>Back to the one that used two transistors heated to 50C. I built
>that many years ago when it forst appeared, because it was charming.
>Although it does work, plain transistors are awful. There is too
>much thermal mass. It takes a minute to stabilize and response to
>changing flow rates was wobbly.

But using SMT transistors such as the MMBT range would reduce the mass, and 
such packaging probably has a better thermal contact between the chip and 
the packaging than something like a TO99 package. 

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