How Not To Build A CPU Hand Warmer

Winter is coming, along with mittens, cold hands, snow, and jackets. Now that we’re all carrying around lithium batteries in our pocket, wouldn’t it be a great idea to build an electronic hand warmer? That’s what [GreatScott!] thought. To build his electronic hand warmer, he turned to the most effective and efficient way to turn electricity into heat: a ten-year-old AMD CPU.

Building an electronic hand warmer is exceptionally simple. All you need is a resistive heating element (like a resistor), a means to limit current (like a resistor), and a power supply (like a USB power bank). Connect these things together and you have a hand warmer that is either zero percent or one hundred percent efficient. We haven’t figured that last part out yet.

Because more power and more retro is more betterer, [GreatScott] pulled an AMD Sempron out of an old computer. Finding and reading data sheets is for wimps, apparently, so [GreatScott] just poked some pins with a variable power supply until the CPU was drawing about 500mA at 5V.

The video continues with some Arduino-based temperature measurement, finding some new pins to plug the power leads into, and securing all the wires on this heating element with hot glue. For anyone in the comments ready to say, ‘not a hack’, we assure you, this qualifies.

With the naive method of building a CPU hand warmer out of the way, here’s the pros and cons of this project, and how it can be made better. First off, using an old AMD processor was a great idea. These things are firestarters, and even though this processor preceded the 100+ W TDP AMD CPUs, it should work well enough.

That said, this is not how you waste power in a CPU. Ideally, the processor should do some work, with more active gates resulting in higher power consumption. If this were an exceptionally old processor, a good, simple option would be freerunning the chip, or having the CPU count up through its address space. This can be done by tying address lines low or high, depending on the chip. That’ll waste a significant amount of power. Randomly poking pins hoping for the right power consumption is not the way to get the most heat out of this CPU.

Of course, the above paragraph is just theory. The eating is in the pudding, or some other disfigured colloquialism, so here’s a quad-core 386 coffee warmer. This project from [magnustron] uses four 80386 CPUs powered via USB to make a nice desktop hotplate for your cuppa. Of course being powered by USB means there’s only 500mA to go around, and the ΔT is comparable to [GreatScott]’s AMD and hot glue hand warmer. Thus we get to the crux of the issue: 5V and 500mA isn’t very hot. Until cheap USB-C power banks, with ten or twelve Watts flood in from China, the idea of a USB powered heater is a fool’s errand. It does make for some great AMD firestarter jokes, though, so we have to give [GreatScott] credit for that.

47 thoughts on “How Not To Build A CPU Hand Warmer

  1. And once you got that 500 mA thing sorted out (perhaps by virtue of USB3), don’t forget that some non-negligible fraction of the dissipated power will happen at the batteries: remember to hot-glue *them* to the whole contraption…

        1. Apple has yet to steal that and patent it.

          The all new iPhone 8, features a single use hand warmer:
          The worlds most expensive and effective.
          Now without speakers, microphones and screens:
          Because who needs old tech anyway?

  2. False assumption here. We’re not talking about a computer USB port.
    Most USB power banks will put out 1A. Plenty even support 2.1A output.
    10W should be more than doable. AFAIR GreatScott chose 500mA for battery life reasons.
    Also given that he’s produced more legit hacks than the majority of people commenting, I think people need to cut him some slack for what was, to everyone but the OP, just a fun bit of goofing of from the parts drawer.

  3. This is horrible writing. Terrible article. I’m embarrassed to read it.

    I would do myself a favor by not patronizing this website anymore.

    You do not need high power draw for a hand warmer. Hence the term – hand warmer, not hand boiler.
    When a simple resistive element is needed for a heater there’s nothing wrong with hooking up pins on an otherwise obsolete and unused processor. The very essence of hacking is TAKING WHAT YOU HAVE AND DOING WHAT YOU WANT WITH IT.

    Not to mention the maximum amperage you can draw from USB is 500ma, it allows the creator to have his heater run off USB battery packs.

    Nor was there any actual mention of the actual effectiveness in testing of the heater. Oh yeah, sure. The terrible writer ‘linked’ a source.

    This is really really quite horrible writing. Hackaday has been lacking in scrappiness and skookum and become a judgmental cesspool.

        1. That’s an artifact of Google. BTW, guess who’s winning in that comparison? Yep, go team Hackaday.

          But anyway, if you don’t like something, just walk away. Think you’re going to effect some change in what and how I write? just walk away. Want to complain? thank you for more views, comments, and user interactions, all of which are metrics for success.. By criticizing me in the comments, you’ve become what you hate: a reason for me to continue annoying you.

