Description:

This project realizes a custom made reflow hot plate controller. I made a fancy graphical user interface for programming the heating curve and displaying the temperature in real time. The programmable reflow curve and the real-time time vs. temperature graph is shown on a 1.8" LCD. All the parameters can be set with a rotary encoder. The temperature is measured with a thermocouple via a MAX6675 module. The system also contains two outputs for fans, so at the end of the heating program, a forced cooling can be applied. The switching of the heater is done by an optocoupler-TRIAC combo.

Features:

I added some gaps to the board to avoid arching at critical places. The high-current traces are at the bottom of the PCB, there is only a few high-voltage (230 V) traces on the top side and they do not carry large currents, they are only used for the switching the TRIAC. The board has 4 mounting pads at the (rounded) corners, plus there is a protective earth connection that allows an easy connection to the metal parts that should be grounded.

Good to know:

• Based on the selected power brick (5 V or 12 V), you have to choose the fans carefully to stay within the specifications of the power brick. Typically, the 5 V brick provides 600 mA and the 12 V provides 250 mA. I would recommend using the 5 V brick.
• You can add a larger heatsink to the TRIAC if you want to use it with high-power heaters. Keep in mind that the TRIAC I picked is "only" a 8 A one.
• You can also attach a tiny heatsink to the BD139 transistor, but probably it is not necessary because it switches relatively low power.
• I suggest to solder the MAX6675 module, the rotary encoder and the display directly to the PCB.
• Keep in mind that the TRIAC-side resistors must be high power resistors; 1 W at least.
• The TRIAC must be isolated from the heatsink. Use a mica sheet is something similar between the TRIAC and the heat sink. Also, leave a little gap between the PCB and the heatsink because one of the 230 V trace is running under the heatsink.
• The varistor (PTC) I picked (SIOV-S05K250) is sized for 5 A. Replace it if you use a larger load.

For all the parts and the values, check the attached BOM file. There is a very useful interactive BOM file in the attached zip file. Also, check my website for extra resources and a video explaining the board.

Check the following video where I explain everything about the board, the heater and I explain the source code line by line: