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I have a USB Block Erupter.  The heat spreader gets very hot, and it seems very close to the USB powered coffe cup warmer the Jalapeno was supposed to be.  The only missing piece is a holder for it.  It should be a very simple design, suitable for 3D printing.  Print, insert Block Erupter and connect.  Perhaps with space for a metal plate on top to protect the Block Eruptor from cofeee spills.

Unfortunately I have no experience in 3D design, no 3D printer, and very limited time.  So I'll just leave my idea here, and hope someone picks it up.  Actually, I'll donate a bitcoin to the first design I'm happy with, if it is published under a free licence!

Most of the 3D printers I have seen so far, use plastic as the print media.  I beleive it begins to melt around 190 Degrees F.  Not a good choice for a "coffee warmer" unless you don't mind it melting all over your desk.   ;D

I agree with the OP this does seem like a nifty novelity to have for an erupter having one or 2 inside a coffee warmer. Would be cool to see this in the future being sold.

I had an old USB-powered soda can cooler, exactly like the one pictured below:

http://www.ign.com/articles/2007/08/09/coolit-systems-usb-beverage-chiller-review

Under the cold plate is a peltier device and a heatsink. It did a poor job at cooling soda cans.

I have since gutted the peltier device and heatsink and replaced those with a Block Eruptor. I also had to replace the cable with a proper USB cable. Let me tell you that it does a worse job at warming coffee mugs than the original cooler did at cooling soda cans.

Sorry I don't have pics as I left it at the office.

In any case, this experiment convinced me that 2.5 W is not sufficient to adequately warm a coffee mug. Maybe 2 or 4 Eruptors might do the trick?

Not even close. If you think a kettle uses 3KW to boil 1litre of water in 2 minutes.... then 10W wouldn't even get close to overcoming the convected loss to atm.

Kettles use 1000 watts typically. 1500 watts max. In North America anyway.

Edit: 1000 watts is 3400 btu, the math from there is simple. MC delta T.

3kw in EU/UK, 230V ftw.

Its not quite MCdT as that's only heat in, in one direction. You'd have to assume the design of the insulating materials was 100%, and doesn't take into account the heat lost by convection from the open surface of the team, and through the cup to atm.

I'm not going to boil water in it.  I just want to keep it warmer for longer.  Even 2.5W is better at that than 0W.

Thats a pretty good idea, maybe you could try making just a simple frame to hold the cup and an usb extension cable?
IF you're going to print it with a Reprap, MakerBot (or a 3d printer using the extrusion deposition method) make sure to use ABS instead of PLA. The ABS softens at around 90 degrees C while as the PLA is going to be about 50. I'm saying softens, not melting :)

Give it a try and let us now? You could post it on http://www.thingiverse.com/.
I'd love to make me a coffee/tea warmer for the winter haha.

That link is nasty. it wouldn't accept being closed before it was done loading and i ended up with 3 incarnations of it before i managed to close them all.

That won't be a problem.  The coffee is only that how when it is freshly brewed, and my cup will be cool enough after the short walk from the coffee machine to my office.  The cup isn't a good heat conductor anyway, so the outside surface will be kept colder than the inside by the colder air on the outside.  I'm more worried about the Block Erupter being able to withstand > 85°C, but I expect that hot coffee will cool off quickly enough.  The heater will only start to have a measurable effect on the temperature from about 60° C, or thereabouts, making the temperature decline slower than it would when the cup is placed on a normal cold desk.  The temperature will converge to a floor higher than ambient.

interesting project.. probably better for someone who works with sheetmetal to do something interesting

if you get/make a usb hub that can have them go vertical like a circular fence (atleast 4 of them) and then have the sheetmetal spread the heat around the bottom 1/3 of the cup, that might be better than just a flat plate

I'm seeing the idea of a new bitcoin mining related product being born in this topic. haha!  :D