That Cheap USB Charger Could Be Costly

[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.

This isn’t, of course, the first phone charger teardown we’ve seen. This probably isn’t as deadly as the USB killer, but we still wouldn’t want to risk it.

31 thoughts on “That Cheap USB Charger Could Be Costly

  1. Somewhere in my parts bin is a Sony wall wart that has a reversed polarity digram molded into it. There was a bit of speculation at the time that they were trying to sell their own replacements since the device would not operate on a traditional “positive tip” connector that was indicated, but wasn’t harmed by it. So it goes.

    1. I’ve got two Sony power supplies that are identical in every way, except for connector polarity.

      Should check if the model numbers match, that would be amusing.

      Whatever happened to the idea that tip diameter determined the voltage (bigger = more volts) or yellow tip = 5v. (And yes, I remember the RadioShack weird tip colours.)

    2. Ahh screw Sony and their reverse polarity jacks, I smoked a Sony D-50 (first portable CD player, a miracle of engineering for 1984) by not paying attention to that….

    1. There was also a generation of the pTouch that went for a *7V* center negative just to be extra obnoxious. Still used 6xAA, so they went out of their way to have their special dumb 7V wall warts that cost as much to replace as buying a brand new labelmaker.

      I’ve got one from a garage sale on the workbench waiting for the external plug to get routed to the battery input so I can use a normal wall wart on it.

      1. That does make me wonder if the 7V was required though… like it would probably work with anything from 6V to 12V. Not that I’d try that without checking the circuit first.

        1. The battery compartment wires actually run up to the same baby PCB that the DC barrel jack sits on, but into separate input points that run through a separate set of components before the common output to the mainboard. Another option would be to measure what that common output is (probably 6V) and go straight to that… but I’ve got lots of 9V adapters to pretend they’re 6xAA.

        2. The Brother P-touch printers (PT-18R) we use at work have come with a few different power supplies over the years, including a transformer 100V-only input 12VDC output, universal SMPS input and 9VDC output and the latest are universal SMPS input and 12VDC output.

          After one of the 100V transformer types was plugged into a 200V outlets and smoked itself and the vreg in the printer, I was able to replace it to get it working again.
          The vreg was a standard-ish part (although reverse pinout to the most commonly available pinouts, of course…) and good for something like 18V-ish supply voltage IIRC.

    2. Center negative is quite common. Its used widely with music gear like synths and pedals. Its also very common with Japanese made equipment.

      The reason for center negative is the switched contact in the socket. It allows battery powered equipment to easily switch between internal battery and external DC power.

      1. > Switched Contact in the socket

        Are you thinking of headphone jacks that ground the sleeve or a ring when no cable is inserted? Your standard barrel jack plug doesn’t have this functionality so I’m really stretching my brain to determine why a relay or other external control mechanism to switch between battery and barrel jack power would care what polarity the barrel jack has.

          1. That works the same with the negative…. just as cheap and no additional components either.
            Just installed the DC socket switched outer part in the negative line instead of the positive (and of course the center tap to positive instead).

    3. Oh wow this is good to know. I got an old ptouch from a garage sale and could never get it to turn on with one of my 9v wall worts. Never bothered to look at polarity at the plug just figured that input was blown just used batteries.

    4. KORG uses center negative for their older gear, with a typical size DC barrel. And 9V center positive like almost every guitar pedal, etc but use a smaller barrel connector that nobody else uses. It’s great, old KORG wall wart to fry your pedals, and new KORG that doesn’t plug into anything non-KORG.

    5. I fried my Ptouch label printer. The labeling of the polarity is small/unclear and due to poor eyesight I ended up attaching a center positive wallwart and let out the magic smoke. Damn those Ptouch engineers.

    6. I didn’t have the correct adapter for mine, nor was I about to stick 6 or more AAs into it. So.. I found a generic 9V supply and soldered it right to the battery contacts.Been working great for 10+ years.

    7. A few weeks ago, I couldn’t get any of the pTouch units from my big box of them (accumulated whenever I find used ones cheap) to work, including some that had worked the last time I used them. At best the tape would not advance while printing, and end up printing nothing but a little black bar. All of them, even with different tapes. The batteries all measured 1.5V, so if they were the problem then the device was really crappy. I’m getting tired of alkaline batteries these days anyhow.

      The only non-pTouch I had in the box was a “Casio Label It!”, and two unopened tapes. It wanted 8 AA batteries (ugh) but it had a barrel jack. It was +5 center, but an odd size. I found one matching that size, and it worked. I don’t mind it being tethered to my work area as long as it actually works.

  2. I once seen the entire 24 volt supply pass through the 5 volt regulator, because a pin on the QFN chip regulator missed the solder. Every FPGA on that board became a short.

  3. Once, in the 486 era, burned out a computer with an odd power supply that used red wires for +12V and yellow for +5V, instead of the other way around like the rest of the world.

    And once lost an entire dimmer rack to brand new three-phase cord that had a few wires crossed. That was an expensive mistake to make.

    Off course, like Al said, I should have tested. But you never do.

  4. Pin positive only makes sense around mobile situations. The first gen of transistorized radios were germanium based PNP and were negative supplied with a positive ground. That first gen barrel jack may have been mounted on a metal chassis or box. Hence the reversal but back then wall warts were rare so anything to customize the connection was done. I remember totally custom connectors on some old gear.

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