"Fred McKenzie" wrote in message
...
The temperature of the oven is chosen to match each specific crystal. The
thermal fuse should be sufficiently high that it won't open in normal
operation, but will if there is a problem. The original part for my
oscillator
was rated at 108 degrees C. The revised part is rated at 115 degrees C.
It is
HP/Agilent part number 10811-80008. Last year the price was $10.50 plus
tax.

Thanks for the info, Fred. Since the oven temperature is controlled by
a thermistor circuit, the selection of the thermal fuse should have no
effect on the normal operating temperature or oscillator stability (unless,
of course, the fuse opens). Digi-Key's price for these devices is 45 cents
each, a considerable cost savings.

The NTE part is definitely too big, but the smaller Panasonic F-series
should fit perfectly.

I wanted to understand why the original thermal fuse opened if its rating
was 105C. It turns out that the behavior of these devices depends both
on the temperature, the rate of change of temperature, and the amount of
current flowing through it. The spec sheets list three or four cutoff
values for each device, depending on the test conditions, spanning a very
wide range (30C or more!). The 130C unit I selected has a long-term
maximum temperature rating of only 80C at 5A DC current. I figured this
was a reasonable compromise given the nominal 82C operating temperature
of the oven.

The thermal cutoff point will also change due to soldering heat. All in
all, these are pretty complicated devices...

-Henry