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Temperature switch

ntewinkel

Hi all,

 

I'm hoping someone may have some ideas that I haven't seen yet.

 

I'm trying to run a little USB-powered (5v) heater to keep a small area outside from freezing or getting too cold. I'm aiming for around 15 degrees Celcius.

It would ideally also be somewhat smallish.

 

Right now I have a couple of KSD9700 temperature switches. It's rated at 15C, but the hysteresis seems to give it +/- 3 degrees. So it fluctuates all the way from 12 to 18 degrees. And the two I have vary widely in their behaviour, which is probably a symptom of a cheap eBay purchase. But it does the trick for the most part.

I've also seen KSD301, but they look to be much bigger. Not sure if their behaviour is similar.

 

These Texas Instruments ones seem to be ideal: TMP709, so I'm planning to pick up a few of those. They appear to have a much smaller hysteresis (2 degrees), which means much less temperature fluctuation.

 

Maxim also has some: MAX6501 that look to be the same kind of thing.

 

It looks like I can combine those with a transistor to power the 5v heater.

Both are smd, so a small PCB could hold all that's needed in a tiny footprint.

 

I'm trying to keep things really simple, so I'm trying to avoid using a microcontroller + separate sensor etc. Although I recall the very first of the TI Launchpads came with a chip with a built-in temperature sensor that was part of their demo. But that does require each chip to be programmed, which adds complexity.

 

Am I missing anything really easy and obvious? Or maybe something less obvious but much more ideal?

 

ps, does 2 degrees hysteresis mean it would be 15 +/- 2 = 13 to 17 degrees, or is the 2 the sweep from high to low, ie 14 to 16 degrees?

 

Thanks!

-Nico

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  • Nico,

     

    Hopefully you are heating a 'very' small area.  Current limits on USB are not going to leave you with too much heat.  Given the following statement of USB current limits, you are going to have somewhere between 2.5 and 7.5W of heater.  Not a lot of heat to work with. Hopefully enough.  Good luck!

     

    "In the USB 1.0 and 2.0 specs, a standard downstream port is capable of delivering up to 500mA (0.5A); with USB 3.0, it moves up to 900mA (0.9A). The charging downstream and dedicated charging ports provide up to 1,500mA (1.5A)."

     

    Gene

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  • Nico,

     

    Hopefully you are heating a 'very' small area.  Current limits on USB are not going to leave you with too much heat.  Given the following statement of USB current limits, you are going to have somewhere between 2.5 and 7.5W of heater.  Not a lot of heat to work with. Hopefully enough.  Good luck!

     

    "In the USB 1.0 and 2.0 specs, a standard downstream port is capable of delivering up to 500mA (0.5A); with USB 3.0, it moves up to 900mA (0.9A). The charging downstream and dedicated charging ports provide up to 1,500mA (1.5A)."

     

    Gene

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  • in reply to genebren

    Thanks Gene,

     

    Yes, it's very small. It's using a USB hand warmer powered by a dual-port cell phone charger, and it's wrapped in insulation. Sorry, at this point I can't give too much specific details yet.

     

    It's actually quite amazing how roughly 1 amp of power (= 5 watts, right?), combined with good insulation, is more effective than older methods.

     

    Here's a graph, which shows the heater is having a good effect, switched with the KSD9700, but I think the insulation could use some work!

    I'll add a blog post soon about the ESP-01 data logger I set up to gather that data. ps, that url is still live, and showing the other same setup which is not nearly as neat a graph.