Cheap, quick and dirty reflow oven

I got this cheap toaster in a “crap shop” for 11 Euros. It’s perfect for reflowing PCBs and small enough to fit in my lab:

Reflow-ToasterI tried to use only components I had at hand, including:

  • a DIP PIC 18F2450 (total overkill to control a heater, I know)
  • a relay from a timer AC socket
  • a small USB iPhone charger for the +5V supply
  • a K-thermocouple and a MAX6675 for the conversion (quite expensive chip, but very simple to use and I had it in a drawer)
  • a small LCD screen, two buttons and two LEDs

The PIC is running from the internal RC oscillator. Very slow, but enough for what it’s used for.

The relay comes from this timer I bought some time ago at Conrad:

It’s rated for 2000W, the PCB is easy to re-use and fuse protected. I added an opto-coupler, so the PIC and user interface are fully isolated from the 220V mains (also convenient for debugging). I thought about re-using the timer’s LCD and microcontroller, but it’s directly bonded on the upper PCB and difficult to hack.

I managed to squeeze everything into the toaster’s plastic flange. (Unfortunately the temperature can increase a lot inside, so I think I’ll move the electronics into a box, outside the toaster)

For now, I just have two functions:

Drying – keeps the oven at 100°C

Reflow – follows the standard leaded solder paste curve (2 minutes@150°C – 2minutes@200°C – 1minute@250°C)

The LEDs also act as switches to select between the two functions. I’m still looking for a good method to cut plastic in a clean way. It’s not so easy with my Dremel.

It took me more or less one day to build everything (I wasted some time for the LCD display control).

I’m only using the heating elements from the upper “toast hole”. The bottom stays cool enough for the toaster to sit on a table without the need of extra feet or thermal protection.

And of course, the oven works just fine for the small PCBs I have to solder!

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iTeadStudio PCB test: Populating the board

After I received the PCB ordered form iTeadStudio last week, I soldered the components today.

So far, everything related to the quality of the PCB looks fine. The footprints match, the FR4 substrate handles the temperature well (1 mm thick option) and the HASL finish is ok and easy to solder on. It changes from my hand-etched PCBs.

GIF animation of an USB board soldering

I successfully programmed the PIC with Microchip’s USB device example firmware.

Unfortunately, the board wasn’t recognized once plugged. The LEDs were blinking, but nothing in the USB device list. After some investigation and noticing that the LEDs were brighter when the board was powered by USB than with the external power supply jack, I found the problem: I have dyslexia.

IteadStudio PCB Test: USB board TOP

Or rather, let’s say: “Don’t name you schematic nets with names that one could mix up” Indeed, in the schematics I inspired myself from, there were two different nets with very close names. One called “Vbus” and the other “Vusb” all written with caps / small caps. And of course, I connected the Vbus to the Vusb and when powered with USB cable, the 3,3V regulator was by-passed and the PIC was fed +5V directly to its “VUSB” pin.

IteadStudio PCB Test: USB board BOTTOM

Luckily, the PIC didn’t die, even if +5V is over its maximum voltage rating. The PCB was easy to fix by cutting a track, adding a wire and de-soldering a diode. Unfortunately now I have to be careful to not plug an external power supply when my board is plugged to a computer (no more protection preventing back feeding of current to the USB cable). Well, I should live with it.

USB dev board iTeadStudio PCB

This board is inspired form Microchip’s FS USB plug-in board. I added a Texas Instrument’s RS232 transceiver (MAX3221), three LEDs, three push buttons and connectors with the six GPIO ports from the PIC.

I’ll play with it now to see what I can do with USB and all those GPIOs.