Conversion

Archiver

Stampers,

A little off topic, but someone may suggest a way to do this with a stamp
I need to take the resistance of a NTC thermistor and convert it to a LINEAR
0-5volt output.
The NTC Thermistor is the same type used as the coolant temperature sensor
used in many cars
The resistance is NOT LINEAR and I need to convert it somehow to a linear
output.

Alternativley, does anyone know of a good source (Australia) of relatively
cheap temp sensors for use in HOT, SALTY water that could be interfaced to a
0-5volt analogue input??

Regards
Stocky


[noparse][[/noparse]Non-text portions of this message have been removed]

Archiver

Hello,

NTC thermistors have to be individually calibrated unless the data are
supplied by the manufacturer.

Some can be mated with a resistance bridge in order to give approximately
linear resistance as a function of temperature. There are also similar
schemes with 2 thermistors and a resistance.

Otherwise, a formula is used (r and R in Ohms, t and T in degrees Kelvin) :
t = 1/(a +b*ln(r) +c*(ln(r)^3)
ln(R) = K+L/T+M/T^3

When calibrated carefully at 3 points (to yield the 3 constants a,b,c or
K,L,M) good thermistors can easily achieve 0.1 degree Celsius precision.

For Stamp use I'd make a lookup table or organize a linearising bridge. It
all depends on the range of temperatures you want to measure and the
precision needed.

Some thermistors are sold encased in various housings or plated by epoxy,
glass or whatever.

If you contact me off list I may be able to give you more tips.

Regards,
Dov

---

Original Message

---

From: "Tim Stockman, STOCKMAN ELECTRONICS"
<stockman.electronics@b...>
To: <basicstamps@yahoogroups.com>
Sent: Tuesday, June 25, 2002 9:36 AM
Subject: [noparse][[/noparse]basicstamps] Conversion


> Stampers,
>
> A little off topic, but someone may suggest a way to do this with a
stamp
> I need to take the resistance of a NTC thermistor and convert it to a
LINEAR 0-5volt output.
> The NTC Thermistor is the same type used as the coolant temperature
sensor used in many cars
> The resistance is NOT LINEAR and I need to convert it somehow to a
linear output.
>
> Alternativley, does anyone know of a good source (Australia) of
relatively cheap temp sensors for use in HOT, SALTY water that could be
interfaced to a 0-5volt analogue input??
>
> Regards
> Stocky
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>

Archiver

>Stampers,
>
> A little off topic, but someone may suggest a way to do this with a stamp
> I need to take the resistance of a NTC thermistor and convert it
>to a LINEAR 0-5volt output.
> The NTC Thermistor is the same type used as the coolant
>temperature sensor used in many cars
> The resistance is NOT LINEAR and I need to convert it somehow to
>a linear output.
>
> Alternativley, does anyone know of a good source (Australia) of
>relatively cheap temp sensors for use in HOT, SALTY water that could
>be interfaced to a 0-5volt analogue input??
>
>Regards
>Stocky

Hi Stocky,

I have an example of thermistor linearization (interpolation in a
table) posted at

http://www.emesys.com/BS2math3.htm#thermistor

As to the subsequent conversion to 0--5 volts, you could use PWM if 8
bits accuracy will do, or for better accuracy use a D/A converter.
Either method is going to require an external chip, an op-amp buffer
for PWM or the D/A chip.

The good old LM34 or LM35 have linear voltage output (10mv per 'F or
'C respectively), and I'm pretty sure you can get them down under.
You would have to mount them in a glass or SS tube or at least shrink
them inside some melt liner tubing.

-- best regards
Tracy Allen
electronically monitored ecosystems
http://www.emesystems.com
mailto:tracy@e...