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'[EE]: 1N4148 as temp probe?'
2002\05\22@045542 by Tal Bejerano - AMC

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Hi

I saw a circuit on the web that use 1N4148 as car water temperature probe.
is it accurate? never heard about this method before.

Regards

Tal Bejerano
AMC - ISRAEL

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2002\05\22@052850 by Alan B. Pearce

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>I saw a circuit on the web that use 1N4148 as car water temperature probe.
>is it accurate? never heard about this method before.

Any transistor or diode junction can be used as a temperature sensor, as the
voltage drop across the junction has a well defined change in voltage with
temperature.

Companies like Maxim make chips that will use a transistor as a temperature
sensor for use inside PC's, as the Pentium chips have a transistor
temperature sensor built in to allow control of fans.

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2002\05\22@061255 by Jinx

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> Companies like Maxim make chips that will use a transistor
> as a temperature sensor for use inside PC's, as the Pentium
> chips have a transistor temperature sensor built in to allow
> control of fans

Like this

http://www.national.com/ads-cgi/viewer.pl/ds/LM/LM84.pdf

from

http://www.national.com/appinfo/tempsensors/

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2002\05\22@065537 by Tal Bejerano - AMC

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10x a lot!
so as I understand I can measure the voltage drop in any temp ? should all
diodes react the same or should I calibrate each circuit?

Regards

Tal Bejerano
AMC - ISRAEL


{Original Message removed}

2002\05\22@070753 by Alan B. Pearce

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>so as I understand I can measure the voltage
>drop in any temp ? should all diodes react
>the same or should I calibrate each circuit?

Depends how accurately you want to measure the temperature, and over what
temperature range. For most uses (like measuring the ambient temperature) it
is probably not worth individually calibrating them.

As an aside I suspect that the cheap LCD temperature probes you can get that
measure inside and outside temperatures use this method.

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2002\05\22@071002 by Rick C.

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They all need to be calibrated independently because all junctions (diode
drops) are not exactly the same voltage. If you want more "gain", put two or
more in series. I use a simple circuit in my bus as a thermostat for air
temperature. Two diodes in series feeding an op amp comparator/schmidt trigger
to drive a relay via a transistor.
Rick

Tal Bejerano - AMC wrote:

> 10x a lot!
> so as I understand I can measure the voltage drop in any temp ? should all
> diodes react the same or should I calibrate each circuit?
>
> Regards
>
> Tal Bejerano
> AMC - ISRAEL
>
> {Original Message removed}

2002\05\22@083226 by Francisco Ares

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I used the forward biased base-emitter junction of a BC548 (any general
use transistor will work). It gives something like -2mV/Celsius degree.

You can find out the exact gain with a prototype and just keep an
adjustment for the offset. The forward biased junction voltage drop is
too different among several transistors of the same type, but the
temperature drift is not.

I built several thermometers using transistors as temperature sensors,
and in 0 to 100 Celsius, I always got errors smaller than 0.5 degree.

Hope this helps
Francisco


Tal Bejerano - AMC wrote:

{Quote hidden}

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2002\05\22@093750 by James Paul

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All,

FYI, yes this works and rather well I might add.  We used this exact
method when I worked at TI to temperature compensate a microcontroller
clock frequency so as to maintain correct time as an RTC.   It worked
well.
IIRC, the change for a 1N914 (1N4148 too I'm sure) is 1.4mV/degree C.
So you can tell a temperature within about 1.5 degrees if everything
is ideal.   A more practical value is about 3 degrees.   This was
determined from our experimentation with the diode as mentioned above.

                                               Regards,

                                                 Jim


{Quote hidden}

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2002\05\22@231613 by Spehro Pefhany

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At 12:07 PM 5/22/02 +0100, you wrote:

>Depends how accurately you want to measure the temperature, and over what
>temperature range. For most uses (like measuring the ambient temperature) it
>is probably not worth individually calibrating them.

If you measure the voltage *difference* of a diode-connected transistor
at two currents 10:1 different, you get a predictable output voltage from
most GP transistors of 200uV/K. This is about 10% of the output sensitivity
of the junction itself, but it is predictable to within about 1'C typically
at room temperature without calibration using garden-variety 2N3904
transistors. This method is used to measure CPU chip temperatures. IIRC,
the main error term is the base spreading resistance parameter.

>As an aside I suspect that the cheap LCD temperature probes you can get that
>measure inside and outside temperatures use this method.

No. They use a very nonlinear thermistor and some smarts- no voltage
measurement
at all, nor a reference. The clinical thermometers are essentially the same,
but narrower range.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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