Help with safely hacking a coffee roaster - Page 1
Forum section: electronics / projects

author: dprotter
author_rank: Newbie
timestamp: December 10, 2024, 05:56:21 pm
content: (Tried this in the beginner section, but didnt get any traction...)Hi all, I'm hoping to get some advice on how to safely proceed on a passion project. I am trying to modify a pretty popular coffee roaster (FreshRoast SR800) so I can control it from my computer where I currently log temperature data. Unfortunately, I didnt study as an EE, so I've been trying to move through this project slowly. After learning more about how this power supply works, Im especially happy I didnt end up seriously electrocuting myself. (I'm sure plenty of comments would simply be "if you dont know enough to do this safely, you shouldnt do it at all..." which... fair!)What I really need is help deciding if there is a safe way to keep experimenting with this project.The basic way the roaster works is to use TRIACs to control the fan speed (resistive coil for voltage drop --> bridge rectifier --> 30V DC motor) and the heating element (resistive coil). The control is clunky and not very fine grained.The roaster is split into a power board that houses the TRIACs (and the optotriacs that do the actual triggering), a capactive drop -> bridge rectifier to generate 12V, and a LM368 to generate a zero crossing signal. The 12V and the 0-x go to a logic board that then takes user inputs via a rotary dial and sets the TRIAC firing delay. The optotriacs supplied from the 12V supply, and triggered by a transistor on the logic board connecting them to ground through a 1k resistor.I had just gotten to the point where I had successfully measured the delay times at various temperature and fan settings, and used an arduino analog pin to read the 0-x data. I was able to successfully set consistent delays after zero-crossing and trigger a MOSFET when desired. I also was able to send serial commands to the arduino to change that delay, which was tantalizingly close. I was just starting to try triggering the optotriacs, but nothing seemed to be happening when I did.At some point I came back to working on things and realized I had lost my zero-x signal, and that the power board was no longer producing 12V. In trying to learn more about how to diagnose what might have been going on with the capacitive drop power supply, I learned about how dangerous those circuits are, and how under the right failure modes (although I'm a bit unclear as to what those are) they could expose the arduino, my computer, and most importantly, ME, to 120V AC. I dont necessarily intend to do much diagnosing of the capacitive drop circuit, but I might consider buying another sacrificial roaster to experiment with.So, again, my question is: is there a safe way to proceed?I would likely want to have the arduino plugged into my computer for the simplest serial connection. Would it be sufficient to have the arduino isolated from the zerox signal by an optoisolator? And, similarly, putting those in between the arduino and the MOSFETs used to control the optotriacs?Thanks for the advice, and I am preparing myself for the (likely reasonable) safety lectures.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 11, 2024, 04:45:10 am
content: Install an ESP32 inside, the Wifi link provides the isolation and no need to drill additional holes in the enclosure. Always unplug the device from the mains before opening the enclosure, including to flash firmware. (The ESP32 will be powered from USB power in that case.)

author: TheoB
author_rank: Regular Contributor
timestamp: December 11, 2024, 08:41:02 pm
content: You should use an isolation transformer. The price depends on the wattage. I have one of 100VA which might not be sufficient for heating elements. But those can be replaced by a 40Watt light bulb to test (if you still get your hands on one). The transformer output is still dangerous if you touch both leads, but it's safe to connect the circuit ground directly to a PC. Using WiFi is also an creative idea 💡Capacitive droppers are very simple. For experimenting they can be replaced by a lab supply. The voltage follows indirectly from the capacitor rating.