Ass blown out of arduino, powered USB hub and device with ungrounded AC to DC  - Page 1
Forum section: electronics / beginners

author: KriLL3
author_rank: Newbie
timestamp: February 14, 2015, 06:10:27 pm
content: Long time watcher of the show, first time posting.I'm by no means a novice when it comes to DC electronics but I got a rude awakening to my ignorance about AC today, I'm not quite sure of the mechanics but I got a vague recollection of Dave talking about the dangers of connecting something with an AC coupled ground to something floating, and well:Real fake chinese Arduino UnoBlown polyfuse and "E01/EOI" 8 pin package (what is that anyway? I can't find it labeled on a schematic, though the IRC channel think it's the dual opamp package. top right inhttp://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdfChinese wireless doorbell receiverBlown "resistor" that had a "0" printed on it, I assume it's a short? I've measured the other "0" resistors and gotten less than 1 ohm resistance, I assume that's from the probes etc.Cables near the left side go to a set of european mains prongs but no ground pin.On the other side :Backstory:This was part of a project of mine, my parents bought a robot vacuum cleaner but unfortunately my dad being my dad bought it without much research or asking anyone else to do the research, so he got a pretty good one, with all features like the remote and scheduling (!) stripped as a cheaper SKU, my idea was simple, it still has the IR receiver for the remote functionality and I managed to track down the IR codes to remotely control it, I've successfully tested them already, but instead of building some kind of conventional remote I figured a wall mounted button next to the main door is a better solution, push the button if you're the last man leaving to trigger the robot. The main entrance/exit to the house is on another floor though, so some wireless solution would be required unless I wanted to drill through the floor and run a lot of cable.I looked around for the cheapest possible way to send a wireless on/off signal and settled on a $5~ chinese wireless doorbell thing, it's a plastic wall-wart looking box you plug into a wall outlet and a small square thing with a clicky button meant to be put up outside, my plan of attack was simple, remove the speaker and find some pin inside that can be probed to tell if it's trying to make a sound, connect an arduino pro mini to that pin and make the arduino spit out the IR code though an IR LED if sensed. And possibly connect a RTC and LCD + encoder to do scheduling etc, aim LED at robot's charging cradle.I probed around with my multi-meter on the chips inside the receiver while I pushed the remote's button and found a pin that went up to 2.6v~ each time the button was pushed (orange lead connected to it) also found a 3.3v (red) and ground (black) pins (yes they were shorted in that pattern out of the box). That I meant to power the arduino pro mini from, Wanted to test my idea using an arduino uno to see if I could sample that 2.6v on the digital inputs or if I had to use an analog input, connected grounds, that orange lead to one of the digital pins with the wireless doorbell connected to mains and connected the USB cable to the Uno, flash and darkness, tripped ground fault interrupter and my room's breaker.Haven't opened the powered 7 pin USB hub yet but nothing I connect to it shows up in windows and devices connected to it only get power if the external 5v source is connected, sadly the hub has no exposed screws at all so opening it might be a bit of a struggle:Hopefully the black bits just pry off with some clips but with my luck they'll be glued/heat-welded.The PC itself and the port the hub was connected to seem fine luckily.Questions:So, errm, what do you guys if anything can be salvaged? Odds of it being economical/worthwhile to repair?How do I avoid repeating this in the future? When I'm noddling around with arduino stuff it's much easier to run the arduino of USB power so you can upload modified programs to it without having to swap from some other DC source, how do I keep doing that without blowing up other gear?Any other suggestions on the specific project? Did I do something wrong?Thanks in advance for any replies (that aren't pure derision).

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2015, 06:19:33 pm
content: This could have been avoided by running the doorbell receive from an isolation transformer, stripping out the old capacitive power supply and replacing it with an isolated PSU or coupling the 2.6V output to the Auduino via an opto-coupler.

author: IanB
author_rank: Super Contributor
timestamp: February 14, 2015, 06:24:02 pm
content: I am not able to tell exactly what happened, but the doorbell receiver, was it powered from the mains?Generally speaking, you should not connect two or more mains powered devices together unless they are designed to be connected. (Example: you can connect your DVD player to your TV because it was designed with external connectors to do this.)If you open up the case (of the doorbell thingy) and poke around the insides, you are no longer on safe territory since you are doing what the designers did not intend. For instance that doorbell receiver inside its plastic box could have a dodgy power supply and be floating at mains potential. As soon as you open up the case and start connecting to its insides you could be sticking the mains up your Arduino...In general, when connecting devices together it is safest if they are all powered from the same shared power supply with a common ground and are isolated from anything else that is not on the same ground reference. When it is necessary to provide isolation then opto-couplers or transformers can be used.

author: SeanB
author_rank: Super Contributor
timestamp: February 14, 2015, 06:36:36 pm
content: You connected a trtansformerless power supply, which has the power rails floating on the mains, either via a diode bridge or direct, to a grounded power rail. Lucky for you you only blew up the doorbell receiver and the cloneduino, not the computer itself.With the new cloneduino board ( nothing salvageable on that one that is either worth anything or will have reliability problems) and new powered hub and new doorbell receiver, you just supply the cloneduino with a 12V wall wart and use that to supply the doorbell receiver via a resistor as well. The supply of 12V is applied across ZD1 on the doorbell board with a 100R 0.5W resistor in series with the positive lead.Then it will work with no issues, just cut off the mains wiring plug leads on the doorbell so it is not accidentally plugged in and blows up again. You can also remove the big 22R 5W resistor ( the big white one) and the big brown mains capacitor as well, along with the plug.

