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I want to build an alarm for my soldering iron that goes off if it is hot for too long. Rather than rely on batteries I am wondering if it is possible to generate enough power from the heat of the soldering iron itself.

The device only needs to function when the iron is hot, and it does not need to start right away. The micro controller I using can operate from 2.8V to 5V. It will use 25µA most of the time in sleep mode, only waking up once a second to use 15mA for about 2ms.

I have already confirmed that I can run the MCU from a capacitor using such a small amount of current.

By my estimate it will use ( .002 seconds * .015A ) + (.998 seconds * .000025A) = an average of about ~ 55µA draw.

If it has been on for too long it will make a quick chirp every second. If the iron gets cold it should run out of power and lose its state thus resetting the timer.

My question is can I create enough current to run the chip in sleep mode and charge a capacitor enough to have to do brief awake times using only the heat from the iron?

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    \$\begingroup\$ It is absolutely awkward way of solving your problem, sorry. The fact the iron is remaining hot means that there is a reliable power supply around which your monitor can use. Second - the heat cannot generate electricity by itself. You need a thermocouple. Or a steam engine with generator. I would put a timed breaker on the supplying outlet and forget about it.. \$\endgroup\$
    – Eugene Sh.
    Commented Oct 22, 2015 at 21:08
  • \$\begingroup\$ Might be worthwhile researching how clothes irons do this. I think this is now standard equipment for all irons sold for some time now. Though, these might be all hardware (a bi-metallic strip and a mercury switch, for example). But it might be worth looking into. \$\endgroup\$
    – user65586
    Commented Oct 22, 2015 at 21:09
  • \$\begingroup\$ This would not be a bad feature for a high-end soldering station that already has temperature control. A timer and a "in use" sensor might be overkill, but could be a nice bell/whistle feature. \$\endgroup\$
    – user65586
    Commented Oct 22, 2015 at 21:11
  • \$\begingroup\$ @jdv high-end soldering stations have automated suspend (temperature = 180°C or something) as soon as the tool is in the holder and sleep modes (= no heat at all) after settable times, so this wheel was already invented ;-) \$\endgroup\$
    – Arsenal
    Commented Oct 22, 2015 at 21:16
  • \$\begingroup\$ @Arsenal, heh. Well, this just proves it was a good idea. Back to the drawing board. \$\endgroup\$
    – user65586
    Commented Oct 22, 2015 at 21:22

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As a soldering iron is very hot, thermoelectric generators should easily be able to produce enough power from that. You should be careful, that the generator doesn't get too hot, they have a specified maximum working temperature, so maybe you need a block which dissipates some of the heat.

You will need a power supply circuit, as the voltage of the thermoelectric generator won't fit to your microcontroller.

Energy harvesting power management ICs offer most of the features you want for a project like this, with automated charging of a supercap and outputs which tell you what source is currently powering your circuit.

As concrete product recommendations are rather useless in an answer, I'll just suggest you have a look around at Linear Technologies, they offer some nice solutions for this.

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  • \$\begingroup\$ Thank you. The Wikipedia article says that the efficiency is about 5-8%. Given the tiny amount of power I need that should be plenty. This is a good answer but I am also agreeing with those who are saying this is not a practical power source. I am thinking some button cells combined with a low battery alarm will be a better solution. \$\endgroup\$
    – HighInBC
    Commented Oct 22, 2015 at 21:35
  • \$\begingroup\$ @HighInBC I think it'll be a great learning experience and yes it's probably not very practical, but using the sensor as a power source is quite cool. \$\endgroup\$
    – Arsenal
    Commented Oct 22, 2015 at 21:39
  • \$\begingroup\$ I agree that it would be a good learning experience, that was about 30% of my motivation(the other 70% was not wanting to burn down the house). In another application where outside power is problematic and the environment produced enough heat it could even be practical. \$\endgroup\$
    – HighInBC
    Commented Oct 22, 2015 at 21:41
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    \$\begingroup\$ A thermoelectric generator does not produce power from mere temperature (of course), but functions as a kind of heat engine when a temperature difference exists between its parts. This means that the cold side of the generator would definitely have to be cooled, and depending on the thermal conductivity of the device (which will certainly be larger than zero), a lot of heat might be conducted through without producing any power. It is an interesting idea but completely impractical for real applications. \$\endgroup\$ Commented Oct 23, 2015 at 1:46
  • \$\begingroup\$ @OleksandrR. for that kind of power levels (just a few milliwatts) I'd think that the natural convection provides enough cooling for the cold side to be effective. The heat transfer from the iron to the hot side will also be a bit difficult to manage, it would probably not be connected directly, so the low thermal conductivity of the air between iron and hot side will prevent the flow of a lot of heat. \$\endgroup\$
    – Arsenal
    Commented Oct 23, 2015 at 9:04

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