Related
Recommended
Engagement
  • Author Author: tariq.ahmad
  • Date Created: Date Created
  • Last Updated Last Updated:
  • Views 21018 views
  • Likes 7 likes
  • Comments 12 comments

Mastering Oven Control: Precision Resin Curing with DIY Modifications - How Hard Can it Be? -- Episode 616

In this element14 presents episode, Katie tackles the challenge of revamping a budget-friendly toaster oven and transforming it into a dependable resin-curing tool with precise temperature control. She begins by explaining her need for clear epoxy resin with specific curing requirements, as well as the dilemma she faces regarding temperature settings. Intrigued by the 'for best results' instructions and a desire to reduce the curing time, Katie turns to an economical toaster oven she found online. Although acknowledging its lower quality, she hopes to modify it for her purpose. 

Watch the Video:

Katie's attention shifts to the toaster oven's temperature control mechanism, which she suspects might not accurately reflect the actual temperature output. The oven's operation involves adjusting the desired temperature and time, but Katie aims for more accuracy. To address this, she opts for a Raspberry Pi Pico and a spare thermistor. However, the thermistor's leads fall short of reaching outside the oven, necessitating soldering. Since the solder's melting point is near the curing temperature, Katie anticipates potential connection challenges. 

 image

The project's crucial stages encompass adapting a kitchen thermometer probe for precise temperature measurement and managing the oven's heating elements. Katie delves into the inner workings of the toaster oven, discovering the series connection between the timer and temperature control. She plans to replace the original control system with her design, integrating an Omron power relay to handle mains voltage switching. She acknowledges the requirement for a separate 24V power supply for the relay coil. 

 image

Constructing a detailed circuit and programming the Raspberry Pi Pico are fundamental steps in Katie's project. She successfully incorporates temperature readouts, buttons for setting and initiating oven functions, and a relay for heating element control. During testing, Katie observes that the temperature remains within a 10-degree range of the set value, marking a significant improvement over the original controller. Impressed with the results, she highlights the potential for wider applications beyond resin curing. 

 image

Subsequently, Katie addresses practical issues, such as preventing the K-Type thermocouple from coming into contact with grounded metal, which could disrupt temperature readings. She develops a protective insulation approach and further optimizes her control setup. Replacing loose connections with Wago block connectors enhances reliability and safety, mitigating grounding concerns. Throughout her journey, Katie demonstrates resourcefulness in troubleshooting and determination to create a refined and user-friendly DIY solution. 

 image

Ultimately, Katie successfully crafts a compact and functional control box for her toaster oven project. While acknowledging potential aesthetic improvements, she celebrates her accomplishment in developing a cost-effective and dependable tool for resin curing. She encourages fellow makers to explore this versatile modification, adaptable to various oven systems. Katie's experience showcases the potential of elevating everyday appliances with technical skills, while also addressing the challenges she overcame in achieving her goal. 

Bonus Content:

Bill of Material:

Product Name Manufacturer Quantity Buy Kit
RASPBERRY PI PICO RASPBERRY-PI 1 Buy Now
High Temperature Sensor, Gravity, I2C, K-Type, 800℃, for Arduino and Raspberry Pi Board DFROBOT 1 Buy Now
Thermocouple, Nozzle, K, 0 °C, 350 °C, Stainless Steel, 6.6 ft, 2 m LABFACILITY 1 Buy Now
GROMMET, BLACK M20 PK50 PRO POWER 2 Buy Now
Power MOSFET, P Channel, 20 V, 4.2 A, 0.04 ohm, SOT-23, Surface Mount DIODES INC. 1 Buy Now
Plastic Enclosure, Box, With Lid, Multipurpose, ABS, 83.1 mm, 177 mm, 120 mm MULTICOMP PRO 1 Buy Now
IC, V REG +5.0V, 7805, TO-220-3 ONSEMI 1 Buy Now
Power Relay, SPST-NO, 24 VDC, 10 A, G2R Series, Through Hole, Non Latching OMRON ELECTRONIC COMPONENTS 1 Buy Now
 

Additional Parts:

Toaster Oven

Actions
Parents

  • The standard way of getting more precise control (used by millions of temperature controllers) doesn't need any extra parts.

    To get a fast warm up and low overshoot the controller needs to regulate the power by adjusting the time the relay is closed compared with the time it is open. These toaster ovens are often rather over-powered (and under isnulated) so you may find it very difficult to control the power without using quite short switching cycles. If you go for a mimimum on time for the relay of 1 second then at 5% power the cycle will be 20 seconds, ideally the temperature of the oven should not change much (compared with the required control precision) in that time.

    It helps if you use a zero crossing solid state relay because you can happily use a horter on time, maybe as low as 100ms. Your standard power relay won't last long like that.

    Once you have proportional control over the power you can use a classice control algorithm like PID to get much better control inside the oven. This will work much better in the long run than attempting to compensate by table.

    My own experiments with toaster ovens have been limitied by their lack of insulation combined with excessive power to thermal mass ratio. Out of the box mine would overshoot by 100C when set to 200C.

    THe good news about trying PID is that you can make a start using no more parts, just smartercode !

    MK

  • in reply to michaelkellett

    Good suggestion about changing the power relay, as you might be able to tell as it's on a board with a load of other connections, it was stolen from a now defunct project!
    The lack of insulation was fairly impressive, I was mostly surprised by there also being no sealing around the door, I could poke a reasonably sized into the oven with the door "closed".
    The lack of insulation didn't seem to make much difference at lower temperatures, at ~80c it holds the temp well, but think it'd be a much bigger problem at higher temperatures.
    When I next get a few moments to tinker I'm planning to have a play with PID control code, it should be fairly easy to do with the Pico microcontroller.

  • in reply to hifromkatie

    Yeah, as you dial in more accuracy by modeling the characteristics of the oven and element you'll want to use something like PID, since the relay will end up clicking on and off more and more frequently as you get close to the set temperature.

    The other thing, that dougw hinted at below, is with the small thermocouple you'll want to make sure the temperature it measures is more representative of the whole oven's temp - optimally a convection fan, but I suspect that would push the easy-hard meter much further right.

    Finally, you might be able to avoid earthing the thermocouple with a pair of silicone washers. I think the max temp would be ~200ºC in that case tho

Comment
  • in reply to hifromkatie

    Yeah, as you dial in more accuracy by modeling the characteristics of the oven and element you'll want to use something like PID, since the relay will end up clicking on and off more and more frequently as you get close to the set temperature.

    The other thing, that dougw hinted at below, is with the small thermocouple you'll want to make sure the temperature it measures is more representative of the whole oven's temp - optimally a convection fan, but I suspect that would push the easy-hard meter much further right.

    Finally, you might be able to avoid earthing the thermocouple with a pair of silicone washers. I think the max temp would be ~200ºC in that case tho

Children
  • in reply to rmd6502

    I did start looking into a adding a fan, so getting a more consistent temperature throughout the whole oven, and the possibility of being able to bring in cooler air to help with cooling down when required, however I decided I just needed to get it working so I could get on with other projects. But maybe there's scope in the future to revisit and make a more complex but better version!