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Thread: Wall Warts

  1. #21
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    Just to shed some light on USB power supplies and how they work. This is my understanding of what is both a simple idea in concept, and a rather complex one once you throw in manufacturing and cost issues, as is the fact that most end-user are not expected to have a lot of knowledge of power supplies.

    1. The primary job of a USB power supply is to deliver 5V. That's the primary specification of all USB power supplies. Anything that the power supply plugs into is expected to perfectly happy if it sees 5V and to work safely with it. Check your power supply and see if it says "5V"; if it says anything else then technically it's not a USB power supply, regardless of what plugs or sockets it has on it.

    2. Where things go squirrelly is that there is no hard specification for current limits. USB 1.x was specified as drawing up to 100mA. USB 2.0 is 500mA, while USB 3.0 is 900mA. Then we have devices like phones and tablets that have very large batteries. If you have a 5500mAh battery in a tablet, it doesn't take a genius to work out that it's going to take 11 or 12 hours to charge that thing with a 500mAh charger, so the manufacturers of the tablet design it to charge at 2 amps, and provide the "USB charger" to do it. Technically it's not a USB charger because it's over-spec on the current rating, but that doesn't matter so much because remember the main specification for the power supply is that it is 5V DC, which any "USB" device has to be able to use. Similar to the way lower coil resistances will draw higher amps from a fixed voltage, the device you plug the charger into will determine how many amps are drawn from the power supply. The job of the power supply is in the first instance to try to output 5V no matter what.

    3. Theoretically in a well designed power supply, if you plug it into something with too low a resistance (eg plugging a 500mA supply into a device designed to charge at up 2 amps for example) then the power supply should limit the current output to stay within it's design parameters. This will result in less than 5 volts at the plug however, and should rightly be considered that the power supply is in an incorrect operating condition. Some devices like phones don't like this error condition so they will stop charging, only to start charging again as soon as the voltage goes back up to 5V. That's bad, for the phone, the battery, and the charger.

    4. Now we get into manufacturing issues, and cost. Typically your wall-wart USB charger is going to cost $10 and be the lowest common denominator, these days only able to supply 500mA. Plugging this into a low resistance (high current) device will normally be relying on a diode to limit the current. This 7 cent component won't be really happy about doing duty as a current limiter, and will start to get hot. The efficiency of the power circuit will also be less, resulting in a lot more heat. If that diode fail in the exact wrong way (and it's only matter of time really) then result can be catastrophic. Reports you've heard of USB chargers launching themselves out of the wall socket in a show of hot sparks are probably due to this kind of failure. If the component does not fail, sometimes the heat can cause melting of the plastic which compromises the insulation properties of the design, for example allowing the high voltage section to short out - same result.

    As you imagine, cost is huge factor in how safe a power supply is designed. The more you design safety into something, the more components you will need and the better their quality needs to be. Your $10 "wall wart" just turned into a $70 power supply, so you might as well make it a 2 amp charger anyway.

    So, if people ask me if they can use their 500mA USB charger to charge their 1 amp or 2 amp device, the answer is that it's not advisable. It will probably work but you are relying on a 7 cent component (in the cheapest case) and running the charger at 100% capacity. Go and buy a 2 amp charger.

    Have a look at your chargers folks: they all have the output current rating printed on them somewhere. Use them within their design limits. If it doesn't have the specs on it, chuck it in the bin and buy something decent. (yes the printing is damn tiny, but it's important).

    To put it into another context that many vapers will understand: A USB charger rated at 500mA is designed to output 5v x 0.5A which is 2.5 watts. Not much, right? if you were to try to draw 2 amps from the same charger, you're asking it to deliver 5V x 2A or 10 watts. It won't, due to that diode and the voltage drop, but it's still a bad thing to ask a $10 wall wart to do. The resulting voltage drop may also put extra stress on the device you are trying to charge, by running the charge circuit out of spec.

    None of this is a problem with properly designed devices and power supplies, but we don't know what's really in there. A cheap component doing double duty as your only safety, or a proper limiting device running within design specifications?

    --------------------------------

    Lastly, and just as aside, there's the related issue of switch mode power supplies and Australian 240V current. Most or all of these power supplies have an electrolytic capacitor for smoothing the voltage on the rectification side of the AC mains. Cheap capacitors of this type are typically rated to 220V and this is defined by the quality of the electrolyte and dialectic material. The next step up are 400V components which are significantly larger and more expensive. Designers and manufacturers in most countries therefore prefer to use the 220V versions. Many distributors do not understand (or do not care) about this issue so will quite happily ship power supplies rated at 220V to Australia. Retail stores are not allow to sell them, however you'll still come across cheap ones that slip through the net.

    Now you put these units into service at 240V. The capacitor is destined for a very short life. Even shorter if it is subjected to 250 or 260V because of your location on the grid or at certain times of day. Talk to a sparky if you want to know more about how this happens.

    A normal failure mode for electrolytic capacitors is a momentary short circuit inside the capacitor, right there on the AC line with just a power diode between them. The resulting surge normally destroys the capacitor with rather noticeable bang and cloud of smoke.

    I'd advise anyone to check the voltage rating on the panel on their USB power supplies as well. If it's missing or is rated at 220V, seriously plan on replacing it with one rated for Australian power.

    ---------------------------------

    So sorry, another book entry from me, in the interests of safety.


