I have a bunch of devices (two e-readers, two cell phones) that are charged through what I call a mini-USB port, the one that is smaller than the one on my computer and with one pair of corners rounded. They come with cords that have plug-in charging blocks on one end and the mini-USB ports on the other. Except one that just has full-sized USB on one end and the mini-port on the other. My wife’s bedside clock has a USB outlet. So my question is, is it safe to use any of these chargers for any of the devices?
Yes, but some may charge faster than others (some chargers can supply more than the nominal 500mA that USB requires).
by the way, that connector is Micro USB; Mini USB is different and seems to be falling into disuse.
The short version? You won’t blow anything up. If your phone/e-reader/doohickey is 10 years old, they may not work with newer USB ports. Pretty much anything newer than that, and practically everything recent, will work just fine - you’ll probably get a faster charge out of the wall chargers than a computer or clock, but the chance you fry something is virtually nil.
The virtually is included because I’m sure there’s one jackass manufacturer in Dumbfuckistan that hasn’t adopted the Battery Charging Specification. If your phone doesn’t pop up Cyrillic lettering during it’s boot sequence, you should be safe.
By “USB sources”, I have to assume by context you mean “USB power sources”, because USB was originally envisioned as a data communications method; i.e., a data source.
The charging capabilities of USB are codified in USB standards. A standards-compliant USB device will adhere to voltage (always 5v) and current (variable, depending on which standard and capability definition was chosen). Form factor has nothing to do with it, except that Type C supports a higher charging maximum than Type A/B.
Type C has a fixed connector definition (symmetrical flat oval). Type A (host end) has the full-sized rectangular connector. Type B (client end) has the variety of form factors you were noticing (large squarish standard size, asymmetrical pinched D-shaped mini, and asymmetrical flat D-shaped micro).
In theory, you can plug a device with a mini or micro connector to a cable and plug the cable into a device (like a clock radio) with a Type A socket and, by doing so, draw an appropriately regulated charging current. (Probably a 100 milliamp slow-charging current, but devices can be designed to deliver more charging current than that.)
As far as I know, you won’t have a problem with micro-USB (except taking forever to charge if you use a port that can’t provide more than the 500mA spec).
There have been problems with non-standard USB-c cables because that standard is designed to deliver more power, which raises the risk of excessive power draw if some cheapskate manufacturer cuts corners. A Google engineer by the name of Benson Leung has done yeoman work in testing lots of cable types; if you’re shopping on Amazon (which has now banned non-standard USB-c cables, but some might slip through) look for his reviews (5-star for the good ones, 2-star* for the non-spec ones).
*He does this on the theory that people tend to ignore one-star reviews, because too many of them are from nitwits who are shocked to discover that stuff doesn’t work if you’re too stupid and/pr lazy to use it properly.
Not directly related to the question, but also note that some cables that come with charging adaptors only have the two power wires in the cable; the data cables are missing. This is not a problem if you only use the cable for charging, but the cable won’t work if you try to use it for data. There’s no easy way to identify such cables visually, as they use the same connectors and look identical to cables with all four wires, so I always label my power-only cables when I find them.
IIRC - there was an original USB spec for the 5V 11mA; some devices need a lot more power, so newer USB devices and ports(on PC, etc.) have the ability to negotiate higher power levels.
My original iPad, for example - I plug it into a PC and it says “not charging”. However, it is charging, just slowly, because it does not negotiate more power. (And the old iPhone connector end is totally non-standard, too.)
They say that when the guy who invented USB dies, they will put his coffin in the ground, then pull it out, flip it over, and put it back in. (And then pull it out, flip it back the way it was first, and put it back again.)
I recall a somewhat more off-color version of this joke, which I presume you can reconstruct for yourself from that description. 
Actually it’s 500 mA.
When Apple designed the iPad, they designed it so it would draw up to 1A (1000 mA) power - but only if it was connected to a proprietary Apple USB power supply that could supply 1 A. The iPad determines this by sensing the voltage on the data signal lines, if I understand correctly. If it was connected to a standard USB port, it would restrict itself and draw only 500 mA.
But when other manufacturers decided to do the same with their tablets or phones, they couldn’t just copy Apple’s proprietary design and make it compatible. They came up with different ways of determining whether the USB power source could supply higher current.
