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.

ATTENTION:
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.

Comments

[naufanmaker]

if you use epoxy to attach

[pbawesome]

I realize this is an old post, but what kind of temperatures did you get with this setup?

I have a project where I'm trying to generate about 350F (I don't plan to do this with a USB port, but I'm just wondering how much heat this particular setup got you...

[livesteamfan]

Even though it's smaller, this works well with my old 90Mhz 486 processors

[zack247]

im making this with my i486 dx processor i pulled from a really old ibm computer.

[zack247]

so... is it the processor or the wire between the resistors that is heating up?

[looking4ideas]

How hot does it get

[ReCreate]

near 3 million degrees

[knektek]

mine went up to 20 million degrees celcius.

[golpen]

Easy reading - very helpful, bravo man!

[codongolev]

somebody with an overclocked cpu put their motherboard in oil and played some game (I think it was WOW) for like, 2 hours. they used it to fry potatoes.

[robmeofmyboredom]

why dont you just buy a peltier? the advantage of that is with a reverse in polarity you can cool your drink as well as heat it. then again shipping from hong kong isnt quite green...

[DELETED_craz meanman]

(removed by author or community request)

[robmeofmyboredom]

haha true i guess a processor is way cooler than a peltier. i just love pelts... theyre so much fun. i cant even do this instructable, my pentium 4 is way too good of a hairbrush.

[jappet]

its nice, thanks for the idea for our thesis

[conrad2468]

so....if i used something like copper wire and did a squiggily thing....kind of like a stove top where it is spiraled.....wouldnt that work the same if it was in direct contact of the coffee cup? I really dont want to waste money (even if a few cents) on a processer.....

[mman1506]

what about thiscally steking the heater to the bottom and jattaching a usb port and putting a case on so all you do is plug in th usb to the cup

[marc00703]

Hey i wonder if 2x 8ohm resistor be ok ?

[bombmaker2]

it would not get as hot but it will work

[bombmaker2]

as long as the w is correct

[marc00703]

Ok thanks.

[andrew101]

if you plan to use other usb devices depending on the configuration there might not be enough power for it all.

[bombmaker2]

no problem

[andrew101]

lol. i took some magnet wire (like 20 ft) and i put it in a spiral. i found it had around 1k ohms (multimeter) (this was some thin wire) so i hooked it upto 3 volts. and it works pretty good.

[daniel!]

uhh, wouldnt it make much more sense to make one with a peltier unit, which is actually meant for heating and cooling. dont get me wrong, this is a great idea and really original, i would never have thought of making a heater of of an old processor, but using a peltier would probably make it heat better.

[pickle1]

about how hot does this get?

[Kagetsuki]

Uh, why not use resistance wire? You know, like the wire used in the coils of heaters? It's usually not that expensive and if you scraped off the pins from the CPU you could lay it out in a nice zig-zag pattern. Also, some basic protection for the USB port may do you good, but as long as you have a good non-dynamic load you should be fine.

[killrsheep]

mmm doesnt resistance wire require a higher power output than USB's are capable off (5V*0.5A=2.5W) i really dont know...
what do u mean by non dynamic load?

[Kagetsuki]

There are just as many varieties of resistance wire as there are resistors, it is sold in a variety of diameters, coatings, and different mixtures of materials for different resistances over length. The reason I suggested resistance wire is because resistors are usually NOT made with the intention to give off heat, you won't get nearly as much spread over an area with the resistors (you can get more heat over area by adhering wire across a flat surface), and resistance wire is usually made to get hot as it is often used specifically for that purpose. If you are asking about dynamic loads I guess you have never dealt with them. USB ports often have internal protection, but devices like inductors when used improperly can easily overcome such protection mechanisms. A dynamic load could take in 5V 100mA and a minute later could be returning 20V or a few A back into the system, which would probably easily overcome your protection circuitry.

