Hey guys
I found this reflow oven controller on Tindie, which in my opinion is a great design in both terms of hard- and software to achieve the goal of an own reflow oven in an efficient way (e.g. no additional solid state relays needed):
https://github.com/0xPIT/reflowOvenControllerHowever, I do end up with one stupid issue I am not able to solve so far. To power the thing, I use one of these cheap ass 5V, 1A switching wall warts from an old USB hub. I shortened the cable to about 10cm and also put some ferrite beads (the straight leads with a bead around it) in series of both leads to power the controller.
When I have it running, suddenly the thermocouple controller goes into error mode saying that the probe is shorted to either VCC or GND, which it is definitely not. This happens anywhere between 0s to 2min from when I have the thing switched on. I also noticed, that the Arduino's timer runs really fast (always speeding up and slowing down to about double on average as it should).
What have I tried so far: When opening up the PSU, I noticed that there is only a single 22uF elcap on the secondary side. So I clamped in an additional 100nF ceramic cap next to the 22uF. When measuring the ripple with my DSO, everything was running smootly. In some conditions of the triacs running at about 33% duty cycle, I get 150mV of ripple, but other than that, the PSU is pretty stable. The ripple is not the problem, since the errors occur about the same amount whether or not the triacs are on. Anyways, the problem didn't occur anymore with these measures. But as soon as I took away the DSO probe, all happened again. Clamping two windings of the PSU cable into a ferrite bead didn't bring any change either.
Next try was to build an easy LC filter out of a ferrite bead I desoldered from an old motherboard with a couple of windings around a torroid bead, a 100nF cap and a 220uF elcap for good measures. But no help either.
The problem is, every time I try to measure the supply voltage either with a voltmeter or a DSO, the controller works fine. Also if I touch both VCC and GND with my finger, the damn thing works without any issue. Haha, I thought...try a high Ohm parallel resistor (had a 10M lying around) or a very low cap (100p and 22p). But no, this both doesn't help. Also drawing more current from the PSU doesn't make the whole thing more stable.
Does anybody have a good idea what to try next? Thanks for your reply!
Cheers
Andy