          1. While “This is terrible writing”‘s reply was unnecessarily rant-ish, I also found the article to be unnecessarily judgemental. It really does not matter at all how you get the CPU to draw the power, it will always be just as efficient as a heater. And it sure is easier to just randomly find a few pins with suitable power draw than trying to figure out how to get it to run something without proper power regulation, memory etc. attached.

          2. how is it an artifact of google?
            the data references your own popularity so all that plot shows is that both hackaday and make suffer a similar relative to themself fall in popularity.

    1. It may not last that long though. If the current is going through a path where normally much less current flow it may kill one of the components in the path due to local spot heating or the path itself due to electromigration.

    2. With all due respect (which, considering the amount of respect you’re showing, ought to amount to just about nothing): People like you are what’s wrong with the world. You looked and a funny article – one that literally made me laugh – and got so enraged by it you decided to bash the entire website in the comments. Frankly, I’m with Brian on this one – if you don’t like it, leave. You have contributed nothing positive. You ARE contributing nothing positive. If you continue in your present behavior you WILL not contribute anything positive. If you want to see Hackaday become a better website, you can help – you could submit projects, tastefully and politely point things out in the comments, but failing those, you can make Hackaday a better place by leaving. I can’t speak for the entire community, but I can speak for myself when I say that I would enjoy this website better if you weren’t a part of it.

    3. >Not to mention the maximum amperage you can draw from USB is 500ma, it allows the creator to have his heater run off USB battery packs.

      Weird, since I have a battery pack here rated for 2000mA. Given that most USB-charged mobile devices need at least 1A to charge, it’s not surprising.

      >Nor was there any actual mention of the actual effectiveness in testing of the heater. Oh yeah, sure. The terrible writer ‘linked’ a source.

      Using an old CPU as a heater isn’t a new thing. Even then, it’s trivial enough that you really don’t need a demonstration.

      CPUs get hot. When you draw 500mA at 5v you produce ~2.5 watts of heat.

      >The very essence of hacking is TAKING WHAT YOU HAVE AND DOING WHAT YOU WANT WITH IT.

      I agree. So why is it your mission to chastise people for doing things you approve of?

      >Hackaday has been lacking in scrappiness and skookum and become a judgmental cesspool.

      I agree. People like you with your judgmental, angry CAPSLOCK comments have really driven this site downhill.

  4. Despite the copyright date and statement from the narrator in the video the processor is not from 2001, it is from 2006. Still old to ancient by technology standards though.

    Just wanted to correct the record. In 2001 AMD was using the Socket A/462 in both notebooks and desktops, which itself came out in 1999.

    1. I was thinking more of setting up a heat extractor like a liquid cooling rig on an existing desktop, and instead extracting the heat all the way out into a radiator over the keyboard with a fan on top. Capture your new cpu heat rather than antique cpu heat. Newer heat is more compatible with today’s operating systems, you know.

    1. Melting point of ‘hot glue’ is higher than the pain threshold.
      From teh Wiki:

      “Glue guns come in low-temperature and high-temperature (hot-melt) versions. Low-temperature glue guns operate at approximately 120 °C (248 °F) and are well suited when high temperatures are undesirable, such as gluing lace and cloth. High-temperature guns operate at approximately 190–210 °C (374–410 °F) and produce a stronger bond.”

  5. A shop near me sells the sodium acetate hand warmers for a pound. Probably a better deal all round. Cheaper, safer, easier. I realise that’s not a hack, but using USB power sticks for heat is pretty pointless. I’m guessing you’d get more useful warmth just from a pair of gloves.

    I’m a bit surprised the CPU gives off heat without a clock signal. Still anything’s a heater I suppose if you find the right bits to connect to.

  6. A CPU’s Vcore is well below 5v, so if you hooked across the ‘correct’ pins I’d imagine it will likely suck the supply straight down if not pop the core with an overvoltage. Also bothering to hook this to the ‘correct’ power pins, then tying some address pins high or low.. why would this make the CPU do anything? You haven’t given it a clock. That adds more circuitry and bulk, now the neat little square heater is no longer as compact as it was.

    I think his approach was acceptable for a CPU that will never be used again, it keeps it dead simple and didn’t require scouring datasheets and building clock sources. While someone could do that (and if you do, I’d totally encourage that and wanna see it here!) it’s not a viable option for the masses, it adds complexity so it’s no longer a DIY project for many, more of a show-off.. which I still enjoy seeing, but alas the author hasn’t either sadly.

    This article is on the judgmental side, without the author doing any legwork or thorough analysis themselves, so it comes off as mildly bigoted to me.

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