I'm looking for a small single temp. normally open thermal switch that I can mount under my laptops to turn on a litlle squirrel cage fan to blow air under the computer. I see the PCB Mount Thermal Switches TO-220 and DIP on page 2139 of the catalog but they only start at 45 C (113 F). Would prefer something around 90 F trip point since they are not monted in the labptop but outside underneath. Could find nothing else. I know one could use an adjustable circiut with electronics etc but I.m hoping to keep it as simple and cheap as possibe since I have several PCs to equip. The fans run off the 5V of the USB port. Any ideas.
I didn't find the thermal switches you are talking about in the Digi-Key catalog. Are you using the current U.S. version US2011? They are likely on a different page if you are using an older or different country version of the catalog.
Thermal switches like that can have a pretty wide actual operating range around the switching temperature they spec (i.e. not really all that accurate). As you mentioned, it's also hard to even find them centered around a specifically desired temperature (such as 90 F). If you want something that is more sure fire (excuse the pun) around a specific temperature you may need to add some modest circuit complexity to do this. With an adequate power rail, an IC temperature sensor, a comparator, a MOSFET, and a couple of appropriately sized resistors I believe you should be able to make yourself a much more accurate temperature switch for your fan.
If you want to get fancy, then the MIC502 chip will do continuously variable speed based on temperatures measured with a thermistor. This is the same chip that modulated fan speed in many cpu motherboards. Comes as an 8 pin DIP and is dead simple to wire-up.
As Alec already ittereated building a custom circuit for your needs is best here. I designed this circuit for a board I'm currently working on. An NTC resistor decreases with increasing restance which causes the temp signal to increase in voltage. Once the temp signal decreases below the voltage divider of R10, and R11 the comparator will turn on the mosfets controlling the fan and the led.
WHy not use a simple temp sensor IC , for instance
Then mount the package under the laptop so it touches. Read the temperature and activate a relay with an i/o pin to activate the fans when your chosen temperature is reached. The sensors basically act like a pot and are read with an A/D converter, calibrated with a map function.
See my site for a bunch of examples of using them. www.jeselectronics.com.
I should have noted there are instructions to calibrate it at the makeitlast datalogger page, linked to on the post for it.
The temp sensor ought to work fine as well. You don't even need a microcontroller with an ADC. You can do the same thing as I posted above. just use the temp sensor instead of an ntc resistor voltage divider. In that the control here is bang bang (either on or off). The complexity need not be greater than a single comparator.
I included one (MCU) to increase functional ability for future enhancment. He may find out that laptop one gets warmer then laptop2 , or decide to cool it at a lower temperature. This is all very easy to do if you do use an MCU. Just a bit of C code . #TWiPad
Sure would be unfortunate if you bump the pot and the fans dont turn on. Just playing devils advocate and explaining the thought that went into my suggestion. I don't think there is a right or wrong way and the complexity issue is really based more on what the op knows, not necessarily the route chosen.