Reflow oven controller

liyf

Reflow oven controller                       

by robots » Sun Feb 13, 2011 2:34 pm

                                                I have bought this oven - . The main reason for this was to bake some PCBs. As with every reflow oven, this one needs temperature control as well.
I have baked several pcb without problem with unmodified oven. But that might be luck, as the temperature profile is nowhere near perfect.
I have therefore decided to make some simple temp controller to take care of the whole soldering process. As my oven features fan, I will probably want to control it as well. The contoller should not to need any special programmer (mega can by programmed with BP) and the board should be etch-able at home.

Schematic:

• The whole thing is based around mega8 chip. There should be enough space for some pid, ramp generator, and menu. Oscillator is internal RC 8MHz.
• User output is through 8x2 lcd display. Tiny display that will fit to the control panel nicely. This display is driven in 4-bit mode, to limit number of IOs. Backlight can also be controlled by the mcu.
• User input is encoder with button. Both encoder pins are interrupt. Button is polled to save some nasty logic needed to multiplex.
• Temperature is sensed by thermocouple and converted to digital using max6675, that i had lying around. Datasheet is saying that data is available every ~200ms. Little slow, but enough for this purpose. Big plus is cold junction temperature compensation and polynomial compensation.
• Output is 2 channel PWM, 0-Vin. Both to be used to drive SSR (solid state relay). These relays can be bought off fleabay for about 3-6dollars/pc (free s/h).
• Serial port is used to talk to bootloader, and to transmit temperature for further analysis (pid tuning)
• Powered by 5.70-12V, voltage is regulated by micrel mic5202-5.0 (same as on BP)
  

There is no programming port for the atmega8. It needs to be programmed with bootloader before the max6675 is soldered. Reset and MISO pads are for this purpose.

The board:

The board is a bit larger than the lcd itself. Most difficult part is to solder the header. That needs to be replaced by smd version in next release. The board is designed to be piggibacked on the lcd. Small hack is Pot to set the lcd contrast. There was no smd pot available near, so I decided to solder miniature trimmer to the pin header directly from bottom side.
Board is single side pcb, with 2 jumper wires.

Finished board:


This board was not etched at home :-). It was made some local company that does UV etching, and they also apply silver coating to protect the copper.
There are few modifications not present in the picture above - gnd on middle pin of the thermocouple connector, voltage divider for serial port. Some parts are missing, and are not needed for lab testing (mic5205 voltage regulator, and big through-hole capacitor).

Thermocouple used is from sure electronics:

The thermocouple does take a while to cool down and I wonder if it is suitable for this purpose.

For user input I use incremental encoder, bought at local store. This seems to be not a good quality build. Tends to jump randomly back and forth.


The software:
Probably the hardest part. This part is still in development. So far the every peripheral can be controlled.
Finished is: encoder, button, display, max6675 temperature reading, PWM output, pid controller.
There are still missing parts like: ramp generation, user interface.

I will provide board/schematic files here on request. The software will be provided as well, once finshed.

卓钢锴

高人啊，真是好东西啊！！

robter

感谢楼主提供这么好的交流平台

gl542400

三路采集！真是不错！