"Arfa Daily" wrote in message
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"Gareth Magennis" wrote in message
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"James Sweet" wrote in message
news:Kn4Qj.6369$Mm1.4242@trndny07...



One thing to watch out for is the hysteresis in these devices. The
device I pointed out will turn the fan on at 30 degrees, but it won't
go off again until the temperature drops to 20. If your room is warmer
than that it will never turn off!


Gareth.



Could build something. I recall seeing some temperature controller
circuits that used a diode as the sensing element. Should be easy to
adjust the hysteresis by selecting component values.



Well this is the range of those devices:

http://uk.farnell.com/jsp/level5/mod...=en/212166.xml


Note that the opening and closing temperatures on some of them are the
wrong way around, but I reckon that the normally open switch that will
switch on the fan at 55 degrees C and turn it off at 35, part no.
1006853, would be ideal. 55 degrees is not going to cook any electronics
and it is unlikely that room temperature is ever going to get to 35.
Case solved, one component, no design work required, and extremely
reliable.



Gareth.


If you do go down that route, I think that the sensor needs to be inside
whichever piece of your equipment generates the most heat (with the
attendant potential electrical safety issues which that may cause on a
piece of kit with a switch-mode power supply ...) because if the
temperature is going to reach 55 deg inside the glass-doored cabinet, then
it's going to be a whole bunch hotter than that inside the equipment, and
trust me, that *is* electronics 'cooking' temperature. Sort of
electrolytics on toast if you like ... :-)

For me, your original power slave switch idea was the better option.

Arfa


There are probably other temperature switches without such a wide
hysteresis, like central heating room thermostats for example, or one from
an old room heater / electric fire.

I am very much of the opinion that the best solution to a problem is the
simplest, most reliable, and cheapest. A single switch fulfills all these
requirements admirably. That is all you need. There is not even the need for
a clean power supply for any electronics - the fan could just use a cheap
Wall Wart - that is a grand total of 3 components and a bit of wire.

Finding a much more complex solution, building, debugging, testing, and
re-designing it may well be a lot more fun but will ultimately always be the
second best solution to a really very simple problem.


Gareth.