The transistor body provides thermal mass, true.  However, it may be an
asset depending on the application and the range of temperatures and the
rate at which the temperature changes.  "Dithering" temperature sensors
making the electronics (and whatever they control) go on and off "quickly"
may not be the best for the application.  So quick response as an asset may
or may not be advantageous.  (Thought I'd stir up the pot a bit).  

In my experience thermal responses of several to tens of seconds have been
more than adequate.

Rich 

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Dario Greggio
Sent: Monday, February 08, 2010 4:15 PM
To: Microcontroller discussion list - Public.
Subject: Re: [PIC] Measuring flow using a temperature sensor (and maybe a
heater)

Bob Blick ha scritto:
> A hot wire exposed directly to the air is pretty responsive. The wire
> material is also probably a relatively poor conductor of heat, so the
> temperature is not heavily influenced by the connecting wire. As opposed
> to a transistor, encased in plastic, with copper leadframe and
> conductors muddling the accuracy by bleeding heat into the thermally
> isolated parts of the device.

I see Bob.
I was considering a BD139-or alike, metallic case.
Since the sensor woudl be smaller (the aforementioned SOT), I should use 
something else. Or maybe move to a TC74 (TO220) and use a BDX53 BJT.

-- 

Ciao, Dario
--
Cyberdyne
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