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-----
> From: Jose S. Samonte Jr. [SMTP:dyowee@USA.NET]
> Sent: Wednesday, April 11, 2001 3:55 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [[EE]: Transient response of a thermistor...]
>
> 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?
> Please help me.
>
>
>
> Bob Ammerman <RAMMERMAN@PRODIGY.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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