    TLDR: Check the ratings of your power supply/charger to see if it's suitable. It should state 240V input, 5V output, and have a current rating that is at least as high as what your device will want to draw (usually 500mA or 2A)
    Last edited by fabricator4; 29-11-17 at 01:07 PM.
    Fatman, Danny76, rockmoose and 1 others like this.
    Chris: Tobacco free since 17:00 15th March 2013.

  2. #22
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    Quote Originally Posted by fabricator4 View Post
    So sorry, another book entry from me, in the interests of safety.
    Books are welcome, thanks Fab for your input. My electronics is reasonable, but somewhat dated.

    if people ask me if they can use their 500mA USB charger to charge their 1 amp or 2 amp device, the answer is that it's not advisable.
    My advice too. But mainly because I'm thinking of a dumb load, which is not necessarily correct. There are more ways to skin a cat.

    Typically your wall-wart USB charger is going to cost $10 and be the lowest common denominator, these days only able to supply 500mA. Plugging this into a low resistance (high current) device will normally be relying on a diode to limit the current.
    When wall warts were transformers and rectifiers, and loads were loads, they would typically fail from this or more commonly melt the primary winding. These days everything is PWM, micro-controllers and (AC->DC / DC->AC / AC->DC) switching regulators. A heap of obscure chips is cheaper than copper and iron, and there are more failure modes possible. Such as throttling the PWM to reduce the voltage supplied. (I'm ignoring the data lines here)

    manufacturers of the tablet design it to charge at 2 amps, and provide the "USB charger" to do it. Technically it's not a USB charger because it's over-spec on the current rating, but that doesn't matter so much because remember the main specification for the power supply is that it is 5V DC
    The fact that people DO plug all sorts of "USB" devices into all sorts of "USB ports", and the shops insistence that the supply would never need to provide more than 500mA, made me consider that there may be a common procedure to prevent catastrophic failures.

    Similar to the way lower coil resistances will draw higher amps from a fixed voltage, the device you plug the charger into will determine how many amps are drawn from the power supply.

    A USB [power supply] rated at 500mA is designed to output 5v x 0.5A which is 2.5 watts. Not much, right? if you were to try to draw 2 amps from the same [power supply], you're asking it to deliver 5V x 2A or 10 watts. .... The resulting voltage drop may also put extra stress on the device you are trying to charge,
    Or the charging module in the mod could recognize the under voltage condition and adjust its parameters accordingly. After all the device being plugged in to the supply is usually a computer of some sort and the charging is under software control. In other words, change the load presented to the supply.

    This doesn't require much of the supply but places all the obligations on the device being powered.
    __________________________________________________ ____________________________________

    BTW Been a while, but I though electrolytics were rated by peak V not RMS, makes the situation even worse.
    Last edited by mattrix; 29-11-17 at 03:23 PM.
    gurishka likes this.

  3. #23
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    Yup.. Pwm and all the fun stuff.. My issue is a lot of people will read that and blink blankly.. Lol. It's like when I talk about valve amplifiers and so forth.

  4. #24
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    Quote Originally Posted by mattrix View Post
    Or the charging module in the mod could recognize the under voltage condition and adjust its parameters accordingly. After all the device being plugged in to the supply is usually a computer of some sort and the charging is under software control. In other words, change the load presented to the supply.

    This doesn't require much of the supply but places all the obligations on the device being powered.
    This is true, but we don't know which devices have this facility, unless the specs specifically state it can use a 500mA charger but will charge at up to 2 amps.

    How it is going to do this except by monitoring voltage drop though, in which case the "safety" feature of the power supply has already in play. There might be something there, but I'm not convinced that will be jammed into something the size of a mod.

    BTW Been a while, but I though electrolytics were rated by peak V not RMS
    Yes I believe so, but I think it's the fritzy way the MEN AC systems works. When you rectify 240V AC, you get 320V DC with ripple coming out of the rectifier bridge, hence the requirement for 400V caps because that's right where they are sitting.

    I don't know, but as it was explained to me, the voltage rating on the caps needs to exceed the AC rating of the source, at least for our AC power system. 220V caps work, but they run hot and have a higher failure rate.

    Back the 80's when I was in IT support, we supplied some 10 base 2 de-muxers into government IBM sites in the city (won't even say which city). There were over a hundred of these things installed before they started to fall over left right and center. I spent a lot of time after that replacing capacitors in the power supplies as they failed. Yep, it was American hardware, and I was replacing 220V caps with 400V versions. It wan't a space issue, they just went cheap on the PS caps. Often the circuit board under the caps was dark brown from the heat. Failure mode was usually the devices failing to start (or just stopping) when the ripple caused the power supply to go out of spec and the CPU wouldn't work, but some of them failed more spectacularly. By that time we were trying to find all ones that hadn't been upgraded and doing them before they failed.

    Years later I had the same thing but with Samsung monitors - that was back in the day when Samsung was still a dirty word. Again it wasn't a space issue, they'd just used the cheaper components without taking into account our power system.
    Last edited by fabricator4; 29-11-17 at 03:45 PM.
    Chris: Tobacco free since 17:00 15th March 2013.

  5. #25
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    Quote Originally Posted by gurishka View Post
    Yup.. Pwm and all the fun stuff.. My issue is a lot of people will read that and blink blankly.. Lol. It's like when I talk about valve amplifiers and so forth.
    LOL, that's why I did the TLDR. I'm fully aware that's all some people want to see.

    I like to say the WHY as well the WOT, if it helps someone or leads to further dialogue. Style is likely to come along and put me in my place any minute, for example.
    act180, gurishka and Danny76 like this.
    Chris: Tobacco free since 17:00 15th March 2013.

 

 
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