How a USB Mug Warmer works

Sketch of a USB mug warmer

As the festive period comes to an end I am amazed by how many little toys and gadgets are strewn about the house. Amongst these I found a USB Mug Warmer.

As the name suggests it is designed with the sole purpose of being plugged into a spare USB port on your computer and using the supplied current to keep the cup of tea, or other warm beverage, at a slightly higher than ambient temperature.

The features boasted on the its packaging were:

  • Easy installation, no driver required, plug & play.
  • Contains an extended USB Port on its side.
  • USB powered, no batteries needed.
  • Quality insulation material around the metal hot plate prevents burns.
  • Light up USB plug.
  • On/Off switch
Even before reading the features my curiosity had been peaked as to how it uses the power provided by the USB port to heat the Mug. But now after learning of the 'On/Off Switch' feature I could wait no longer before undoing the 4 screws located on the base of the unit and prising off the panel to see inside...

Information about USB Mug Warmers and the like...

The USB Mug Warmer is not unique in its use of the USB port for a pretty pointless, yet fun, purpose. These types of USB gadgets come in many forms...

Sketch of a USB missile launcher Sketch of a USB fridge Sketch of a USB owl

From a USB Missile launcher, or even a USB Fridge, all the way to a USB Owl! Most USB gadgets, along with being used for 'fun' also don't use the communications aspect of the USB port.

The official name these gadgets have acquired is USB Decorations. If you wish to read more about USB Decorations and their possible problems with power consumption then head on over to Wikipedia and check out their ever informative article.

Taking a peak inside the USB Mug Warmer

Before even opening the USB Mug Warmer the first thing you notice when you plug it in, other than the hot plate getting surprisingly hot, is that the USB plug lights up blue! They've thought of everything!
On closer inspection of the USB plug you can see how this is achieved.

USB Connector

Sketch of the wiring to the USB connetion

Attached to the USB connector you have 4 wires. The Red (Supply Voltage), White (Data-), Green (Data+) and Black (Ground).
Within the USB plug there is a tiny blue LED in series with a resistor connected to the Red+ and Black- cables. All the cables then carry on into its body.

If the computers USB port was only being used as a source of power for the USB Mug Warmer then just the Red (Supply Voltage) and Black (Ground) wires would be needed. However this particular USB Mug Warmer contains an extended USB port on its side. So even the communication (White and Green) cables are needed.

USB Mug Warmer's Body

Sketch of the USB mug warmer's casing

To the right you are able to see each layer of the USB Mug Warmer's insides. From the plastic casing that holds all the insides right down to the fastenings that hold the Hot-Plate and heating element firmly in place. The casing on the bottom of it has been omitted from the image, due to the fact it would detract attention from the most useful parts.

The Plastic Casing

The top layer is the plastic casing, containing two holes on the front for the On/Off Switch and On/Off Light and two holes on the back. The two back holes are for the USB cable to run into and for the Female USB plug on the circuit board.


The second layer is a circular piece of metal, called the Hot-Plate.
When it's switched on the heating element that can be seen below the plate of metal heats up and transfers the heat into the Hot-Plate, thus its very apt name.
It is the Hot-Plate on which the mug of tea (or any other delicious beverage) sits.


The circuitry inside it is very simple, and consists of 3 sections.

Sketch of the USB mug warmer's internals

To the right of the circuit you are able to see where all the cables come in from the main USB cable. All the cables attach directly to the female USB plug found on the side of the USB Mug Warmer.
Plugging anything into this extra USB plug is essentially the same as plugging it directly into your computers USB port, as all the wires have simply been extended.

The Positive (Red) and Negative (Black) wires continue on to the rest of the circuit. Following the Positive cable along you are able to see that it connects to both the Female USB Plug and a Switch. The switch controls the current going to the LED (which lights up when it has been switched on) and the Heating Elements.
In series with these components are resisters of various values. Resistors are used to limit the amount of current going into an electrical component.

Heat resistant fastenings

The heat resistant fastenings do nothing more than hold the edges of the the Hot-Plate tight against the casing, and hold the heating element tight against the centre of the Hot-Plate.


Although apparently complex from the start, like many things, once it has been stripped down to its most basic elements it is always easier to understand. The USB Mug Warmer is no exception.