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transistor equals temperature

transistor equals temperature

transistor equals temperature

(OP)
Hello:
I am trying to get a general purpose transistor e.g. 2n4124,CA3046, or 2n2222, to yield 1 degree accuracy of the enviromental air temp. Using a delta Vbe, My Eq. is:       T=(q(Vbe1-Vbe2))/(K(LN(Ic1/Ic2)). The circuit plus Eq. yields roughly 40 deg. at room temp 22 deg. Am I missing something that the highly predictable results of a transistor can give.  Yes I know there are predeigned IC for this application; However, I would like to use a GP transistor.

thanks a bunch, too hot

RE: transistor equals temperature

The most likely thing you are missing is that you MUST use
low enough current, so the series resistances don't cause significant error.

I don't understand the 40....22 ???

If you use conputer interface, the computer can take care of the calibration factors, too.

 

<nbucska@pcperipherals.com>

RE: transistor equals temperature

(OP)
Thanks for your response on this small problem. The transistor is operating at about 30 to 40 Ua so as not to heat the device internally, so I thought. The set up yields an ambient temp. of 40 deg C. when in a room of 22 deg. I would like to hear more about the computer interface or direction on the net to see the set up.
Cordially;

Xray

RE: transistor equals temperature

The simplest computer interface is the parallel port.
There are plug-in breadboards for PC with interface

Look up www.maxim-ic.com ;   or linear, dallassemi etc
how they do it. and their AP notes.

<nbucska@pcperipherals.com>

RE: transistor equals temperature

(OP)
Well back to the question at hand. The circuit a transistor set up as a single diode drop, collector hooked into the base, and the equation as listed above yields a 40 deg. C temp. when the ambient temp in the room is only 22 deg. C.
Any help is appreciated.

RE: transistor equals temperature

Do you use a SINGLE transistor ? The eq. measures the difference between two diodes with different current.

<nbucska@pcperipherals.com>

RE: transistor equals temperature

(OP)
Yeah single transistor: The circuit switches two diferent resistors from the source to the collector giving two different trasistor currents. I don't see that the equation distinguishes between two different PN junctions or a single junction with two different currents and therefore two different junction voltages.

RE: transistor equals temperature

Should work, what about the rest of your circuits ?
S/H leakage, offset etc. error, non-linearity ?

<nbucska@pcperipherals.com>

RE: transistor equals temperature

xray:
A silcon junction as the base to emitter in a bipolar transistor is a good choice as a temperature sense element.
It has good liniarity from -55 to about +150C. With a current of about 1 mA, it changes about 2.2 mV per deg C, with the voltage drop decreasing as temperature increases.  Calibrate with ice water for 0 deg C, and boiling water for 100 deg C (assuming you are close to sea level).  This method of temperature measurement is commonly used in many systems because of its simple accurate and inexpensive approach.
Elecmec

RE: transistor equals temperature

Suggestion: Check
John Markus, "Electronic Circuits Manual," McGraw-Hill Book Company, 1971
Substrate Temperature Control on page 833

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