> There was a discussion on using incandescent micro-lamps as
> temperature sensors two years ago with Wagner Lipnharski and Roman
> Black.

> I experimented a little with 60V-50mA lamp - small glass tube with two=20
> wires:
> - 120 Ohm at 25C;
> - 155 Ohm in boiling water;
> - At 250C, I guess, its resistance would be aprox 200 Ohm;
>
> At 10ma current the voltage range would be ~ 1mv=E2=80=932mv =E2=80=93 =
pretty enough
> to get it amplified by some Op Amp to, say, 2v=E2=80=934v suitable for =
PIC's
> ADC.

> Glass tube provides better stability than exposed element, I think.
> Time constant of temperature drop was about 10s.

That sounds like an excellent idea. But the delta V will be far larger th=
an=20
that at constant current.
At 200 ohms, 10 mA V =3D 0.01 x 200 =3D 2 volts. At 25c and 120 ohms it's=
 1.2=20
volts.
Presumably you meant 10 microamps, where your figure would be correct.
I imagine you want to keep the current so low that there is no significan=
t=20
self heating.
At 10 mA you get about 20 mW which is not vast, but probably noticeable.
It would be interesting to know how well they work at mA level currents a=
s=20
the A2D can then be used directly.
Some processors have an eg 20x internal ADC gain giving a 0.128V full sca=
le=20
range which would allow about 0.5 mA excitation.


        Russell McMahon

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