Mug Warmer

A portable solution for keeping your drinks warm!

Overview

This project uses a heating element and various voltage converters to allow USB-C or battery powered heating of hot drinks.

Description

This project keeps hot drinks hot by using a PCB to power a polyimide heating element, which connects to a conductive piece of metal. With USB-C or battery power and a 3" x 4" footprint, this PCB allows for a portable mug warmer!

Features

ESP32 microcontroller
USB-C or LiPo battery input using PMOS logic
1 Cell LiPo battery charging
12 V step up for heating element
3.3 V step up for ESP32
Tactile buttons for temperature control
Up to 80°C heating capability
IR sensor for closed loop feedback control
M3 screw holes for interface with mechanical setup

Schematic includes USB-C, step up circuit to 12V, step up/down circuit to 3.3V, battery charging circuit, heating element, temperature sensor circuit, and ESP32 circuit.

PCB Layout Front Side (Major Components and ICs)

PCB Layout Back Side (All SMD Passives)

Full PCB Layout

Total Cost: $56.90 (2 Boards)

3D Model of PCB

Progress

PCBs arrived from JLCPCB

Hand Soldered/Hot Air Soldered Components

Vbus/Vbat to 3.3V subcircuit

Using test points to check subcircuits before moving forward

3.6 - 5.5 V step up to 12V/800mA subcircuitt

Completed PCB

Challenges and Issues

Fluctuating Voltage - 12V step up

PAM2423 which converts from 3.6 - 5.5V inputs to 12V output had noticeable fluctuations between 13 - 14 V
Considerations: capacitor tolerances, resistor and capacitor values
Solution: resoldered the board with new capacitors

Fluctuating Voltage - 3.3V converter

MIC5504 which converts from 3.7 - 5V inputs to 3.3V outputs had noticeable fluctuations only after 4.5 V input range
- - Voltage would spike from anywhere between 3.3V to 5V intermittently
Considerations: components out of tolerance, IC not rated for low load capabilities, capacitor's temperature rating too low
Solution: on full board (not in isolation), output is stable

Heating Element Current Draw

When testing heating element by manually connecting 3.3V logic into NMOS, we damaged the PAM2423 (12V step up IC), inductor, 3.3V converter, and possibly more
Considerations: heating element draws too much current, trace widths too small for given current
Potential solutions: perform in-depth current calculations and replace components with higher current ratings and increase trace widths appropriately for a given current

Future Plans & Lessons Learned

Design for Manufacturing - While designing the board, we did not consider how we would assemble the board, making certain parts very difficult to solder. For instance, since we had a double sided board, we could not use the hot plate or hot air to solder components since they would affect the components on the other side. Additionally, we did not realize the scale of some of the parts, like our USB-C and ESP32, making them very difficult to hand solder.

Fixing Current Issues - We can test our logic by testing the NMOS in isolation with a 3.3V gate voltage, 12V source, and heating element at the drain. We should also replace our components that got damaged, such as the two step up ICs and the inductor with components that are rated for higher currents.

Future Plans for Completion - We need to manufacture the PCB's enclosure using copper plate and heat resistant outer structures. Furthermore, we need to write the code to obtain the IR sensor's values and convert this into a closed feedback loop to maintain a certain temperature output. To expand the project, we could add a display and allow the user to choose the temperature they want their drink to be heated to.

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