This information was provided courtesy of Mo, who was an active CAD_CAM_EDM_DRO list member in the beginning, in a follow-up email he sent me from a discussion on the CAD_CAM_EDM_DRO mailing list hosted on Yahoogroups
On the subject of temp control
TYPE 1
Let me say the first item is one that I swear by it is from a company
called Airpax and is a power transistor looking device designed to bolt onto
heatsinks costs a couple of dollars and is available in a wide range of
preset trip temperatures.
Why preset? because the device has a micronic little bimetallic strip inside
the chip and is very accurate. You can get them in normally open or normally
closed. That way you can switch a high or a low signal to the circuit pin
used for disable or even a buzzer direct !
One guy here in the UK uses several different temp ranges on the heatsink
e.g 40,50,60,70,80 'C and gets each one to light a LED with the final one
sounding a buzzer and driving the disable pin. Neat idea no electronics
knowledge needed other than to remember the current limit resistor to each
LED! Because the unit contacts handle over 40V at 1 AMP you can kick in
your backup fan directly leaving it off when not needed to save its life.
At a couple of dollars it is FLEXIBLE!
you'll find it here
TYPE 2
Then there is the electronic adjustable type - this does not have the
contact rating of the 6700 as it will give a logic signal output high or low
good for driving the disable pin on the stepper logic.
For that the choice is between National Semiconductor
http://www.natsemi.com and good old Analog Devices at http://www.analog,com
See files TMP01.pdf TMP12.pdf AD22015.pdf and LM56.pdf
TYPE 3
There is another type of temp sensor which does not have a built in trip
circuit but merely outputs a small voltage which increases with temperature
this would normally be interface to your own trip electronics design e.g a
cheap opamp chip (sorry to assume you dont know what an op amp is if you
already do) this is simply a unit with two inputs if the input on the first
input is higher than the input on the other, it will flip its output from
high to low. The output being high or low is an indication of which of its
inputs is at a higher voltage. So we would use a voltage reference on the
first output e.g a potentiometer to set a voltage and the other input would
have a temp sensor as the output from the temp sensor exceeds the voltage
set on the first input the op amp output flips from high to low.
Op amps cost a few cents and the temp sensors cost a few cents also.
Why did I mention this way of doing things if a single chip as in type 2
above is the equivalent of the sensor and the opamp needing only a pot to
adjust the trip point?
Because instead of using a pot on the first input and a temp sensor on the
second, you could use a temp sensor on both inputs and this would then
switch when there is a preset difference between them such as putting on in
the case and one in the airflow, when the fan is working , the voltage of
the fan airflow temp sensor will be lower than the other sensor in the
higher ambient temperature.
But when the fan stops the sensors will both be at the same temp which with
a single resistor to give the ambient sensor an slight advantage will cause
the output to flip - EMERGENCY, FAIL FAILED it will cry :)
As you can get a chip with 4 separate opamps (or comparators because they
compare one input with the other) for about a dollar and the temp sensors
for a few cents you could have three heatsink temp switches and a fan fail
sensor for a few bucks.
See the file LM135.pdf page 12 - ignore the fact that the op amp uses +15
& -15 supply you can have a single supply 5v or 12v type
If you want to go this route let me know and I will give you a simple
design.
Then there is finally the control freak method, the guy that wants to see
what is happening even when the unit is working OK i.e he want a temperature
readout. This method is easy with type 3 as you can attach a meter across
the temp sensors or use a bar graph chip that lights up a set of LEDs as the
input voltage gets higher, the chip to do this is about 1 to 2 dollars plus
the cost of a few LEDs, you could have the first few LEDs green then a few
amber then the top ones red and the last one flashing! Let me know if you
want to go this route and I will set you up with a circuit don't use the
one in the similar circuit in the LM135 is not as good as it could be.