transistor equals temperature

[xray]

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

 

[nbucska]

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>

 

[xray]

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

 

[nbucska]

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

Look up

http://www.maxim-ic.com

or linear, dallassemi etc
how they do it. and their AP notes.

<nbucska@pcperipherals.com>

 

[xray]

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.

 

[nbucska]

Do you use a SINGLE transistor ? The eq. measures the difference between two diodes with different current. <nbucska@pcperipherals.com>

 

[xray]

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.

 

[nbucska]

Should work, what about the rest of your circuits ?
S/H leakage, offset etc. error, non-linearity ?
<nbucska@pcperipherals.com>

 

[elecmec]

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

 

[jbartos]

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