[killrsheep]

Ohh i see, yeah i just wasnt familiar with the term, thanks for the info :)

[killrsheep]

Nice instructable!, Simple yet effective!, i had been wanting to know the MAX current rating of a USB port. im going to try this one :), i have all the materials within my room... err somewhere in this mess however, a resistor with a higher power rating would result in less heat transfer to the proccessor, wouldnt it? since a higher power rating means its designed to dissipate heat more effectively. i think heat loss around the resistor (the part not directly in contact with the proccessor ) would be increased as power ratings for the resistors increase, id try to use the lowest rating possible, always considering that heat has to be transfered entirely to the proccessor "cooling" the resistors as much as possible. wow... i did learn something at the "heat transfer" course! LOL

[gb78]

Well, thanks a lot. I was so concerned about USB safety that I didn't think about that (actually, I have never used a resistor for heating purposes). I still don't know much about heat transfer, but all the time I kept thinking what could happen if used a smaller power rating resistor (I mean, wouldn't it possibly burn at some point? Maybe cause a short and damage USB?) I wanna test these possibilities on the 'Mark 2' :)

[killrsheep]

What i would do, if i wanted to make those tests would be to use a 5V power supply, i made one at school, if you dont have one maybe a buddy of yours has one (or you can make one ), then plug in the circuit and monitor the current on it, that way you can avoid damaging the USB port, since a power supply can handle more current, i dont think youll get a short circuit from a burnt resistor (im not 100% sure tho) , a smaller resistor will be more likely to burn , but thats where the thermal paste and the processor come in... so yeah, basically try testing it with a power supply

[gb78]

Nice tip about the power supply, thanks. I believe the most common is that resistors get 'open' when they burn, just like a fuse, but I've seen cases when they got 'short circuited'.

[killrsheep]

I just bumped into this again and re-read it, i can only say one thing man: NICE JOB!!!! it looks so complete and umm thorough,

[micahduberstein]

The common cycle is: Step 1 - heat up some, but not excessively, Step 2 - approach excessive heat, Step 3 - enter "thermal runaway" (where current rapidly goes to max, and component burning occurs, Step 4 - Short circuit, short duration, as this is where heating REALLY hits the ramp, Step 5 - Smoke, burn, open, no more current. Its during the thermal runaway portion that other components are most likely to fry. I speak as a "fry baby", from personal experience. And 35 years in component repair, replacement, troubleshooting, and test. Fried a bunch, fixed less. Micah

[bob_shaftenkluger]

"Fried a bunch, fixed less." I know all about that one. My skill levels improving now though, maybe as much as 25% sucsess!

[gb78]

Thanks for sharing this information. It is good to know from who really saw it happening from, you know, a (long) road (man, I really mean it, 35 years is older than me). By the way, do you know how safe would it be the watts rate for our resistors to heat the coffee without damaging the circuit itself or the USB's?

[micahduberstein]

If you are familiar with Ohm's Law, it allows you to calculate the current (I or Amps), the Voltage (V) and the resistance (Ohms or R), and from them, the power the resistor needs to dissipate. Since USB is 5 Volts, and USB current gives you 100 ma, unmodified, or 2500 ma, after appropriate handshaking, you can use the formula as follows
V / I = R, so 5 / .1 (100 ma) = 50 Ohms, and 5 / 2.5 = 2 Ohms
So a 2 Ohm resistor will work IF the handshake is done, and the USB port makes the full power available. If not, I understand the USB standard just current limits it by dropping the available voltage, in which case, it won't even heat significantly. So, to make it possible to use all the power the port will provide, use a 2 Ohm resistor. Now
the formula for power is I(Sq)(Amps) X R(Ohms), so 2.5 (Sq) = 6.25 X 2= 13 Watts. Big resistor physically. And you CAN use more than one resistor, in parallel, to achieve greater power in a smaller (set of) package(s), but that requires recalculating the resistance, etc., and the math is more than I want to attempt teaching here. Use a 15 Watt 2Ohm resistor, and it should work, and WILL be safe. And 15 Watts is pretty hot. Put your hand on a 15 Watt light bulb that's been burning awhile. That's how hot the resistor will get. But it won't burn up, so it won't short, and it won't open.
Micah

[bob_shaftenkluger]

15 watts can be VERY hot. My first soldering iron was 17 watts.

[killrsheep]

where did you get the 100 ma figure? gb78 had said that it was 500 ma, i haven't tested this yet but the power rating on resistors only means the power the resistor is capable to dissipate, in other words, a resistor rated 15W will stay relatively cool under a 15 watt load, again im looking for confirmation on this cuz i haven't run any tests. also, yeah it would probably be better to make sets of resistors to make a better distribution of the heat source on the microprocessor.