Electric Cup Warmer

During the cool months of winter, I always find it warm and cozy to have a nice hot cup of tea whilst I'm watching TV. However, as I drink more of my tea, I notice that it cools off faster when there is less in the cup. Before long you find yourself sipping on cold tea, which is undesirable to say the least. In this instructable I have set out to find a solution this very annoying issue;

This project works by using a large metal washer as a 'hot plate' that is heated by 2 large ceramic resistors. The cup will sit on this hot plate and will be heated when current is fed through the resistors. There will be a potentiometer and a thermistor to control the heating element in order to prevent thermal runaway as well as allowing for different temperature set points. I have powered my cup warmer with 3 Li-ion battery cells however any 12 volt source would be suitable.

This project would have likely been easier to do by using a microcontroller like an Arduino board however I opted to try something different and experiment with useful electronics.

For this project I used:

1. 1 x STM 3055VL MOSFET *
2. 1 x 1KΩ 0.5W Thermistor
3. 1 x 200Ω 0.5W Resistor
4. 1 x 10KΩ potentiometer
5. 1 x 50mm 'mudflap' washer
6. Sheet of thermal transfer tape
7. 2 x 33Ω 5W resistors
8. 3 x Li-ion batteries and holders (any 12v 1A source will work well)
9. 2 x 25mm m3 nuts and bolts

*Any N channel MOSFET will work if it fully opens at >10V

These are the minimum wattage values which you can get away with, the higher the wattage of each component the less likely they will burn out in use. Due to the variability of all the parts which can be used I created a Microsoft excel spreadsheet that can tell you what parts you need for your project will if you want to change it.

I looked at lots of different materials to use for the heating element and here is a list of some which I found along with their individual pros and cons:

1. Peltier modules:

These modules are pretty cool and are readily available near where I live however, even though they are efficient heaters, I don't think they will be suitable for my cup heater project. This is because they are expensive, very power hungry and need a require good airflow to heat a cup of tea up to a suitable temperature.

1. Nichrome Wire:

Nichrome wire is a very good DIY heater due to its high resistance and efficiency at heating. I initially brought some of this wire to experiment with and found out that I would need a meter of it wound into a 50mm circle without any sort of short circuit between itself. I tried to accomplish this however I could not get it to work and along with its inability to be easily soldered I opted for another heater.

1. Resistors:

I choose to use resistors because there were easy to solder and could withstand the high temperatures along with the wattage of the heating element whilst being cheap and as power hungry as you choose. Non-ceramic case resistors have better heat transfer however I could only find high wattage ceramic resistors which perfectly fit within the metal washer.

Once I discovered that I was going to use resistors I had to determine the resistance values and the power of each resistor. This can easily be done with a few calculations and approximations. The first of these steps is to approximate the amount of heating power we want:

1. 5W is a bit low for keeping tea warm but is more energy efficient
2. 10W is a better balance of heating and energy efficiency
3. 15W will get hotter faster but will draw more current and will likely get too hot to be useful

I choose to make a 10W heater. To determine the resistance we need, we will have use the value of power we want and solve the following equation:

R = (V^2) / P

Where R is resistance, V is the max voltage and P is the power.

For example, I am using 12.6V (from the Li-ion power source) and want a power of 10W:

R = 12.6^2 / 10

R = 158 / 10

R = 15.8Ω

This means the total resistance of my heating element should be about 15.8Ω. With this information I can then buy whichever resistors I would like as long as it adds up to above 15.8Ω and is rated for at least 10W. I choose to use two larger 33Ω resistors in parallel to give a total resistance of 16.5Ω. Each of my resistors have to be at least 5W in order to handle the amount of power in the system. I could only buy ceramic case resistors, but these are not the best choice to use as heating elements because the ceramic case insulates the hot resistive core well, decreasing the heat transfer to the cup.

1. To start off we will have to trace around the outside of our washer onto the heat transfer tape. Then we can cut out the resulting circle.

1. The heat transfer tape has 2 sides both of which are sticky, we can peel the protective cover off one side and stick it to the washer.

1. Now peel the other side off and stick the resistors onto it.

The heat transfer tape should provide a good connection between the washer and the resistors allowing a lot of the built-up heat within the resistors to transfer to the washer. This aids with quick and efficient heating of the cup.

1. Now twist both ends of the resistors together and solder them onto some wire to ensure a good connection and add some shrink wrap to top it off.

In Fusion 360 I designed a simple holder which will act as the core of the device. Its purpose is to hold the heating element, the electronics and the wires together in one piece. This core is designed to be printed using Cura. It is intended to be printed from Petg and it has no overhangs or bridges. The standard settings in Cura should work well however using an infill like cubic subdivision at 10-15% or similar should help with heat dissipation within the core.