So for a while, high-power USB power supplies came in different flavors, Apple or Samsung. Then Qualcomm came up with the Quick Charge 2.0 standard, but I don’t think it ever caught on. Eventually, they started making smart USB power supplies that could supply high current to most Apple and Android devices (i.e. successfully convince most devices that they are connected to high-current power supplies and are allowed to draw high current, up to 2.1A or so).
TL;DR version: Any device with a micro-USB port can be safely connected to any USB port, but it may charge more slowly if it is connected to a standard (500 mA) port, or thinks it is.
And not part of **Hari Seldon’**s question, but the USB ports on the wired Apple keyboard only supply 150mA. Ok for a mouse but not enough for some USB flash drives
Another warning is that not all of the sockets you plug the rectangle into are created equal. Some of the really cheap ones (especially the ones that look almost like the real name-brand plugs) do a really terrible job of supplying a steady 5V. I’ve seen some for which the “5 volts” is actually a sine curve ranging between 0 and 10 volts, for instance. This usually won’t damage a device, but it can, and it also often has effects like rendering the touchscreen unusable while it’s charging.
I know I took the Apple plugs overseas (Australia, Hong Kong/China) where it would use 240V not 120V. In some instances, the tiny cube charger did not work; I plugged it into assorted wall plugs over and over until it finally deigned to charge my iPhone or iPad. The larger more rounded charge blocks had no such issues.
Those tiny Apple cubes are miracles of engineering as it is-- It’s probably asking too much to expect them to adapt to 240V vs. 120V.
Not really, they are switching power supplies, the integrated circuit that chops up the 240V input into the correct voltage should be able to manage 120V (or 110), the principle of operation and circuitry is the same for both instances. In fact, AFAIK, all the power supplies I have like those work between 110 and 240V.
As an aside, last week I was doing some testing with a DC-DC Buck-Boost converter module I bought some time ago, now those are neat! Input voltage from 5 to 36 volts, output voltage adjustable from 0 to 35, independently of input voltage, so for example the output voltage can be set at 24 volts and it will remain there while the input varies between 5 and 36 volts.
For people who don’t know switching powers supplies (the step-down or “Buck” converter type that reduces the voltage) work on a principle similar to a bucket with a hole in it, let’s say you want to keep water coming out of the hole at a constant rate, but the source of water that fills the bucket is much higher than the output you desire, what the switching power supply does is basically turn on and off the input to keep a steady output, as if you would be quickly opening and closing the tap feeding the bucket to maintain a more or less constant amount of water in the bucket, and thus a constant pressure at the hole leading to a constant rate of discharge.
No not miracles of engineering.
The 120 V components are cheaper, and so to make a cheaper cube, they can make it 120V only. I dont know what the guy was doing plugging it into 240V if it didnt say “240 volt compatible”. ALWAYS ALWAYS ALWAYS CHECK the device says (is designed for) “240V” before plugging it into 240V!
It’s not a miracle of engineering to make a switching power supply (or rather, it is, but it’s a very common miracle). The miracle is in making one of that high quality that’s that small. There are supplies that are higher quality than Apple’s, and supplies that are smaller, but very few of either, and none that is both.
It’s not a “miracle.” It’s just that Apple doesn’t cheap out on them.
“Miracle of engineering.” sheesh.
Thanks to all, but especially for the link above. I don’t think I have ever seen the one called mini. Like they say, the great thing about standards is there are so many to choose from. Oh, I get that the later ones are improvements over the earlier ones.
Just for the record, all the chargers I have seen (especially for laptops) claim to work on 100–240 volts, 50 or 60hz. 100 volts is used in Japan, 50hz in the east, 60hz elsewhere, with US standard sockets.
Actually md2000 was saying that the battery charger was rated for 240V, in that case it’s safe to plug it on 120V, it may not work, but it won’t burn.
I knew I had the innards of one of this charging cubes laying around somewhere, not too much stuff in them, a tiny transformer on the top board, a power transistor to chop up the input voltage and a few capacitors to smooth out the output. Under the board with the transformer there’s a small tiny chip that I think is in charge of running things.