[Big Clive] picked up a keychain battery to charge his phone and found out that it was no bargain. Due to a wiring mistake, the unit was wired backward, delivering -5 V instead of 5 V. The good news is that it gave him an excuse to tear the thing open and see what was inside. You can see the video of the teardown below.
The PCB had the correct terminals marked G and 5 V, it’s just that the red wire for the USB connector was attached to G, and the black wire was connected to 5 V. Somewhat surprisingly, the overall circuit and PCB design was pretty good. It was simply a mistake in manufacturing and, of course, shows a complete lack of quality assurance testing.
The circuit was essentially right out of the data sheet, but it was faithfully reproduced. We should probably test anything like this before plugging it into a device, but we typically don’t. Does our phone protect against reverse polarity? Don’t know, and we don’t want to find out. [Clive] also noted that the battery capacity was overstated as well, but frankly, we’ve come to expect that with cheap gadgets like this.
[Doug]’s newly-installed Yaesu FT-891 mobile transceiver failed to power up despite a careful installation, and it turns out to have ultimately been caused by a reversed cable. There’s a happy ending, however. Since the only real casualties were a blown resettable fuse and a badly-burned resistor that damaged the PCB, [Doug] was able to effect a repair. Things could have been worse, but they also could have been better. Damage could have been prevented entirely with some better design, which [Doug] explains during his analysis of what went wrong.
The main problem was that the generic RJ12 cable that [Doug] used to connect radio components had its connections reversed. This would not be a problem if it was used to connect a landline telephone to the wall, but it was a big problem when used to connect the radio components together. According to the radio schematics, the two center wires carry +13 V and GND, which meant that a reversed cable delivered power with reversed polarity; never an optimal outcome.
Once the reversed power arrived at the other end, [Doug] discovered something else. Diodes whose job would be to protect against reverse polarity were marked DO NOT INSTALL, probably to shave a few cents off the bill of materials. As a result, the full 13 V was soaked up by a 1/8 W surface mount resistor which smoldered and burned until a fuse eventually blew, but not before the resistor and pads were destroyed. Thankfully, things cleaned up well and after replacing the necessary parts and swapping for a correct cable, things powered up normally and the mobile radio was good to go.
We’ve been meaning to get around to this one for many weeks now. It’s been hard to find good fail write-ups… it’s as if hackers are afraid to admit that sometimes projects fail. We hope you’ll shake off that opinion as failure is the fastest path for learning and true understanding!
[xymax] was working on a control system for a chandelier with 150 bulbs which use 5 Watts each. This project was being readied for the NYC Resistor Interactive Party which [Adam Fabio] attended last month. As deadline for the show approached, the last piece was put in place late into the night… but it was connected backwards. In a tale worthy of a slapstick movie, the reverse polarity caused a chip on all seven controller boards for this module to blow like the one seen above. But that’s not all, the laptop being used during prototyping was connected by USB and started smoking!
All of us feel the pain of this type of equipment failure. Luckily [xymax] looked for lessons to learn instead of dwelling on the mistake itself. Use protection diodes, keyed connectors, and write about your failures. Hopefully reading this will help others avoid a similar equipment-destroying mistake.
Fail of the Week is a Hackaday column which runs every Thursday. Help keep the fun rolling by writing about your past failures and sending us a link to the story — or sending in links to fail write ups you find in your Internet travels.