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'[PICLIST] [[EE]: Transient response of a thermisto'
2001\04\11@105509 by

Hello again sir Bob!
I found out that the thermistor I have takes 1.5 minutes to register a total
change in temperature.
But why does in my thermistor circuit, with a LCD readout, why does it take
more than 5 minutes for the reading in the LCD to stabilize?

Bob Ammerman <RAMMERMANPRODIGY.NET> wrote:
Thermal 'circuits' are very similar to electrical circuits. You can use a
lot of the same analysis tools (like ohms law, kirchoffs law, etc) by just
changing the terminology:

Masses(things that store heat) == capacitors

Connections between masses == resistors

Temperatures == voltages

Heat flows == currents

So, for example if you know the thermal mass of the thermistor (its
pseudo-capacitance) and the thermal resistance of its connection to the item
being measured (the pseudo-resistance) you can compute the corresponding
time constant RC (just like in an electrical circuit).

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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What is the current in the thermistor, what is the dissipated power in it,
how do you measure it, and how do you define 'stabilize' ?

Peter

--

Jose,

You have to be very carefull when using thermistors that you do not pass too
much current through them which will caus self heating.  The equation P=I^2
R shows that the self heating effect of the current increases with the
square of the voltage.  If this self heating effect is too high, then not
only will your thermistor measurement be inaccurate, it also takes much
longer to stabilise.

Thermistors have a parameter called "Dissipation constant", which is usualy
stated in units of mW/degree C.  This tells you what temperature rise you
would expect from the amount of power the thermistor is dissipating.  I have
a very good PDF from a company called Betatherm that describes almsot any
aspect of thermistor measurement you will ever need.  If you would like me
to send this file, then email me.

Mike

> {Original Message removed}

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