thermistor in the lower part of the arm. (I'm not sure
this makes much difference but I feel more comfortalble when the NTC
is touching ground.) I replaced your ADC with an instrument
amplifier.

5V---------------+
| |
R1 R2
| |
| |
|--In Amp--->pot
| |
NTC |
| R3
| |
0V---------------+


Then Johns original suggestion looked like this,
(I think)

5V---------------+-----+
| | |
R1 R2 |
| | |
| | |
|--In Amp----+-R4->pot
| | |
NTC | |
| R3 |
| | |
0V---------------+-----+

Which looks like a loaded pot, except I'm not sure what R4 is doing.
(calls for a bit of analysis.)

George Herold

R4 reduces the effect of the pot to a small trim of the (stable)
voltage set by precision resistors R2 and R3.

 

[Bernhard Kuemel]

I use a circuit like this for temperature control of laser diodes.
I've put the thermistor in the lower part of the arm. (I'm not sure
this makes much difference but I feel more comfortalble when the NTC
is touching ground.)

I put it on top because that gives me a non inverted voltage response to
temperature changes. Not actually important, though, since the voltage
is going to a uC. Hmm, but if there are leak currents between case and
NTC it might be better on ground/case potential.

I replaced your ADC with an instrument amplifier.

I'll be using an in-amp, too. It has to arrive, yet. Currently I only
have normal op-amps.

Bernhard