To make the next stage easier I recommend having wires of approximately 4-5cm soldered to each of the following:

1. The heating element
2. The thermistor
3. The potentiometer

Now you should insert the thermistor into the core by sliding the 2 soldered wires through the holes closest to the center in the core so that it should look like the following:

The next step is to insert the finished heating element into the Core by pushing its two wires through the 2 outermost holes, this may require some fiddling and bending of the wires to achieve this:

Now that the basic components have been connected, we can start soldering in the electronics required to make it work. Below is a schematic of the basic elements and how they should be soldered together

*Note the 16ohm resistor is the heating element:

Why These Components and What Do They Do?

If you cannot obtain any part of this circuit for example the 1k Ω thermistor or you have to use a different power supply with a different voltage some of the electronic components may have to be swapped out. A brief summary of each component's use is given below:

Components connected along the blue cable:

1. The MOSFET:

In this circuit the MOSFET acts like a voltage detector. When the potential difference (voltage) between the gate pin (left most pin) and the ground pin (right most pin) is above 10V the drain will be fully open and have the maximum voltage possible. When the potential difference is between 2-4V the drain pin will only be partially open limiting the voltage supplied to the drain pin, any voltage below 2V will leave the drain pin closed where no voltage will be measured. The drain pin is connected to the heating element thus any change in the drain pin's voltage will have an impact on how hot the heating element becomes.

1. The Thermistor:

The thermistor has a changing resistance based on how hot it gets, the hotter it is the closer to 0 Ω it becomes. At room temperature it has a resistance of 1k Ω. This is the core element of the control circuit where the MOSFET is essentially measuring the voltage drop across the thermistor. As the thermistor becomes hotter, and its resistance decreases the voltage measured across it also decreases. When the voltage measured by the MOSFET drops below 2V the heating element cuts off, this occurs at when the thermistor is at approximately 100°C.

1. 10K Ω Potentiometer:

This functions as a variable resistor to change the temperature cutoff of the heating element. This is because power is supplied to the potentiometer through one of the outermost pins (any of the 2 will work) and it will travel through the inner slider which determines the resistance of the potentiometer depending on the rotation of the knob. This varies the total resistance of the circuit impacting the current throughout the circuit and subsequently it will impact the voltage drop measure across the thermistor.

1. 200 Ω Resistor:

This functions to prevent thermal runaway when the potentiometer has a reading close to 0 Ω. This is because if only the thermistor was in the circuit, it will be supplied the full 12V no matter its resistance thus the MOSFET will constantly be turned on heating up the hotplate to high temperatures. A resistor value that is too large or small has a significant impact on the voltage readings across the thermistor.

I have also included an excel file (saved as a .txt file) so that you can adjust each value and work out which components that you may need to use.

I printed the base components in eSun PLA+ because it is unlikely going to be exposed to high temperatures unlike the core component. The default settings of cura should be fine to print these components with. Note that these images are from the prototype, the switch opening will look different in the attached model, but the method is the same.

1. To start; we will have to thread the battery holders through the bottom (larger) base component, so that they protrude out of the rectangular slot:

1. Next, we have to thread the rest of the components through this hole so that the core lies flush with the upper surface:

1. Then, we will have to fit the switch and the potentiometer through the available openings in the case, the switch opening may need to be drilled out to a larger size, or you may need to print the case without the switch opening and leave it loose below the batteries if the switch size is too large:

1. Then we have to tighten down the potentiometer with the nut that is provided with it:

1. Press fit the upper base component over the core such that the 2 mounting holes line up and bolt it down using 2 25mm m3 bolts and nuts (the bolts must be added to slots found underneath the base component:

1. Finally secure the battery holders with hot glue or double-sided tape.

The last thing to do is to add a knob for the potentiometer and the cup warmer is finished.

To use the cup warmer all you have to do is place a cup on the metal washer (a metal cup will work better than a ceramic mug due as the ceramic will act like an insulator) flick the switch so that the circuit is on. The warmer should start heating up, you can adjust the final temperature using the control knob.

Now the cup warmer will keep your tea warm so you can stay warm whilst watching TV, even when your down to your last few drops of tea.

Some improvements to this design would be to:

1. Include a light to show whether the warmer is turned on or off
2. Include a light that turns on once the target temperature is reached
3. Include a battery level indicator to show what charge the batteries are at
4. Use a larger washer as I found the 50mm one to be limited in size

Due to my desire to keep this project short and simple since I was trying something new, I did not incorporate any of these things. If you would like to add any of these improvements let me know in the comments and I'll see if I can help. I would also be inclined to make a version 2 if I get enough feedback about this project and any improvements I could make.

I would like to thank Autodesk for sending me a T-shirt and stickers from my involvement in the ‘Make it Move’ contest. I have placed them on one of my new 91" RC planes along with plenty of 3d printed parts to try and keep the wires neat! I will be sure to parade them off at some of the upcoming IMAC competitions here in Perth. If you'd like to see more of what I did for this plane, make sure to leave a comment and I'll be happy to write up a new instructable explaining it.