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Rescue a defunct phone charger Answered

I have an old cell phone charger that is very handy for charging my Kindle Touch and a set of Bluetooth earbuds. It quit working one day after someone used it to charge a high demand device.

I read that the capacitors in these often fail. I got two electrolytic capacitors at a surplus store for less than a dollar total. I had to use a fine saw to open the plastic case. The new capacitors were the same electrical size, but not the same physical size. Closing the plastic case was not an option. I covered all exposed electrical contacts with hot glue, which is an excellent insulator. It looks ugly, but I have a good versatile charger again. (The high voltage connections are buried deep down where my fingers can not reach. As best I can tell, the terminals covered with hot glue for insulation are all low voltage.)

Discussions

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rickharris

3 months ago

Electrically speaking this is a poor idea. Mains electricity and open circuit - even covered in hot glue isn't good.

Especially for an item that costs very little to buy. Indeed if you go to the reception of a hotel they will have dozens of chargers under the counter for free that guests have left behind.

However well done for fixing it.

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Phil Brickharris

Reply 3 months ago

Our mains are half of the voltage of yours in the UK. I have been using this repaired charger for months with hot glue as the insulator. This arrangement was not my first choice, but it has worked very well with no hint of a shock. My experience with electricity is not as extensive as yours, but we are not far apart in age and I have worked safely with it throughout my whole life. My first experiences and training was to work with my father as his helper in his electrical business.

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rickharrisPhil B

Reply 3 months ago

Fair enough. Anything is safe if you respect it and understand what - might - happen.

IMHO you sound safe :-). Alas many people arn't!.

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Jack A Lopez

3 months ago

Wow! Nice one, Phil!

Your successful repair of this phone charger gizmo illustrates several things I have noticed about these devices before, but that I have never bothered to write about, uh... until now.

The first thing is the fact that, for many of these phone charger power converters, the plastic case has been intentionally welded shut, thus necessitating some kind of sawing, cutting, or cracking, just to get the case open. In some ways this action is similar to cracking open a nut. Some cases are easier to crack than others, just like some nuts are easier to crack. E.g. pistachios are easier than walnuts, and walnuts are easier than pecans.

The usual tools I use to crack open one of these sealed power converters, are a bench vise and a hack saw. The bench vise is to hold the plastic case steady, while using the saw to make cuts. Also usually I like to make the cuts on the existing seams where the two shells of the case connect together.

The compressive forces applied by the vise jaws, can help sometimes too. Cranking down the jaws just enough, and in the right direction, can actually crack the plastic case in places that are helpful, for to get the case to come apart.

However, continued cranking of the vise jaws, without subtlety or caution, risks crushing the circuit board and, or, the components on it, and that would be bad.

Although accidentally breaking the circuit board would not be the end of the world, because this kind of charger is cheap, especially if you buy it from a thrift store, which is often where these things go, when deemed junk by their owners, i.e. when the charger gets separated from the phone or other device it used to charge.

Another thing I have noticed about these charger gizmos, is often there is a wide variety of different shapes for the connector that plugs into the device itself. So that the funny shaped connector is much like Cinderella's glass slipper, and it strangely becomes the most rare, and necessary, part of the charger.

Although, with the advent of USB, I am hoping funny Cinderella charger connectors are going to become a thing of the past. Since it is easy enough for device manufacturers to just use some form of USB, instead of these funny proprietary shaped connectors.

The final thing I was going to comment on, regarding filter capacitors, is that it almost always safe to err on the side too much capacitance. I mean, for example a 1000 uF filter capacitor could be replaced with 2000 uF or 4700 uF, etc, with equal or greater voltage rating, and the circuit will work just as well, or better, with less ripple noise, than before.

Although one trade off to bigger electrolytic capacitors, is physical size. You might encounter the circumstance where the capacitors are too big to fit inside the case.

However, it looks like you solved that problem too, just by putting some hot glue on it.

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Phil BJack A Lopez

Reply 3 months ago

I used a thin cutting wheel on a Dremel rotary tool to open the case. I did nick the output wires from the circuit board to the external output cable, but repaired that. The fitting to a device is micro-USB. The replacement capacitors are the same 330 micro-farad, but a higher voltage rating compared to the originals. I wanted to protect against line voltage shocks.

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Jack A LopezPhil B

Reply 3 months ago

I was guessing it might have been some form of USB, just based on the what you wrote about this charger being used with a plurality of devices, erm, three specifically, "Kindle Touch", "set of Bluetooth earbuds", and a third, unnamed, "high demand device". I mean, if it was a charger with a Cinderella connector plug, it probably would have been just charging one device exclusively, the one with the matching jack.

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iceng

3 months ago

There is a worth while concern that too much extra capacitance may stress the rectifier by the inRush charging current...

A situation that needs addressing on audio power supplies.

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Jack A Lopeziceng

Reply 3 months ago

You know that is a good point. I was thinking a kind of, sky is the limit, i.e cannot pick one that is too big, philosophy, is good for picking filter capacitors, and I said as much in my comment. However, I was not thinking about the instant in time when the power supply is switched on, and the capacitor resembles a short circuit, for a brief time. Or, I mean the duration of that "brief" time, and the duration of the initial current surge, is, I think, proportional to the size of the capacitor.

In defense of Phil, he was not making the same kind of sloppy assumptions I was. He replaced that circuit's capacitors with ones with the same capacitance, but higher maximum voltage rating.

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Phil Biceng

Reply 3 months ago

The replacements are the same nominal 330 micro-farads the originals were, but physically larger because of a much higher safe maximum voltage rating.