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In article <C0Int7.4wu@max.physics.sunysb.edu> danube@max.physics.sunysb.edu writes:

>   I need to control the temperature of a small infrared window to 
>0.1 degrees C or better.  The actual value need not be so precise, but
>changes in temperature must be kept to a minumum.  At present, the
>window is heated by a ring of brass wrapped with heater wire. Adjusting
>the current changes the temperature, which is typically 100-200 C.
>The temperature is measured with a thermocouple (5 mil Chromel-Alumel wire,
>"type K").

	Thermocouple amplifiers are usually good instrumentation
amplifiers (uA725 being one inexpensive model), and must be operated
in a temperature-stable environment (read: closed metal box)
with some kind of 'cold junction' compensation.  There are 
dozens of application circuits in the (for instance) National
Semiconductor Linear Applications Handbook for this kind
of gizmo.  Analog Devices' AD509 is a handy cold junction
compensation device, as well, and there's application support
from Analog Devices for it.

>   Finally, though TC's are prefered, I can also use a silicon diode
>to measure temp (though not to 600 C).  

	And, the diode forward voltage (nominally 0.7V) is proportional
to temperature, so the output voltage varies by circa 2 mV/C; the
signal is a lot bigger, and the corresponding precision of the
amplifier is less.  Don't use a simple switching diode, though;
use a transistor B-E junction (collector shorted to base) instead,
because switch diodes are FAR from perfect diodes (due to the tricks
used to gain either higher current capacity or lower forward capacitance).

	A medium power transistor, such as 2N3019 or 2N3053,
is rated for normal operation at 200C; plastic package
transistors, glass or plastic diodes, are not, usually.

	There's really no hard trick in making the circuit up; you
know how to add voltages (summing junction), how to difference
voltages (difference amplifier) and how to amplify by a constant
voltage gain; once you have a constant voltage source and
a source with a known temperature coefficient, the thermocouple
amplifier and cold junction compensator are easy to build.

	John Whitmore