USB Powered Warming Mug

Coffee is one of my two primary beverages, the other being water. I sip on two to three cups of coffee throughout the day. Ambient temperature coffee suits me during most of the year here in Florida. In the winter though, I'm starting to prefer drinking it warmer. I decided to make a warming mug that can be powered by a beefy USB power supply or power bank. Please follow along and make one for yourself.

Parts and Materials:

• 1 - 3-1/8" (80mm) Diameter Ceramic Mug
• 2 - 90mm Bamboo Coasters
• 1 - 90mm Cork Coaster
• 1 - 70mm Bamboo Coaster
• 2 - 1 Ohm 5 Watt Ceramic Resistors
• 1 - 16" to 36" Length of 2 Conductor 18 AWG Wire
• 2 - 4" Lengths of 24 AWG Solid Copper Wire
• 1 - USB-A Male Connector
• 1 - 5/8" X 3/8" Galvanized Compression Spring
• Silicone Caulk/Sealant
• Heat Sink Compound (Optional)
• Aluminum Foil Tape

Tools:

• Cordless Drill
• 1/8" Drill Bit
• 3/16" Drill Bit
• 13/32" Drill Bit
• Dremel or similar Rotary Tool & Bits
• Soldering Iron & Solder
• Hot Glue Gun & Glue Stick
• Wire Strippers/Cutters
• Scalpel or Razor Blade
• Small Clamp or Vise
• Pliers

To promote good bonding and heat conduction, grind away any ceramic glaze from the mug's bottom with a Dremel or similar rotary tool equipped with a grinding stone bit.

1. Strip the ends of the 18 AWG 2 Conductor Wire as shown in the images.
2. Solder the USB A Connector to the wires as shown.
3. Temporarily clamp the connector shell halves together, then carefully enlarge the wire hole using a 3/16" drill bit.
4. Snap together the two halves of the connector shell.

1. Using aluminum foil tape, fasten the 1 Ohm 5 Watt Resistors together as shown in the images.
2. Shape and solder the leads of the resistors together as shown.

Temporarily connect the USB power cable to the heater leads by carefully bending the leads around the wires as shown in the images. Then briefly plug the USB connector into a power supply or power bank capable of delivering at least 2 Amps of current. I advise against powering this circuit from a computer's USB port as it may damage it. If the resistors start to heat up the test is successful—unplug the power source and disconnect the cable from the circuit. If not, make sure the components are wired as shown in step 4, then retest.

1. Center the heater on the mug's bottom as shown in the images, then trace around it with a pencil.
2. Apply heat sink compound (optional) or silicone to the mug's bottom in the traced out area as shown.
3. Using aluminum foil tape, fasten the heater to the mug's bottom as shown.
4. Using pliers, carefully shape the heater leads as shown.

1. Drill two 1/8" holes spaced as shown in the images.
2. Countersink them to a depth of 2mm with a 13/32" drill bit on the bottom of the coaster as shown—to accept copper wire coils.

1. Flood fill around the heater with silicone as shown in the images.
2. Flood fill the recess in the 70mm Coaster with silicone as shown.
3. Center the coaster above the mug's bottom, align the heater leads to pass through the 1/8'' holes, then press down the coaster making sure the gap is uniform around the entire perimeter.
4. Use a finger to tool in any squeeze out around the perimeter.
5. Allow the silicone to cure following the manufacturers instructions.

1. Use the rotary tool to cut a notch in the perimeter ring of one 90mm Bamboo Coaster as shown in the images. This is where the power cable will pass through.
2. Drill two 1/8" holes spaced as shown.
3. Countersink them to a depth of 2mm with a 13/32" drill bit on the coasters bottom as shown—to accept spring terminals.
4. Feed the wires through the holes from the bottom, then bend over the stripped ends inside the countersinks as shown.
5. Cut the spring in half, then insert the two halves into the countersinks.
6. Rotate the springs as needed to seat them square.
7. Solder the bottom spring loops to the power cable wires.
8. Bed the the power cable in with hot glue as shown.
9. Apply silicone around the bamboo coaster's perimeter ring, then center the 90mm Cork Coaster above it and press them together until the silicone sets.

1. Wind two flat coil terminals from 24 AWG solid copper wire, using pliers and your fingers to form them as shown in the images.
2. Fit the coils into the countersinks pushing them in firmly until fully seated.
3. Solder the heater leads to the inner wrap of the coils, then trim the leads off flush with the coils as shown.

The 1 Ohm 5 Watt Resistors are being used as a heating element. Wired in series the initial resistance is 2 Ohms, requiring a maximum of 2.5 Amps from the USB power source. As they heat up their resistance increases providing some self regulation.

Do not plug into power when the mug is empty.

Do not leave it plugged into power unattended.

The second 90mm Bamboo Coaster can be used as a mug lid to slow the heat loss.

It should be safe to hand wash the mug, but it's best not to completely submerge it in water.