Introduction: USB Heater (or How to Upgrade Your Coffee Cup)

I've been visiting Instructables once in a while, and I realized it was time to re-start building stuff. I used to unmount-mod my "toys" when I was a kid - teenager (like blowing out a little train and putting its mottor in a GI-Joe like helicopter to spin its blades, cool), and at some point of my life I forgot how amusing that was.

So this is my first instructable, hope it is usefull.

I was wondering for quite a while how could I build my own coffee heater, for the following reasons:

- I usually bring my coffe to my desktop (not the virtual one), and while I am coding or something it gets colder and colder, sometimes needing replacement (even my old Ni-Cd have a greater dutty cycle);

- Lots of power available through unused USB ports, I have 8 and use only 2 or 3 (think green);

- Usb heaters are cheap and easy to find, but I needed to build one from scratch to satisfy my ego (and impress my friends);

- I have lots and lots of scratch at home, needed to find a way to use them instead of just throwing out (think green, recycle, reuse reduce)

- and, at last but not least, I found someone who actually did it, with the math behind ( check this link ).

This is a prototype! - I actually built it, plugged to one of my USB and felt it going hot. The USB port still runs normally, no power or caffeine issues. But, as you'll see, it is not yet finished, no casing, no ZIF socket, no way to prevent the mug from dropping. I already have some ideas for an improved second version, comments and suggestions are welcome.

Step 1: The Physics Behind

IMPORTANT: I can not be held responsible for any damage caused to USB ports, computer parts, power supplies, coffee mugs, the coffee itself or greenhouse effect. I checked the hole thing for the maximum safety and built mine at my own risks, I expect you do the same.

Some basics on Ohm's Law:

I = E / R

Which means, the current through the resistors (I) in Amperes is equal to the volts applied (E) divided by the resistance (R). Consider E=5 (USB power), R=14 (resistors used):

I = 5 / 14 = 0.357143

So, we're spending around 350mA, safe enough for most USB ports with a 500mA limit.
Now, to check how much power will be sent to our tasty coffee:

P = E x I

We already know the values of I and E, so:

P = 5 x 0.357143 = 1.785714

meaning we get close to 1.8 watts, enough to keep it hot till the last drop. You can calculate your own to give more/less heat, but keep in mind the limits (5V, 500mA for USB ports).

'Some considerations:'

- First things first. The resistors should be able to handle the power they will be dissipating; Thou shall not use resistors smaller than 1 watt or thou shall have unpredictable and unpleasant surprises... I took mine from old ATX power supplies, judging by theyr size they're at least 1w for sure (the body is almost 1 inch long). Guess you'll not spend a dolar to get them on local stores.

- Using 2w or 4w resistors (or bigger) will give you more safety, not heat. Trust me when I say this is technology, not magic.

- You can use a different number of resistors, as long as you keep the final resistance inside the calculated values. A smaller resistance could burn your USB, cause personal injuries or several other situations involving your hot coffee.

Kids, don't try it at home without reading everything carefully!
The resistors are somewhat critical parts, you should stick to the values calculated (by me or you). When you connect two (or more) resistors in series, the final resistance value is increased by summing all their values. So, 6.8+6.8=13.6 (the tolerance of each resistor may take it a bit higher or lower).
Connecting them in parallel DECREASES the final resistance value, drastically, so don't do that, unless you really know all the formulas.

ATTENTION: (part 2 - thanks RetroPlayer)
If you connect this project to a non-powered hub (meaning no AC/DC adapter plugged in), the 500mA from the USB is actually split by the number of ports, so this probably wouldn't work on that type of hub, unless you use the other ports for unpowerable purposes (like connecting digital cameras, scanners, printers and other stuff that doesn't depend os USB power to work). Anyway, a powered hub will ensure 500mA to each port, therefore being a better idea in case you really need to use a hub (or eventually would forget and plug all your tiny USB stuff).

- Ensure your favorite mug can handle extra heat, you will not be pleased if it melts over your stuff. The same goes for the casing if you build one. By the way, good ideas for casing are wood or some thick plastic, I believe metal should be avoided because it could dissipate ALL the heat and/or eventually cause a short circuit.

Step 2: Requirements

Now that we've done the math, let's start it. These are the parts needed:

- 1 Pentium processor (166 Mhz was used but any clock should do the trick);
- 2 resistors x 6.8 ohms, 5% tolerance, 1w (2w if possible);
- USB cable with male A plug;
- Soldering stuff, termal paste, chopstick, multimeter, electric tape, patience, etc.

I assume you've read everything untill here very carefully, so i won't keep saying about USB safety. If you still have any doubt, post a question.

The processor is not critical, as long as you don't need it anymore. Any clock rate might be good enough for our purpose... ;-)

The two resistors stripes are blue, gray, gold, gold. Make sure both can handle 1w. If you decide to use a single resistor (15 ohm recommended), the stripes are brown, green, black, gold, and this should be able to handle 2w (ok, no more safety talking from now on).

USB cables don't require great specifications, any old one should be enough.

Step 3: Heat It!

Clean the processor, check for dust or other stuff that could prevent the thermal paste apllication or smell bad when heated.

Cut the USB cable to expose the wires, isolate green and white. These are USB data, we only need the power, which comes from red and black.

Solder the two resistors in series, then solder the black and red from the USB cable. No polarity issue here, so don't worry about who goes where, as long as you respect the design and keep the wires distant enough to avoid a short circuit.

The chopstick was used to spread the thermal paste under the processor. You could also use some thermal adhesive, heard about it but never seen these. I did some soldering between the processor and resistors terminals to hold them together with a small piece cut from them (a poorman's solution for lack of thermal adhesive).

Step 4: Grand Finale


It is done, takes a few minutes to warm up, and can also be used to warm your hands on cold days.

But at this you should be asking yourself: Why does it keep saying yellow goes to red? Well, I wanted to add extra cabling in case I needed the USB cable for something else, and I ran out of red wires from my old power supplies...

Thou shall be ready to make it (in case you've read everything). As I told before, I am already developing ideas for an upgrade, so I would appreciate any ideas, comments, suggestions, coffee recipes, etc. This was done under the concept of doing the most with the least, but I think it can be improved.

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