author: cosmicray
author_rank: Frequent Contributor
timestamp: February 14, 2015, 07:17:02 pm
content: I see the same cloneduino board, shipped from HK for $16USD. Do you want to buy another one, or get an authentic one from Italy ?

author: SL4P
author_rank: Super Contributor
timestamp: February 15, 2015, 06:37:37 am
content: I don't want to sound like a smart-arse, but IMO, you're lucky to be alive.DC experience or not.Clear as day - as mentioned above - the doorbell RX is running on non-isolated mains (i.e. floating Vcc).Not nice, and probably illegal to sell/use in most countries if the case is not designated and spec'd as DOUBLE-INSULATED. (WHL branded unlikely)You can see the mains input on left, straight into cap, R, and diode bridge - then across the 'isolation barrier' to the LV circuit. Shocking - literally. Theonlysaving grace is that the TX is wireless, and has no voltage relationship to the RX !

author: KriLL3
author_rank: Newbie
timestamp: February 15, 2015, 01:52:12 pm
content: Quote from: Hero999 on February 14, 2015, 06:19:33 pmI'll have to look into opto-couplers, and yeah I guess supplying DC from a less... dangerous? source is a idea I can get behind.Quote from: IanB on February 14, 2015, 06:24:02 pmWell yeah in retrospect it wasn't the best thing to do ever, I've done stuff like this before on non-floating stuff though just fine.Quote from: SeanB on February 14, 2015, 06:36:36 pmI wondered why I couldn't find the transformer coil... thought it might be one of the larger components.Quote from: cosmicray on February 14, 2015, 07:17:02 pmI'm not partial to any particular kind of cloneduino, can get cheaper newer ones from china that doesn't pretend to be the real thing usually branded "Freeduino" or similar.Quote from: SL4P on February 15, 2015, 06:37:37 amWell I've been zapped by 240v in the past, it's unpleasant but I doubt it's lethal unless you manage a prolonged exposure or got a bad heart, and I wasn't in the loop. I thought the AC part looked dodgy but I had no idea it was that dodgy, does it basically run on 240v and 235v compared to ground to give a 5v differential?Got the USB hub opened, was glued but with some glue that set real hard but not elastic, just needed a tug to snap, can't find any obvious damage but devices connected to the hub only get power, windows doesn't register anything. It has a fuse which isn't blown, only thing of note I can find is this:Those 2 traces on the big (and only) chip are shorted, not sure that happened then or was like that from the factory though.Thanks for your replies people, this will teach me to consider how the AC circuit is built not just measure voltages on the DC side thinking 5v is 5v...

author: Seekonk
author_rank: Super Contributor
timestamp: February 15, 2015, 02:04:58 pm
content: Now might be the time to consider moving to the MINI328 platform that only costs $2-3 shipped and smaller than two postage stamps. Pinout is the same spacing. can do whatever the UNO can do except communicate via a USB port. For that you nrrd an additional $4 FTDI board to program or get out data (they can be programmed with another UNO). Then you can make all the dodgey projects you like.

author: Monkeh
author_rank: Super Contributor
timestamp: February 15, 2015, 05:00:21 pm
content: Quote from: KriLL3 on February 15, 2015, 01:52:12 pmThat is the sort of attitude which will get you killed.

author: KriLL3
author_rank: Newbie
timestamp: February 15, 2015, 05:30:44 pm
content: Quote from: Monkeh on February 15, 2015, 05:00:21 pmI obviously don't seek it out but accidents do happen, I've gotten zapped 2-3 times over 20+ years, hurts but not that badly, especially if you just ground a live wire, not complete the live-neutral circuit.

author: Monkeh
author_rank: Super Contributor
timestamp: February 15, 2015, 05:47:21 pm
content: Quote from: KriLL3 on February 15, 2015, 05:30:44 pmUh, ground does complete the circuit.Please learn a bit of respect.

author: CyclicUK
author_rank: Contributor
timestamp: February 15, 2015, 06:04:02 pm
content: Quote from: Monkeh on February 15, 2015, 05:00:21 pmFrom what I've read it takes a surprisingly small amount of current to kill - 100-200ma apparently:https://www.physics.ohio-state.edu/~p616/safety/fatal_current.htmlEven if you're comfortable taking those kinds of risks it might not be sensible suggesting it's ok, especially in a beginners forum - noobs like me might read it and think it's quite safe...

author: KriLL3
author_rank: Newbie
timestamp: February 15, 2015, 06:05:52 pm
content: Quote from: Monkeh on February 15, 2015, 05:47:21 pmYes but nowhere near as bad, especially if the ground path goes along your legs into the floor, a lot higher resistance path than from hand to hand live and neutral.

author: Monkeh
author_rank: Super Contributor
timestamp: February 15, 2015, 06:11:40 pm
content: Quote from: CyclicUK on February 15, 2015, 06:04:02 pmAs little as 3-5mA across the heart, actually. 30mA across the chest can easily achieve this.Quote from: KriLL3 on February 15, 2015, 06:05:52 pmThat's not the point, and is still potentially adequate to drive fatal current.The moment you start thinking of 240V as 'unlikely to be fatal', it becomes much more likely. Unless you consciously accept this, the only way you'll learn is by damn near dying from your own stupidity, if you survive. Trust me, that's how I learned.