Thermistors

Anyone know of a good app note for these? The ones I have seen are very
vague.

Thanks!

 

Try the manufacturer's web site. If there isn't any, then try another
manufacturer. Lots of info out there.


Best regards,
Spehro Pefhany

 

I think Epcos may have some app notes - also try Vishay BC.

Thermistors *are* pretty vague btw. Considered a semiconductor solution ?

Graham

 

YES

We are talking lost resources here.

The original manufacturers were Philips and Siemens who produce, or
otherwise did produce, what have to be the best data sheets on the planet
for passive components. They still produce good shit for the shit they do.

You will notice that they are European companies.

Not to say that you American or Japanese people don't know **** but, you
couldn't find it in the first place. Wankers.

The Philips stuff got sold off to Beyerschlag(? BC bought up by Vishay) and
the Siemens stuff got sold off to Epcos. Both of which promptly fucked
things up three times over.

So.... it's all shit as defined by some suck cock beany fucker.

DNA

 

That pretty much sums it up nicely- Thnx. And don't forget those British
eff-yew-see-kays who bought up Gates batteries....

 

Are they ?
Not really. There are some with 1% matching or less.
Then you measue temperatures, you have some problems
anyway. How are you going to calibrate ?
A PT100 ? With a precision current of voltage source ?
You're good when you reach 6 digits for a current or
voltage source. Then apply the PT100 to the location isn't
trivial either.
On the other hand, calibration with a temperature reference.
Water at the tripple point, outgaz it with vacuum,
apply vaccum to reach the tripple point and hope you have
some good contact. Good. Then you may need a second point.
A temperature normal. Freezing mercury, non-alloyed.
Yuck. Another idea....

I found NTCs good for repeatable 10mK accuracy around
room temperature. Knowing where you are is a different matter
but perhaps not really necessary.

Rene

 

Thermometrics Taunton Devon UK used to be STC they still make precision
NTC and as described they can provide the best temperature information
for many applications. Note I used to employ thermistors with good long
term aging but un adjusted resistance to compensate crystal oscillators
individually matching crystal/resistor/thermistor for best performance.
Ignore demon address try

 

If you want interchangeable thermistors, Yellow Springs Instruments -
YSI - were the original manufacuturers, and when I last looked you
could still get some of their +/-0.05C tolerance parts of the shelf
from Newark (via Farnell if you live outside the USA).

For their data sheets and application notes look at

http://www.ysi.com/temperature.htm

In Europe, BetaTherm in Ireland have a decent range of parts at
+/-0.2C- see

http://www.betatherm.com/

Thermometrics make some very good interchangeable thermistors - see

http://www.thermometrics.com/

If memory serves, their application notes were particularly good.

 

Thanks to all who made datasheet/app note suggestions. After some
reading I'm getting the impression that all thermistors have a
non-linear curve? Am I limited to temp sensor chips (lm35 etc) if I
need a linear response?

Thanks

 

Hello Dave,

No. There are also plain diodes and thermocouples. You have to deal with
rather small signals here but with clever offset nulling (clamping,
chopper amp etc.) it can be done.

Regards, Joerg

 

There are compound thermistor devices that are linear, but with about
1/10 the output, IIRC.

Maybe you could linearize the response over the region of interest,
using analog or digital means.



Best regards,
Spehro Pefhany

 

Precious-metal RTDs are one of the best temperature sensors if you
need an accurate linear response and don't mind a little cost. They
have only a tiny bit of nonlinearity over a narrow range, and most of
that is easily eliminated with a taste of +ve feedback. They are also
made very accurately (initial tolerance) and can be very much free of
hysteresis and drift, depending on the construction you choose. Base
metal types are popular with the comfort heating and A/C crowd, and
for monitoring heavy motors.

There are plenty of other types of temperature sensors.



Best regards,
Spehro Pefhany

 

All thermistors seem to conform to the Steenhart-Hart fitting function,
which is pretty non-linear.

Over a sufficiently small temperature range, a thermistor-based sesnsing
circuit can look reasonably linear.

As Spehro Pefhany has pointed out, there are a number of options available
if you need to measure temperature over a wider range.

A thermistor plus Microchip Technology PIC microcontroller with a built-in
A/D is probably the cheapest way to go if you need linear temperature
measurements over a wider range.

A platinum resistance thermometer is certainly the best way of getting a
good temperature measurement, but you have to dissipate quite a lot more
power in the sensor, and even then the sensitivity in volts per degree is
quite a lot less, to the point where you find yourself stuck with building
an AC-excited bridge circuit to get reliable resistance measurement. That is
the way to go if you want to build a microdegree temperature controller, but
it tends to be an overkill if you are less ambitious.

 

I have in the past simply used the temp coefficient of Vbe with a simple
transistor sensor.

It's linear with temp, you just need to select a device that has a
reasonably consistent Vbe @ a given temp. Depends how accurately you're
measuring.

I understand this method has been used for measuring die temp in modern
CPUs.

Graham

 

Except they don't use Vbe these days, they use delta Vbe at two
different currents, which gives better interchangability. On the minus
side, the signal level is an order of magnitude lower (more like
200uV/K than 2mV/K).



Best regards,
Spehro Pefhany

 

Two 1N914s work quite will to get the difference in temperature. I used
this fact in a controller for the pump in a solar hot water system many
many years ago.

 

The important questions with temperature sensors
are "how many", and "do you already possess some
means of temperature calibration".

If the OP just wants one (or even just a few), then
he goes for an already calibrated sensor, Pt100 or
precision thermistor, at the appropriate price.

If there is a production quantity then he can go for
a cheap homebrew sensor, (of whatever sort is most
convenient), and invest some money in a temperature
calibration test setup.

I like metallic resistive temperature sensors, (Pt,
Ni, or even Cu in some apps), because the Beta-term
is reasonably known and the linearising is simple.

Afair there is even a combination of Pt+Ni where the
correct proportions make the Beta-terms cancel, but
have never seen a proprietry implementation of this.

 

If you want to control at a set temperature thermistors are good, YSI?
Used to do a pair on thermistors that gave a linear voultage over a
linited temperature range.