Cheap ass 5V switched wall wart playing games with me... - Page 1
Forum section: electronics / beginners

author: Highcooley
author_rank: Newbie
timestamp: January 26, 2015, 06:04:38 pm
content: Hey guysI 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!CheersAndy

author: Simon
author_rank: Unknown
timestamp: January 26, 2015, 06:14:18 pm
content: change power supply, if it's junk it is junk.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 26, 2015, 06:14:29 pm
content: Your wall-wart could simply be defective (intermittent).Unless you are stuck on a desert island (or some equivalent supply-chain anomaly),I would simply toss it and replace it with a known-good supply, preferebly a linear one.

author: Paul Price
author_rank: Super Contributor
timestamp: January 26, 2015, 07:12:07 pm
content: Looks to me like you have a grounding problem or you don't have a bypass capacitor at the power supply connections where they connect to Ardruino board. When you connect the scope or even your fingers you are doing something that shouldn't effect the circuit. The power supply is ok, something else is the matter with your connections. Check the grounding and make sure you aren't somehow have a wiring error that is connecting your control circuit directly to the mains. The whole thing looks to me like what is the result of a floating input pin on the Arduino that is connecting to your temperature sensors or some other pin is floating and reacting to voltages created by your proximity and causing the problem.

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 26, 2015, 09:33:43 pm
content: When you are measuring your secundary, you effectively short the negative to earth through your scope. This also provides an easy path to earth for funky noise. Normally, there should be a small Y2 cap (2.2 nF or so) tot het rid of the noise, but cheapass Chinese PSUs tend tot be... Inventive with respect tot that

author: bugs
author_rank: Contributor
timestamp: January 26, 2015, 10:24:28 pm
content: I have had some weird Arduino problems, which turned out to be a dodgy USB cable. Is your Arduino still connected to a PC? You may want to check/replace your cabling.

author: Highcooley
author_rank: Newbie
timestamp: January 26, 2015, 11:05:46 pm
content: Thanks guys for your quick reply.If I power the thing over my bench supply, everything is fine of course. Unfortunately, bypass capping the Arduino didn't help. Ah well, long story short, it's definitely the wall wart, since Y2 caps are entirely missing in there. And no, the controller supply is definitely not shorted to the mains, that's always a thing I triple check before powering up the first time...otherwise I would be dead by now.Ah well, I'll have to check if I've got another supply which will probably be dirt cheap stuff too, but may be a tiny bit better. It's always hard to distinguish what you get if your order online. You probably won't believe it, but Switzerland - although in the center of Europe - is kind of a desert in terms of electronics supply possibilities. Except if I want to pay 15 bucks up for a simple 5V wall wart.I'll keep you guys posted on my results when I've got the time to go on with this little project next week.CheersAndy

author: Monkeh
author_rank: Super Contributor
timestamp: January 26, 2015, 11:39:22 pm
content: Quote from: Highcooley on January 26, 2015, 11:05:46 pmhttp://ch.farnell.com/raspberry-pi-psu/rpi-psu-eu-mk1/stromversorg-raspberry-5v-1a-euro/dp/2254794

author: Highcooley
author_rank: Newbie
timestamp: January 27, 2015, 12:15:04 am
content: Quote from: Monkeh on January 26, 2015, 11:39:22 pmThx for the link. Shipping Farnell to CH is 8 bucks, so we're at CHF 15 againAt least, with the recent drop of the EUR-CHF binding, shopping abroad gets a tad bit cheaper. So we don't want to beef about that.Anyways, I found an unused iPhone charger, which has a lot more parts in it (including an LC filter). And tadaaa, it does the trick! Again, thank you all for your help! I learned a lot today.CheersAndy

author: Monkeh
author_rank: Super Contributor
timestamp: January 27, 2015, 12:23:10 am
content: Ah, I don't speak the barrage of unpredictable lingo, so I can't find their shipping costs. They'll overnight me 20p of capacitors from Belgium for free, but will charge more than the item is worth to ship a little package to Switzerland.. No understanding large companies.Can't go wrong with an Apple one anyway.