I recently purchased a USB mug warmer off of ebay.  It was one of the $2 ones, with integrated 4 port USB hub.  It shipped in from Hong Kong.

At first, I found it fantastic.  It seemed a great use of USB to me.  I often consume warm beverages at my computer, but I usually don’t finish them before they get cold. Unfortunately, reality soon sunk in.  The warmer just didn’t keep things warm.  It barely got warm to the touch.

So, I dissasembled it.  Taking a look at it, it was nothing more then a simple hub with an added resistor circuit on a switch.  The resistors simply dissipated voltage when the switch was thrown.

So, the first “increase the heat” modification was to disable the USB hub that was robbing voltage from the heating element.  I cut a trace on the board by scratching it with a screwdriver.  No more hub voltage.  But alas, even without the drain of the 4 port hub, it scarecly got any warmer.

I set aside the “heatsync” (a 1/8th inch thick piece of aluminum about the size of the bottom of a coffee mug) and let the resistors heat up on their own.  After a 10 minute heatup…they got kinda warm.  So, I deduced the resistors weren’t making good enough contact with the heatplate.  I removed the globs of thermal paste, added a little back, and put a penny between each resistor and the plate.  EUREKA…heat….but only in two very small spots.  The resistors just didnt have the oomph to even begin to heat the plate up.

So, I decided to do some math.  Ohms law to the rescue.

I = V / R

I being current in Amps, V being voltage in volts, and R being resistance in Ohms.

and

P = V x R

P being power in watts, V being voltage in volts, and I be resistance in Ohms.

After scraping off all of the thermal paste, I discovered that the resistors were 17 ohms.  The two in series was 34 ohms of resistance.  After some math, I found out that they were dissipating .73 watts of power.  This is NOT a lot of heat.  So, it was time for a drastic redesign.

I came up with this circuit.

All of the resistors in that circuit are 1 ohm, 1 watt resistors.  The total resistance of that circuit is 10.5 ohms.  Using the above formulas, that circuit would dissipate 476 mA of energy, less then the 500 mA possible from a USB port.  It also would dissipate 2.38 watts of energy, producing a nice, toasty plate.

All of the parts have been ordered and are on the way.  I’ll soon be posting photos of the build process and an instructable.  I’ve already decided to use the housing from my current warmer, removing the built in circuit and adding to some mass to the warmer plate.

4 port usb hub, Amps, circuit, EUREKA, Hong Kong, R I, R P, resistor circuit, USB, V Hide